An update of the lithostratigraphy of the Ieper Group. Version 9 Revision Litho...1 Version juin 2016 1 Proposal Lithostratigraphy Ieper Group 06/2016 2 3 An update of the lithostratigraphy

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Proposal Lithostratigraphy Ieper Group 06/2016 1

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An update of the lithostratigraphy of the Ieper3

Group.4

5

Etienne Steurbaut, Marleen De Ceukelaire, Tim Lanckacker, Johan Matthijs, 6

Peter Stassen, Hervé Van Baelen, Noël Vandenberghe 7

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To refer to the present document in publications use the following reference : 9

Steurbaut, E. , De Ceukelaire,M., Lanckacker T., Matthijs, J., Stassen, P., Van Baelen H., 10

Vandenberghe, N., 2016. Lithostratigraphy Ieper Group. 11

http://ncs.naturalsciences.be/Paleogene‐Neogene/proposals‐and‐discussions. 12

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TableofContents15

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An update of the lithostratigraphy of the Ieper Group. ............................................................ 1 17

Introduction ............................................................................................................................... 7 18

Context of the review. ............................................................................................................ 8 19

The use of geophysical well logs and a compendium of reference logs. ............................... 9 20

Relationship between lithostratigraphy and bio‐chronostratigraphy. ................................ 10 21

IEPER GROUP ............................................................................................................................ 11 22

Authors: ................................................................................................................................ 11 23

Description: .......................................................................................................................... 12 24

Age: ....................................................................................................................................... 12 25

Regional distribution: ........................................................................................................... 12 26

Stratotype ............................................................................................................................. 13 27

Subdivisions ......................................................................................................................... 13 28

Kortrijk Formation .................................................................................................................... 15 29

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Authors ................................................................................................................................. 15 30

Description ........................................................................................................................... 15 31

Stratotype ............................................................................................................................. 15 32

Area ...................................................................................................................................... 16 33

Thickness .............................................................................................................................. 16 34

Members .............................................................................................................................. 16 35

Age ........................................................................................................................................ 16 36

Remarks ................................................................................................................................ 16 37

Het Zoute Member ................................................................................................................... 16 38

Authors ................................................................................................................................. 16 39

Description ........................................................................................................................... 16 40

Regional occurrence and previous name: ............................................................................ 17 41

The volcanic ash: .................................................................................................................. 17 42

Stratotype ............................................................................................................................. 17 43

Mont‐Héribu Member .............................................................................................................. 17 44

Authors ................................................................................................................................. 17 45

Description ........................................................................................................................... 17 46

Regional occurrence and previous names: .......................................................................... 18 47

Stratotype ............................................................................................................................. 19 48

Orchies Member ....................................................................................................................... 19 49

Authors ................................................................................................................................. 19 50

Description ........................................................................................................................... 19 51


Stratotype ............................................................................................................................. 21 53

Geophysical borehole references and Subdivisions of the Orchies Member ...................... 21 54

Roubaix Member ...................................................................................................................... 23 55

Authors ................................................................................................................................. 23 56

Description ........................................................................................................................... 23 57


Stratotype ............................................................................................................................. 24 59

Geophysical borehole references ........................................................................................ 25 60

Aalbeke Member ...................................................................................................................... 25 61

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Authors ................................................................................................................................. 25 62

Description ........................................................................................................................... 25 63

Regional occurrence and previous names ........................................................................... 26 64

Stratotype ............................................................................................................................. 26 65


‘pink silt’ bed ............................................................................................................................ 27 67

Mons‐en‐Pévèle Formation ...................................................................................................... 27 68

Authors ................................................................................................................................. 27 69

On the Formation status: ..................................................................................................... 27 70

Description : ......................................................................................................................... 28 71


Lithological trends and paleogeography: ............................................................................. 29 73

Stratotype ............................................................................................................................. 30 74


Tielt Formation ......................................................................................................................... 31 76

On the position of the Egem Member ................................................................................. 31 77

Tielt Formation ......................................................................................................................... 31 78

Authors ................................................................................................................................. 31 79

Description ........................................................................................................................... 31 80

Stratotype ............................................................................................................................. 32 81

Area ...................................................................................................................................... 32 82

Thickness .............................................................................................................................. 32 83

Members .............................................................................................................................. 32 84

Age ........................................................................................................................................ 32 85

Remarks ................................................................................................................................ 32 86

Kortemark Member .................................................................................................................. 33 87

Authors ................................................................................................................................. 33 88

Description ........................................................................................................................... 33 89


Stratotype ............................................................................................................................. 34 91


Egemkapel Member ................................................................................................................. 35 93

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Authors ................................................................................................................................. 35 94

Description ........................................................................................................................... 35 95


Stratotype ............................................................................................................................. 36 97


Hyon Formation ........................................................................................................................ 37 99

Authors ................................................................................................................................. 37 100

Description ........................................................................................................................... 38 101


Stratotype ............................................................................................................................. 39 103

Biostratigraphy ..................................................................................................................... 39 104

Egem Member .......................................................................................................................... 39 105

Authors ................................................................................................................................. 39 106

Description ........................................................................................................................... 39 107


Stratotype ............................................................................................................................. 40 109


Mont‐Panisel Member and the Bois‐la‐Haut Member ............................................................ 41 111

Authors ................................................................................................................................. 41 112

Name .................................................................................................................................... 41 113

Description ........................................................................................................................... 41 114


Stratotype ............................................................................................................................. 44 116


Differentiation between the Mont‐Panisel and the Egem Members: ................................. 45 118

Gentbrugge Formation ............................................................................................................. 45 119

Author ................................................................................................................................... 45 120

Description ........................................................................................................................... 46 121

Stratotype ............................................................................................................................. 46 122

Area ...................................................................................................................................... 46 123

Thickness .............................................................................................................................. 46 124

Members .............................................................................................................................. 46 125

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Age ........................................................................................................................................ 47 126

Kwatrecht Member .................................................................................................................. 47 127

Authors ................................................................................................................................. 47 128

Description ........................................................................................................................... 48 129

Regional occurrence and stratigraphic position .................................................................. 48 130

Stratotype ............................................................................................................................. 48 131


Merelbeke Member ................................................................................................................. 49 133

Authors ................................................................................................................................. 49 134

Description ........................................................................................................................... 49 135


Stratotype: ............................................................................................................................ 50 137


Pittem Member ........................................................................................................................ 51 139

Authors ................................................................................................................................. 51 140

Description ........................................................................................................................... 51 141


Stratotype ............................................................................................................................. 52 143


Hooglede Sandstone Bed ......................................................................................................... 52 145

Authors ................................................................................................................................. 52 146

Description ........................................................................................................................... 53 147


Stratotype ............................................................................................................................. 53 149

Vlierzele Member ..................................................................................................................... 53 150

Comment on the stratigraphic ranking ................................................................................ 53 151

Authors ................................................................................................................................. 54 152

Description ........................................................................................................................... 54 153


Stratotype ............................................................................................................................. 56 155


Aalterbrugge Bed ..................................................................................................................... 56 157

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Authors ................................................................................................................................. 56 158

Description ........................................................................................................................... 56 159

Boundaries............................................................................................................................ 57 160

Regional Occurrence ............................................................................................................ 58 161

Stratotype ............................................................................................................................. 58 162

Remark ................................................................................................................................. 58 163

COMPENDIUM OF REFERENCE LOGS WITH COMMENTS ....................................................... 59 164

Brugge (023W0454) ............................................................................................................. 61 165

Gent (055W1020) ................................................................................................................. 61 166

Hijfte (040E0373) ................................................................................................................. 61 167

ON‐Kallo 1 (014E0355) ......................................................................................................... 62 168

Kerksken (086E0340) ........................................................................................................... 63 169

Kester (101W0079) .............................................................................................................. 63 170

Knokke (011E0138) .............................................................................................................. 63 171

Kruishoutem (084E1412) ..................................................................................................... 65 172

Merchtem (072E0229) ......................................................................................................... 65 173

Merksplas (017W0280) ........................................................................................................ 66 174

Mol SCK 15 (031W0237) ...................................................................................................... 66 175

Oosterzele (070E0237) ........................................................................................................ 67 176

Pittem (053W0073) .............................................................................................................. 67 177

Rijkevorsel (007E0200) ........................................................................................................ 67 178

Tielt (053E0061) ................................................................................................................... 67 179

Wieze (072W0159) .............................................................................................................. 68 180

Wortegem (084W1475) ....................................................................................................... 68 181

Zemst‐Hofstade (073E0397) ................................................................................................ 68 182

Zemst‐Weerde (073E0359) .................................................................................................. 69 183

Referenties: .............................................................................................................................. 70 184

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Preface 188

189

The working group was installed by the Paleogene‐Neogene subcommission meeting in 190

2014 (S.Louwye chair, K.De Nil, secretar). Working Group Members are Marleen De 191

Ceukelaire, Tim Lanckacker, Johan Matthijs, Peter Stassen, Etienne Steurbaut, Hervé Van 192

Baelen, Noël Vandenberghe (coordination by N.Vandenberghe and M. De Ceukelaire). A 193

first meeting was held 20th December 2014 and a discussion text was drafted 20th February 194

2015. Comments received were discussed by the Paleogene‐Neogene subcommission on 195

13th July 2015. A revised draft was discussed by a working group meeting on 14th August 196

2015. This new version 7 is based on these discussions and has been complemented by a 197

series of geophysical well logs interpreted in terms of the lithostratigraphic subdivisions 198

proposed and discussed in the text. The document is forwarded to the Working Group in 199

October 2015 for approval to submit it to the Subcommission for posting as discussion text 200

on the NCS website. The discussion period on the website ended March 2016. The present 201

text results from a final discussion meeting by the working group in April 2016. The text is 202

submitted in May 2016 for approval to the Subcommission Paleogene‐Neogene. 203

Acknowledgements 204

Michiel Dusar, Katrien De Nil , Rik Houthuys, Thierry Smith and Laurent Wouters and are 205

sincerely thanked for their valuable contributions to the discussions. 206

207

208

Introduction209

The present Ieper Group lithostratigraphy as officialised by NCS presents the 210

outcome of a synthesis work by the Ieper Group working group. The data 211

present the understanding of the lithostratigraphy in mid 2016, based on 212

published data and the evaluation by the working group members. 213

The use of the electronic stratigraphic compendium, as practised by NCS 214

since several years, is replacing the traditional periodic edition of a printed 215

book version. This approach offers the possibility to continuously adapt the 216

lithostratigraphy with new finding , respecting the procedure of NCS 217

(National Committee of Stratigraphy), and hence keeping the 218

lithostratigraphic nomenclature always up to date. This electronic version 219

will therefore in the future be modified as research progresses and all 220

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modifications will always be properly authored so that an appropriate 221

reference can be made to the new contributions. 222

To refer to the present document in publications use the following reference 223

Steurbaut, E. , De Ceukelaire,M., Lanckacker T., Matthijs, J., Stassen, P., Van Baelen H., 224

Vandenberghe, N., 2016. Lithostratigraphy Ieper Group. 225

http://ncs.naturalsciences.be/Paleogene‐Neogene/proposals‐and‐discussions. 226

Briefly discussed are the context of the review, the reason for the emphasis 227

on geophysical wells, and the role of biostratigraphic data in the 228

lithostratigraphic interpretation. 229

Contextofthereview.230

The objective of the present revision is to complement the lithostratigraphy 231

of the Ieper Group published in 2001 (Laga et al., 2001). This last publication 232

reflected the activities in the Tertiary Subcommission at that time. The review 233

published in 2001 framed in an initiative of the National Stratigraphic 234

Commission and was limited to the lithostratigraphy at formation level. The 235

Laga et al (2001) reference document has been the basis for the NCS website 236

until now. 237

The Ieper Group is characterised by clay‐dominated sediments overlying, in 238

most situations, the Landen Group strata and, if not outcropping, underlying 239

the sand‐dominated Zenne Group sediments. According to Laga et al. (2001) 240

in their reference document for Paleogene and Neogene lithostratigraphy, 241

the Ieper Group consists of the Kortrijk, Tielt and Gentbrugge Formations and 242

members in these Formations are only listed. These subdivisions are also 243

used on the 1:50 000 geological maps of Flanders, edited in the last decades 244

of the 20th century. 245

Especially the additional description of the members, and where possible, 246

horizons, identified in the Formations, made the present review necessary 247

and also modifications at the formation level itself arisen since 2001 needed 248

to be integrated in a new synthesis. 249

The present update is based on the earlier description of members in 250

Maréchal & Laga (1988), Geets et al. (2000) and Steurbaut (1998) as far as 251

appropriate. All modifications, discussions and additions are supported by 252

the relevant literature references. 253

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Theuseofgeophysicalwelllogsandacompendiumofreferencelogs.255

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In the present update the use of geophysical borehole logs in the 257

characterisation and definition of lithostratigraphical units is formally 258

introduced. This is a logical evolution as substantial amounts of data on the 259

Ieper Group are derived from subsurface reconnaissance studies. 260

In comparison to common field observations in outcrops and clay pits, 261

geophysical logs in boreholes offer the advantage of characterising the 262

vertical succession of several stratigraphic units and commonly offer a 263

continuous characterisation of the transition and boundaries between 264

lithostratigraphic units. 265

Natural‐gamma‐ray (GR) logs and resistivity (RES) logs are the commonly 266

available geophysical data, but also other logs can serve as proxies for 267

lithology. 268

Continuously recorded data in geophysical borehole logs offer a consistent 269

way to subdivide the stratigraphic column in ‘lithological intervals’ with 270

similar properties. Such intervals can be based upon upward coarsening or 271

fining up trends, levels of changing trends, or any particular log signature. 272

Trends and levels can be correlated between boreholes. Although a purely 273

geophysical stratigraphic subdivision can be made irrespective of already 274

known lithostratigraphic units, in this review it is chosen for the logical 275

approach to accommodate the traditional field and core borehole‐based 276

lithostratigraphy into the newly discussed geophysical log subdivisions. It is 277

also realised that at this stage a one to one relationship between a 278

geophysical log‐defined limit and a field defined boundary between units will 279

not always be possible; in such cases the specific choices made are explained 280

in the text. 281

282

The geophysical‐well‐log‐based lithostratigraphic subdivision and 283

interpretation of the Ieper group interval has benefited from several previous 284

attempts. An early attempt correlated logs irrespective of existing 285

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lithostratigraphic nomenclature: based on trends and events 9 correlation 286

levels were identified in 8 large plates (Vandenberghe et al., 1991). 287

Subdivisions and correlations have been published by Steurbaut (1998) and 288

Vandenberghe et al. (1998). A subdivision of the Kortrijk Formation using 289

resistivity logs was proposed by De Ceukelaire & Jacobs (1998). Van Marcke 290

et al. (2005), Welkenhuysen & De Ceukelaire (2009) and Walstra et al. (2014) 291

applied pattern recognition and correlation in numerous examples across 292

North Belgium. 293

To illustrate the subdivisions discussed in the text a set of 19 well log 294

interpretations is added as a reference compendium. The borehole localities 295

are chosen to cover the whole area of occurrence of the Ieper Group. 296

Brugge 023W0454, Gent 055W1020, Hijfte 040E0373, Kallo 014E0355, Kester 297

101W0079, Kerksken 086E0340, Knokke 011E138, Kruishoutem 084E1412, 298

Merchtem 072E0229, Merksplas 017W0280, Mol 031W0237, Oosterzele 299

070E0237, Pittem 053W0073, Rijkevorsel 007E0200, Tielt 053E0061, Torhout 300

052E0195, Wieze 072W0159, Wortegem 084W1475, Zemst‐Hofstade 301

073E0397, Zemst‐Weerde 073E0359. 302

In an exemplary way on these logs, most, but not always all, units are 303

identified, depending on the quality of the logs. 304

Subdividing logs requires particular attention to boundary levels between 305

units, straightforward in case marked jumps in properties are observed. In 306

the Ieper Group, lithological properties such as grain size are often evolving 307

within units and not constant as the definition of a lithostratigraphic unit 308

intuitively might suggest. In such cases the precise definition of the upper and 309

lower limiting surfaces can be more subject to debate. The guideline in the 310

choice should be the picking of those boundary surfaces that have the best 311

chance of being recognised in other logs, in the field and in borehole 312

descriptions. 313

314

Relationshipbetweenlithostratigraphyandbio‐chronostratigraphy.315

316

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The subdivisions aimed at in this review are exclusively lithostratigraphic. For 317

many current purposes a coherent and consistent lithostratigraphic 318

nomenclature in a region, such as proposed in this review, is a prerequisite. 319

Obviously a deeper stratigraphic understanding of strata needs 320

lithostratigraphic data to be complemented by biostratigraphic data. This is 321

not the aim of this review. 322

On the other hand, in the case of lithologically similar packages, as is the case 323

in the Ieper Group, biostratigraphy can be required to differentiate such 324

packages and eventually confirm suspected hiati or lateral lithofacies 325

changes. Geophysical well log correlation is also helped by paleontological 326

support. 327

Biostratigraphy is also the key methodology to correlate between regions and 328

basins and to situate the deposits in the international chronostratigraphic 329

chart. The Ieper Group strata are all Ypresian or Lower Eocene. Details of the 330

bio‐ and chronostratigraphy are to be discussed separately on the website. 331

Basic biostratigraphic data are given in Steurbaut (1987, 1991, 1998, 2006, 332

2011) for calcareous nannoplankton zonations, in De Coninck (1975, 1991, 333

1996) for dinoflagellate data and in Kaaschieter (1961) and Willems (1982) for 334

foraminifera data. Summary descriptions are available in Steurbaut et al. 335

(2003). Magnetostratigraphy is another means for interregional and inter‐336

basin correlations. In the Ieper Group, clay‐pit sections have been 337

investigated for magnetostratigraphy by Ali et al. (1993). A methodology for 338

integrating all stratigraphic data, and including in particular the obvious 339

cyclicity in the Ieper Group strata, is the sequence stratigraphy approach, also 340

relying heavily on geophysical well logs. Such an approach however is already 341

interpretative and strongly depends on biostratigraphic calibration; therefore 342

it will be dealt with in the chronostratigraphy section of the website. 343

IEPERGROUP344

Authors:345

The term Ieper Group was introduced by Maréchal (1993, p 224), and 346

described by Steurbaut (1998, p 109) and in Geets et al. (2000). The Ieper 347

Group includes all strata previously grouped in the Ieper and Vlierzele 348

formations by Steurbaut & Nolf (1986). The present revision of the 349

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stratigraphic hierarchy between the different formations and members 350

within the newly defined group is based on their suitability for mapping and 351

their lithological and faunal distinctive properties. 352

The Ieper Group is named after the town of Ieper, Ypres in French, in West 353

Flanders; Ypres is also the reference name for the Ypresian global stage. 354

Description:355

The Ieper Group contains marine sediments which consist dominantly of clay 356

in the lower part of the Group and become silty towards the top of the 357

middle part and evolve to fine sandy sediments in the upper part of the 358

Group. 359

Age:360

The Ieper Group almost coincides with the Ypresian or early Eocene age. Only 361

the Tienen Formation of the Landen Group, below the Ieper Group, 362

represents the very earliest Ypresian and the basal part of the Zenne Group 363

above represents the very late Ypresian. Therefore the age of the Ieper Group 364

can be estimated between about 55 and 49 Ma (see Vandenberghe et al. 365

2012 in GTS 2012). 366

Regionaldistribution:367

The Ieper Group occurs in the western, central and northern part of Belgium. 368

The Group outcrops are located especially in northern Hainaut, south and 369

central West and East Flanders, west and southwest of Brabant; the Group 370

occurs in the subsurface of the Antwerp and Limburg Campine. Outliers occur 371

in the Mons basin south of the Sambre river. Towards the east in the Brabant, 372

Limburg and Antwerp provinces, the Ieper Group thins and disappears. Maps 373

of the different Formations in the Ieper Group, recognised at different 374

moments in the development of the Ieper Group stratigraphic research and 375

practice, can be found in Maréchal (1993), (Walstra et al., 2014) and can be 376

consulted at the D.O.V. website: 377

https://dov.vlaanderen.be/dovweb/html/3isohypsen.html#waar. 378

https://dov.vlaanderen.be/dovweb/html/services.html#NPisohypsen or 379

https://dov.vlaanderen.be/dovweb/html/3G3Ddata.html. 380

The Ieper Group overlies the Landen Group or locally Paleozoic rocks. In the 381

Gent area and in the northwest the Ieper group is covered by the Aalter 382

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Formation of the Zenne Group. To the north and the east the Group is 383

overlain by the Brussels or Lede Formations of the Zenne Group. In the 384

exceptional case of the absence of these formations the Ieper group can be 385

covered by the Maldegem Formation or the Sint‐Huibrechts‐Hern Formation 386

in the southeast. In the coastal plain, the alluvial plains of the Leie and the 387

Upper‐Scheldt, the Ieper Group is overlain by thick late Quaternary 388

sediments. 389

The maximal thickness is about 200m and thinning occurs towards the south 390

and the east. 391

Stratotype: 392

The lower boundary stratotype is defined in Steurbaut (1998) at 288m depth 393

in the Knokke borehole (011E0138) at the contact between the Tienen 394

Formation (Oosthoek Member) and the Kortrijk Formation (Zoute Member), 395

topographic map sheet 5/6 Westkapelle with coordinates X = 78.776, Y = 396

226.370, Z = +4,91 m. 397

The upper boundary stratotype is defined in Steurbaut (1998) in the profile of 398

the Mont‐des‐Récollets (Cassel, France) at the contact of the Vlierzele 399

Member and the Aalter Formation of the Zenne Group described in Nolf & 400

Steurbaut, 1990, mapsheet XXIII‐3,Cassel, France with coordinates X = 62.000, 401

Y = 344.500, Z = +143 m. 402

403

Subdivisions: 404

The lithostratigraphic subdivisions recognised have been ranked and 405

represented in a schematic table below ; the two columns in the table 406

emphasize that the units represented in the right column occur in an area 407

lateral of the units listed in the left column; their precise lateral 408

correspondence is not entirely known. The table is schematic and a regional 409

distribution of the units is represented in more detail in the figure 410

“doorsnede” in bijlage. 411

The units are further discussed in the text in their approximate stratigraphic 412

order , from old to young. 413

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414

LITHOSTRATIGRAPHIC TABLE IEPER GROUP 415

416

Zenne Group Aalter Formation 417 418

Ieper Group Gentbrugge Formation 419 Aalterbrugge Member 420 Vlierzele Member 421 Pittem Member 422

Hooglede Bed 423 Merelbeke Member 424

Kwatrecht Member 425 426 ____________________________________________________________________________427 ___ 428 Hyon Formation 429

Mont‐Panisel Member 430

Egem Member Bois‐la‐Haut Member 431 _____________________ ________ ________ ________ ________ _ 432 433 Tielt Formation 434

Egemkapel Member 435 Kortemark Member 436 ____________________________________________________________ 437 Kortrijk Formation 438

Aalbeke Member 439

440 ______________________ ________ ________ 441

Roubaix Member Mons‐en‐Pévèle Frm 442 443 _______________________ ________ ________ 444 Orchies Member Orchies Member 445

(Upper,Middle, Lower ) Mont‐Héribu Member 446 Het Zoute Member / 447

Landen Group Tienen Formation 448 ................................ 449

450

451

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KortrijkFormation. 452

Authors: Geets (1988), Steurbaut (1998). 453

Description: the formation is an essentially marine deposit, consisting mainly 454

of clayey sediments. 455

A standard sequence contains from bottom to top: 456

‐ an alternation of horizontally laminated, glauconiferous clayey sands or 457

sandy clay, and compact, silty clay or clayey silt, locally bioturbated. The base 458

consists of oxidized and indurated clayey sand, with lenses of pure sand 459

(Mont Héribu). 460

‐ a homogeneous deposit of very fine silty clay, with some thin intercalations 461

of coarse silty clay or clayey, very fine silt (Orchies Mbr); 462

‐ a less homogeneous deposit of clayey, coarse or medium silt, with some 463

sand containing layers; fossil rich layers occur; the whole deposit becomes 464

more sandy to the east and the south (Roubaix Mbr); 465

‐ a very fine and compact clay and in addition also silty clay parts (Aalbeke 466

Mbr). 467

To the east, in the Brabant and the Campine, and towards the Mons basin, 468

the deposits become more sandy. 469

Stratotype: 470

The formation is defined by boundary stratotypes in Steurbaut (1998). The 471

lower boundary stratotype is placed at 288 m depth in the Knokke borehole 472

(011E0138) at the base of the Het Zoute Mbr. Sheet 5/6 (Westkapelle). Co‐473

ordinates: x = 78.776, y = 226.370, z = +4.91 m. The upper boundary has been 474

placed in the Tielt borehole (068E0169) at the top of the Aalbeke Mbr. This 475

upper boundary is located at 48.5 m in the compendium (Tielt 053E0061); in 476

earlier versions (Geets, 2000), the Aalbeke top was mislocated at 71 m. 477

Steurbaut (1998, p117) correlated the in‐the‐present‐text defined top of 478

Aalbeke Member (see also further details under Aalbeke Mbr) with the top of 479

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his unit D of the Kortemark silt member (sensu Steurbaut (1998)) in the Tielt 480

borehole located at 46.7 m. Sheet 21/6 (Wakken). Co‐ordinates: x =76439, y = 481

187576, z = +48 m. 482

Area: the formation is found in the western and central part of Belgium. It 483

outcrops in the north of Hainaut, the southern and central part of West‐484

Flanders, the south of East‐Flanders Flanders and the southwest of Brabant. 485

Outliers occur in the Mons Basin and south of the river Sambre. 486

The regional distribution map of the occurrence of the Kortrijk Formation in 487

Belgium is figured in Maréchal (1993, p 221) and Walstra et al. (2014) and can 488

be consulted at the D.O.V. website (dov.vlaanderen.be). 489

Thickness: 125 m in the northern part of West‐Flanders, but the thickness 490

decreases in eastern and southern direction. 491

Members: the formation is subdivided into the Het Zoute Mbr, Mont Héribu 492

Mbr, Orchies Mbr (Lower, Middle, Upper), Roubaix Mbr and the Aalbeke Mbr. 493

Age: early and middle Ypresian. 494

Remarks: the formation is also discussed by Cornet (1874), De Ceukelaire & 495

Jacobs (1998), De Coninck (1975), De Coninck et al. (1983), de Heinzelin & 496

Glibert (1957), De Moor & Geets (1975), Geets (1990), Gosselet (1874), 497

Gulinck (1965, 1967), Gulinck & Hacquaert (1954), King (1990), Laga & 498

Vandenberghe (1980), Maréchal (1993), Ortlieb & Chelloneix (1870), 499

Steurbaut (1988), Steurbaut & Nolf (1986), Vandenberghe et al. (1991) and 500

Wouters & Vandenberghe (1994). 501

502

HetZouteMember503

Authors: based on King (1990), Steurbaut (1998),Geets et al. (2000) 504

Description: 505

Silty to sandy clay, bioturbated and with irregular pockets and lenses of very 506

fine silty sand. Fine grained mica, woody debris and glauconite are present in 507

sieve residues throughout the unit (King, 1990). Coarse grained angular to 508

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subangular grains are identified as degraded volcanic ash. Pebbles occur in 509

the base of the overlying clay. 510

Regionaloccurrenceandpreviousname:511

The Zoute Member is a thin unit of almost 5 m thickness found at the base of 512

the Ieper Group section in the Knokke borehole (011E0138) at the Zoute 513

hamlet and first described in detail by King (1990,p70) and at that time 514

named Member X by this author. The name Het Zoute Member was proposed 515

by Steurbaut (1998, p110). It was erroneously interpreted as Mont‐ Héribu 516

Member by Geets & De Geyter (1990, p25). 517

This unit has no equivalent in other sections of the Ieper Group in Belgium 518

where it corresponds to a hiatus between the Landen and Ieper Groups; this 519

is confirmed in Steurbaut (2006, p77). 520

Thevolcanicash:521

The indication of volcanic activity is a particular property of this unit. The 522

other indication of volcanic grains in the basal sediments of the Ieper Group 523

clays are the heavy mineral types in the basal clays, identified as Mont‐Héribu 524

as reported by Geets (1993). 525

This volcanic activity is related to the ash series at base of the Eocene in the 526

North Sea Basin and correlates to the A1 Division of the London Clay 527

Formation (King, 1990, p 80). 528

Stratotype529

Knokke borehole 011E0138, interval 288 to 284,1 m depth. Geological Map 530

5/6 (Westkapelle) 531

Coordinates: X = 78.776, Y = 226.370, Z = + 4,91 m. 532

533

Mont‐HéribuMember534

Authors : De Coninck et al. (1983, p 98), Steurbaut and Nolf (1986,p 123), 535

Steurbaut (1998), Geets et al. (2000), 536

Description: 537

Alternating horizontal laminae of glauconite bearing clayey sand or sandy 538

18

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clays with compact silty clays or clayey silts. Locally burrows are present. The 539

base of the unit consists of cemented clayey sand and lenses of just sand. The 540

unit occurs at the very base of the Ieper Group, except where the Zoute Mbr 541

is present. 542

The definition of the Mont‐Héribu is limited to the sandy base of the Ieper 543

Group. This sandy base is 6 m in the Mons Basin, maximal 10 m southwest of 544

Brussels but in most boreholes it is limited to 1 to 2m and rarely noticed in 545

most boreholes (see for example sections in Gulinck, 1967). Therefore the 546

interpretation of the extension of the Mont‐Héribu Member in the 1:50 000 547

mapping of Flanders is exaggerated and comprises for a large part the 548

overlying Orchies Member. The definition refers to a grain‐size distribution 549

with only a limited clay fraction and a coarser fraction that gradually evolves 550

upward over a short distance to the larger clay content of the Orchies 551

Member. This short pattern of rapidly fining upwards is the typical signature 552

on GR and RES logs (see borehole logs ON‐Kallo‐1 014E0355, Rijkevorsel 553

007E0200). 554

On geophysical well logs 555

In some logs of the compendium, the very short change‐over interval from 556

sandy to clay deposit as observed on RES, GR logs could be considered as 557

Mont Héribu Member although generally it is considered too short to be 558

individually marked. In the ON‐Kallo well, core control allows to identify 559

1.30m of silty clay interpreted as Mont‐Héribu Member at the base of the 560

Ieper Group overlying the Tienen Formation at 401.35m (core) depth 561

(Mohammad, 2009). 562

563

Regionaloccurrenceandpreviousnames:564

The unit was first reported as ‘Argile de l’ Eribus’ (Cornet, 1874, p 567) at the 565

locality Eribus (‘Mont de l’Heribu’, south of Mons) which geology was studied 566

by Ortlieb & Chelloneix (1870, p 168). In the Mons Basin the unit can reach up 567

to 6m and; its maximum thickness is reported from Bierghes (southwest of 568

Brussels) where it reaches 10m (Geets, 1991, including grain‐size data). In 569

many borehole descriptions this unit is not formally recognised as an 570

individual unit, or supposed to be reduced to just a few cm thickness; King 571

19

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(1991) interprets the occurrence of the Mont‐Héribu Member in central West 572

Flanders and not in the Knokke well 011E0138. 573

What is mapped as Yb on the Geological maps 1:40 000 logically corresponds 574

to the Mont‐Héribu Member. In the Stratigraphic Register by the Conseil 575

Géologique (1929) and translated by the Aardkundige Raad (1932), it is 576

included in the Lower Ypresian Y1a. 577

Stratotype: 578

Sand pit at the Mont de l’Héribu south of Mons between +57.5 en +51.4 m 579

topographic height on the geological map 151 Mons‐Givry (topographic map 580

45/7). 581

Coordinates: X = 119.750, Y = 124.510, Z = + 57,5 m. 582

583

OrchiesMember584

Authors: Gosselet (1874, p 611), Steurbaut (1998), Geets et al. (2000) 585

Description: 586

Compact and heavy stiff bluish‐grey clay occurring at the base of the Ieper 587

Group only separated from the base itself by the underlying sandy Mont‐588

Héribu Member where this latter is present. The Orchies Member is overlain 589

by more sandy or silty clay deposits of the Roubaix Member or Mons‐en‐590

Pévèle Formation. The thickness can be up to 40 m. A pebble layer has been 591

reported occasionally at its base (Ya on the 1:40 000 geological maps). 592

Whereas in the visual description of macroscopic samples, even from cores, it 593

is very hard to see any further lithological subdivision of the Orchies Member, 594

the geophysical log signatures (see reference boreholes in compendium) do 595

show a systematic variability interpreted as grain‐size and mineralogical 596

variations. 597

The top of the very high gamma‐ray section at the base of the Orchies 598

Member, about 10 to 15 m thick, is a correlatable surface (see also further 599

Geophysical logs and Subdivisions). It corresponds to the top of the mistakenly 600

named Mont‐Héribu Member (KoMh) unit in the correlation figures in 601

20

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Welkenhuysen and De Ceukelaire (2009 figs 12,14,16,18, 20, 22, 24) and 602

approximately to the level 1 in the plates in Vandenberghe et al. 1991). 603

The top of the Orchies clay Member/ base Roubaix Member or base Mons‐604

en‐Pévèle Formation, is defined by the top of the very clay‐enriched 605

sediment. This level could be identified with confidence in descriptions of a 606

series of destructive boreholes by G. De Geyter (1990, Archives Belgian 607

Geological Survey) as the transition from silty clay above to heavy clay below. 608

(courtesy Marleen De Ceukelaire). 609

610


The Orchies Member consistently occurs where the Ieper Group occurs in 612

Belgium. In the Hainaut area thickness is between 10‐16m whilst in central 613

Flanders and north Belgium thickness can be over 40 m. Towards the east in 614

Brabant its thickness is reduced to a few meters. 615

Originally the name was introduced by Gosselet (1874, p 611) to indicate the 616

compact and stiff clays at the base of what is now known as the Ieper Group 617

sediments; later, as a refinement of the lithostratigraphy, the sandy and silty 618

Mont‐Héribu Member at its base was individualised as a separate unit and 619

the name Orchies Member was reserved for the compact heavy clays above 620

the Mont‐Héribu Member (Steurbaut, 1998). The later introduced name 621

Saint‐Maur Member (Belgian stratotype area, Geets, 1988; Maréchal, 1993), 622

used in the legend of the 1:50 000 mapping in Flanders is a synonym of the 623

Orchies Member although it (Saint‐Maur unit) was generally used in a more 624

restrictive way, because the lower part of the Orchies Member was 625

erroneously assigned to the Mont Héribu Member; it is preferred to maintain 626

the original name Orchies, a small locality to the southeast of Roubaix in 627

Northern France. 628

On maps 1:25 000 of the Brabant Wallon (Nivelles‐Genappe, Braine‐le‐Comte 629

‐Féluy) the ‘Formation de Carnières’ is used for a unit ‘close to Orchies’. 630

On the legend of the geological maps 1:40 000 the Orchies Member was 631

included in the Yc clayey deposits and in the stratigraphic register (1929, 632

1932) in the Y1a. 633

21

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The ‘argilite de Morlanwelz’ is a lateral equivalent of the Orchies Member 634

(Steurbaut, 1991). 635

The Wardrecques and Bailleul members are reported in King (1991). The 636

lower part of the Wardrecques member belongs to the Orchies Member 637

whilst the upper part and the Bailleul member corresponds to the Roubaix 638

Member (King, 1991). This subdivision is not commonly used in the literature 639

but the position of Wardrecques and Bailleul members is well documented in 640

boreholes of the Moeskroen‐Kortrijk‐Marke‐Ooigem area by King (1991, fig. 641

11). In this area at least 5 glauconiferous beds occur, each less than 15 cm 642

thick (King, 1991). 643

644

Stratotype: 645

The Wahagnies clay pit (“Briquetterie de Libercourt”) in northern France, map 646

sheet XXV‐5 (Carvin). Ortlieb & Chelloneix (1870, p25) had already used the 647

name ‘Argile de Wahagnies’ to indicate the Orchies Member compact clays 648

(Steurbaut, 1998). In the clay pit, the base is defined by the basal pebble bed 649

below about 8 m of stiff clays. Coordinates: X = 649.250, Y = 310.600, Z = +50 650

m. 651

The upper boundary, marking the limit with the overlying Roubaix Member, 652

is proposed in the Kallo well 027E0148 (Gulinck, 1969) at 341m depth 653

(Steurbaut, 1998, p 112) (see also below). 654

GeophysicalboreholereferencesandSubdivisionsoftheOrchiesMember655

Several horizons in the Orchies clay Member can be defined and correlated 656

with reasonable confidence across the whole area of occurrence of the 657

Orchies Member. 658

The base of the Orchies is defined by the appearance of very clay enriched 659

sediments and a corresponding rapid installation of a high GR level. 660

The top of the Orchies clay Member/ base Roubaix Member or base Mons‐661

en‐Pévèle Formation, is defined by the top of the very clay‐enriched 662

sediment. This level could be identified with confidence in descriptions of a 663

series of destructive boreholes by G. De Geyter (1990, Archives Belgian 664

22

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Geological Survey) as the transition from silty clay above to heavy clay below. 665

(courtesy Marleen De Ceukelaire). On the geophysical logs, this level can 666

most easily and reproducibly be picked below the base of the first marked 667

sandy excursion on RES logs (labeled 6 in the compendium) and the 668

corresponding sharp drop in GR. 669

This defined top of the Orchies Member occurs at a short distance above the 670

Orchies/Roubaix boundary defined by Steurbaut (1988) and labeled OR ES88 671

on the logs in the compendium where appropriate; this OR ES88 boundary 672

was put at 331,5m in the BGD‐Kallo borehole. However, this last level is a thin 673

sandy layer systematically occurring close to the now defined top of the 674

Orchies Clay Member, with a corresponding small log signature identifiable in 675

GR and RES. This thin sand layer and corresponding geophysical log 676

signature, can also be traced in the cored Mol SCK15/1974 borehole 677

described by Gulinck & Laga (1975) just below the Mons‐en‐Pévèle 678

Formation. This observation leads to the conclusion that the base of the 679

Roubaix Member and the Mons‐en‐Pévèle Formation develop at about the 680

same moment in that part of the basin. 681

In 1998 Steurbaut (1998) picked another boundary between the Orchies and 682

Roubaix Members in the BGD‐Kallo well 027E0148 at 341m instead of at 683

331,5m as before in Steurbaut (1988). The log signature associated with this 684

1998 definition, labeled OR ES98, can easily be recognised as it is plotted by 685

Steurbaut (1998, Fig.10) on a series of logs including the Rijkevorsel well 686

007E0200 also included in the compendium; it occurs at a marked GR low , 687

slightly more than about 10m below the top of the Orchies Member. 688

Therfore it is agreed to keep this level as a formal subdivision of the Orchies 689

Member, defining the Upper Orchies Member between this level and the top 690

of the Member. 691

692

The lower part of the Member with stable very high GR readings can also be 693

reliably delineated by the top of this high GR reading, where the signal 694

recedes to lower readings. This level defines the boundary between the 695

Lower Orchies Mmeber below and the Middle Orchies Member above it. The 696

23

Version juin 2016

OR ES98 level , defines the boundary between the Middle and Upper Orchies 697

Member. 698

Still other horizons have a reliable correlation potential as illustrated in 699

Steurbaut (1998, Fig 10 ), Vandenberghe et al. (1991), Van Marcke et al. 700

(2005) and others. For example a change in GR gradient recognized in the 701

ON‐Kallo well 014E0355in the compendium at 375m (Mohammad, 2009) can 702

be correlated with the 416,5m level in the ON‐Doel well, in which it is named 703

the Orchies/Roubaix boundary (Van Marcke et al., 2005). However such 704

horizons are not used to further officially subdivide the Orchies Member. 705

706

707

708

RoubaixMember709

Authors: Gosselet (1874), Steurbaut & Nolf (1986, p 123), Steurbaut (1998), 710

Geets et al. (2000). 711

Description: 712

In contrast with the underlying (Orchies Member) and overlying (Aalbeke 713

Member) compact heavy clays, the Roubaix Member consists of more silty to 714

fine sandy calcareous clays. The thickness varies from about 40m in south 715

Belgium to 60m in North Belgium. Calcareous fossils like nummulites and 716

molluscs are present. Glauconite‐rich horizons occur. The more 717

heterogeneous composition of the sediment is shown by layering (see e.g. 718

Marke quarry in Steurbaut, 2006 fig. 7), also well visible in the geophysical 719

well logs. 720

Several of these specific layers, labeled 1 to 6 in the log examples in the 721

compendium, can be recognised and correlated between well logs with a 722

reasonable degree of confidence. Based on the correlation of these levels, 723

Welkenhuysen and De Ceukelaire (2009) have selected a specific level as the 724

boundary level between the Orchies and the Roubaix Member which 725

corresponds approximately to the position of the Steurbaut (1988) definition. 726

24

Version juin 2016

On the geophysical logs in the compendium, this level can most easily and 727

reproducibly be picked below the base of the first marked sandy excursion 728

on RES logs (labeled 6 in the compendium) and the corresponding sharp drop 729

in GR. 730

731


The Roubaix Member occurs over northwest France, north Hainaut, east and 733

west Flanders. Towards the south the occurrence of sandy layers becomes 734

more pronounced whilst to the northwest the Member becomes more clayey 735

and hardly distinguishable from the underlying Orchies Member (Geets et al., 736

2000). Towards the southeast and the east the Roubaix Member evolves into 737

a fine sandy unit, the Formation of Mons‐en‐Pévèle (see further). 738

The later introduced Moen Member (Belgian stratotype area, Geets, 1988; 739

Maréchal, 1993) used in the legend of the 1:50 000 mapping of Flanders, is 740

synonymous with the Roubaix Member. Roubaix is a town in North France 741

and was the original reference for this clay type as described by Gosselet 742

(1874) and therefore the name Roubaix Member is retained. 743

In the 1:40 000 mapping the Roubaix Member was mapped in the Yc unit, 744

however in the Kortrijk area it was erroneously mapped as Yd (Steurbaut & 745

Nolf, 1986; Geets et al., 2000). In the Stratigraphic Register of the Conseil 746

Géologique (1929) and the Aardkundige Raad (1932), the Roubaix Member is 747

included in the Y1a unit. 748

Stratotype: 749

The Roubaix Member was previously exposed along the Bossuit Canal at 750

Moen (near Kortrijk) (Steurbaut & Nolf, 1986) and in the Marke and Heestert 751

clay pits near Kortrijk. As these outcrops are no longer accessible a reference 752

section for the lower boundary is choosen in the Kallo well (Gulinck, 1969) at 753

341m depth (see further) whilst an upper boundary with the overlying stiff 754

clays has been visible in the former Kobbe clay pit (DOV kb29d97e‐B989) at 755

Aalbeke (x= 68.450, y= 164.300, z= 49 m) Steurbaut (1998). 756

25

Version juin 2016

Geophysicalboreholereferences757

The base of the Roubaix Mbr has been defined in the literature in 2 different 758

ways by Steurbaut (1988, 1998). In the Kallo log (014E0355) (Steurbaut,1988) 759

the boundary between the Orchies and Roubaix Members is put at the top of 760

heavy clay at 331,5m whilst in the 1998 definition the boundary is put 10m 761

lower at 341m. The correlation between the Kallo well 027E0148 (without 762

geophysical logs) and the ON‐Kallo‐1 014E0355 with geophysical logs 763

(courtesy Peter Stassen) allows to identify the log signatures associated with 764

the two definitions. The 1998 definition is also plotted on a series of logs by 765

Steurbaut (1998, Fig.10) located in West‐Flanders but also on the Rijkevorsel 766

well – 007E0200. Therefore the two boundaries can systematically be 767

indicated as OR ES 88 and OR ES 98 on ON‐Kallo‐1 – 014E0355 and Rijkevorsel 768

– 007E0200 and on many other borehole logs in the compendium (see also 769

Mons‐en‐Pévèle Fm). 770

The boundary level between the Orchies and Roubaix Members as defined 771

above (see Description)corresponds to the level selected by Welkenhuysen 772

and De Ceukelaire (2009, e.g. in the Merchtem and the Gent boreholes). 773

AalbekeMember774

Authors: De Moor & Geets (1975), Steurbaut & Nolf (1986), King (1991), 775

Steurbaut (1998) 776

Description: 777

A very compact heavy clay without sand fraction of some 10 m thickness 778

sharply contrasting with more silty to fine sandy overlying (Tielt or Hyon 779

Formations) and underlying units (Roubaix Member or Mons‐en‐Pévèle 780

Formation). The Aalbeke Member is mostly non calcareous. Small pale brown 781

to yellow phosphate nodules are common in the Aalbeke Member. 782

It can be pointed out that this pure clay unit is relatively thin and therefore 783

can be mistaken for other even thinner clay units above, namely the 784

Egemkapel and the Merelbeke units. To unequivocally identify these different 785

clay‐rich layers, a complete vertical succession is often required or support by 786

micropaleontological characterisation. 787

26

Version juin 2016

In most geophysical log responses the lower boundary of the Aalbeke 788

Member is sharply marked; at present there is no field outcrop of the contact 789

between the Roubaix and Aalbeke Members. 790

It is strongly suspected that the top of the Aalbeke Member is an erosive 791

contactas it is overlain by different lithological units in different areas: in clay 792

pits in Aalbeke, it is overlain by the Mont‐Panisel Member of the Hyon 793

Formation, in central Flanders by the Kortemark Member, and in SE Flanders 794

and Brabant by the Hyon Formation. Also, at the base of the overlying 795

Kortemark Member in the De Simpel clay‐pit erosion can be observed 796

(Steurbaut, 1998; Vandenbergh et al., 1998). The upper boundary can be 797

sharp (e.g. Kerksen borehole 086E0340 in compendium, data Geological 798

Service Company; Brugge ‐ 023W0454) or more generally the upper part of 799

the clay unit shows a gradual coarsening upward. In the latter case, the upper 800

boundary of the Aalbeke Member in contact with the Kortemark Member is 801

put at the top of this coarsening upwards section. 802

803

Regionaloccurrenceandpreviousnames: 804

The Aalbeke Member is exposed in the hills around Kortrijk, where also the 805

type locality Aalbeke is located, and in the adjacent border area of north 806

France where it corresponds to the ‘argile de Roncq’ (see De Coninck, 1991 807

fig.9). It occurs in the subsurface of the whole Flanders and has an average 808

thickness of about 10 m varying between 5 and 15 m. 809

On the geological maps 1:40 000 the Aalbeke Member was part of the Yc unit 810

and in the Stratigraphic Register of the Conseil Géologique (1929) and 811

Aarkundige Raad (1932) it is part of Y1a. In the Kortrijk area, on the 1:40000 812

sheets it was mapped erroneously as the ‘P1m’ unit (Merelbeke Member of 813

the Gentbrugge Formation). 814

Stratotype: 815

Several clay pits exist in Aalbeke and the De Witte clay pit, the extension of 816

the now filled‐up Kobbe clay pit – DOV kb29d97e‐B989 (X = 68.450, Y = 817

164.300, Z = + 49 m), designated as stratotype by Steurbaut (1998) (map 818

27

Version juin 2016

sheet 29/5‐6 (Mouscron ‐ Zwevegem), is the logical new unit stratotype 819

locality. 820

821


Exemplary log signatures with the identification of a base and a top of the 823

Aalbeke Member are the boreholes logs of Gent 055W1020, Kallo 014E0355, 824

Merchtem 072E0229, Pittem 053W0073, Rijkevorsel 007E0200, Torhout 825

052E0195, Wieze 072W0159. 826

827

‘pinksilt’bed828

Within the Aalbeke Member outcrops in the Kortrijk area a pronounced 829

pinkish silty layer of some dm thickness occurs. It might serve as a 830

stratigraphic marker bed. However the bed is not given an official bed status 831

as it is not yet established that only one such layer occurs in a complete 832

Aalbeke Member section. 833

834

835

Mons‐en‐PévèleFormation836

837

Authors: 838

King (1991), Steurbaut & Nolf (1986), Steurbaut & King (1994, p180), 839

Steurbaut (1998) 840

OntheFormationstatus:841

Although the Formation status is given to the Mons‐en‐Pévèle sand unit , 842

arguments could be forwarded to consider it as a Member of the Kortrijk 843

Formation. A member status could logically reflect the lateral transition zone 844

with vertically alternating sandy layers and clay layers (such as e.g. in the 845

Mouscron borehole in fig.10 as an undifferentiated Kortrijk Formation in King 846

(1991)). The Mons‐en‐Pévèle unit is not included in the Kortrijk Formation 847

because of the sandy nature of the former in contrast with the dominantly 848

28

Version juin 2016

clay nature of the latter. Also the Mons‐en Pévèle sand unit can be properly 849

mapped with considerable thickness in Hainaut where it links up with the 850

Cuise Sand of the Paris Basin; such a map unit usually gets the formation 851

status. Also, the 1:25 000 mapping in Wallonia uses the status ‘Formation de 852

Mons‐en‐ Pévèle’. Therefore in the present review it has been chosen to rank 853

the Mons‐en‐Pévèle sandy unit as a Formation. 854

Description:855

Succession of one or a few m thick laminated packages of pure very fine sand 856

(60‐80µm), often cross stratified, and strongly bioturbated clayey sand; the 857

latter are more important closer to the base. The sand is micaceous with 858

commonly very fine glauconite. Several coarser beds are packed with 859

Nummulites, appearing for the first time in the basin in the Mons‐en‐Pévèle 860

Formation. Locally cemented layers occur, a.o. nummulitic limestone beds. 861

862


The Mons‐en Pévèle Formation is occurring southeast of a line through Lille 864

(North France) (see map in King, 1991), from Mons‐en‐ Pévèle (North France) 865

to Tournai and Ronse and further eastwards. Mons‐en‐Pévèle is a locality 866

south of Lille in North France and the name ‘Sables de Mons‐en‐Pévèle‘ was 867

introduced by Ortlieb & Chellonneix (1870, p 27). 868

Towards the east in Brabant, the Ieper Group thins and a typical clayey basal 869

part is distinguished from an upper fine sandy unit. The basal clay 870

corresponds to the Orchies Member of the Kortrijk Formation whilst the sand 871

has been given a lithostratigraphic name, the Vorst/Forest sand. It was shown 872

by King (1991) that these fine sands are equivalent to the Mons‐en‐Pévèle 873

Member. Logically therefore the Bierbeek sand above the Orchies Member in 874

the Leuven area (geological map 1:50 000 sheet 32 Leuven, Vandenberghe & 875

Gullentops, 2001) can be considered as a decalcified sand of the Mons‐en‐876

Pévèle Member, in a similar way as the sands above a thin clay unit in north 877

Brabant (Rillaar) and Limburg (e.g. Veldhoven, Beringen) as figured by Gulinck 878

(1967) and discussed by Fobe (1989a). 879

From a nomenclature point of view, in the transition zone of laterally 880

interfingering units such as the Roubaix and Mons‐en Pévèle units, the ICS 881

29

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Stratigraphic Guide recommends that a somewhat arbitrary boundary should 882

be chosen in mapping and borehole description, obviously accompanied by 883

an appropriate explanation in the legend or description. In the case of the 884

Roubaix/Mons‐en‐Pévèle limit the present review suggest that if the unit 885

consists of over 50 to 60% sand layers, the unit should be named Mons‐en‐886

Pévèle Fm and otherwise the unit should be classified as Roubaix Member of 887

the Kortrijk Formation. For example the 368‐407 m section in the Mol SCK 888

borehole 031W0237 is mainly described as fine sand with minor clay layers 889

(Gulinck & Laga, 1975) and is therefore to be named Mons‐en‐Pévèle Fm. 890

Localities with Mons‐en‐Pévèle sand are listed in Steurbaut & Nolf (1991, 891

Fig.3) and appear systematically between Ronse and Brussels. According to 892

the lithological description (sand/clay proportion) the log signature in the 893

borehole Merchtem 072E0229 should be classified as the Roubaix Mbr and in 894

the Kester borehole 101W0079 as Mons‐en‐Pévèle Fm. However an 895

inspection of shape of the RES and GR logs allows to consider the boreholes 896

Merchtem (compendium) , Meise (see profile Gent‐Zemst and Meise‐897

Rotselaar),.as transitional signatures between Roubaix and Mons‐en‐Pévèle. 898

The criterion will need further refinement and the study of more wells. Also it 899

has to be recognised that the descriptions of the boreholes, especially if 900

destructive, are often not accurate enough to reliably decide on the number 901

of sand layers. Also if grain‐size data are available, it will be needed to define 902

how exactly to apply the criterion; e.g. at first glance the amount of >62µm 903

fraction in the borehole Kattem (087W0479) south of Aalst (Geological 904

Service Company, 2003) is high enough to describe the unit below the 905

Aalbeke Member as the Mons‐en‐Pévèle Member. The same holds for the 906

application of the GR/RES log values in determining how much sand layers 907

occur in the interval. Maybe the 50‐60% sand layer boundary will need to be 908

changed or maybe it will appear practical to introduce a new transitional 909

lithological unit. 910

911

Lithologicaltrendsandpaleogeography:912

Paleogeographically, from central Flanders towards the east and the south, 913

several clay enriched facies of the Kortrijk Formation are replaced by more 914

sandy deposits (maps in Steurbaut, 2006). The lateral transitions are well 915

30

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documented and figured in King (1991). The Roubaix Member laterally 916

interfingers with the Mons‐en‐Pévèle Fm. More southwards to the Paris basin 917

the closer to the base of the Ieper Group starts the occurrence of the sand 918

unit (profiles in King, 1991). The sands are known as the Mons‐en‐ Pévèle 919

Member and grade into the ‘Cuisian‘ sands in the Paris Basin. The Aalbeke 920

Clay extends over the Mons‐en‐Pévèle Sand into the Paris basin where it 921

corresponds to the Laon clay (King, 1991). Where the ‘argilite de Morlanwelz’ 922

is a lateral equivalent of the Orchies Member (Steurbaut, 1991) more sandy 923

facies in southern direction in the Hainaut province with specific names such 924

as the Godarville sand and the Peissant sand (Steurbaut & Nolf, 1986) are 925

included, without a specific stratigraphic status in the Mons‐en‐Pévèle 926

Member. The Morlanwelz Sand, with a Formation status, is figured in 927

Steurbaut (1998 p 145; Steurbaut et al., 2003 p 11) as a lateral equivalent of 928

the Roubaix Member but as a separate unit underlying the Mons‐en‐Pévèle 929

Sand Formation (see also Steurbaut, 1998 p 110); however this subdivision is 930

not retained in the present review due to a lack of precise data. 931

932

It should be noticed that the reverse lithological trend logically is also present 933

in the north direction leading King (1991, p 361, 370) to introduce the name 934

Flanders member for the homogeneous Ieper Group clays beneath the Egem 935

Member in the Knokke well. In this review this suggestion is not followed as 936

these very clay rich sections can still be subdivided using existing 937

nomenclature such as ‘the Tielt and Kortrijk Formations ‘ (see Welkenhuysen 938

and De Ceukelaire, 2009 fig. 16) (see also Knokke well in the compendium) 939

and as the geophysical well log divisions of these clay‐rich sections can even 940

be recognised further north in the Netherlands (de Lugt, 2007). 941

Stratotype: 942

No formal stratotype has been designated. Logically the Mons‐en‐Pévèle hill 943

south of Lille and north of Douai in North France is the preferred reference 944

area (see Steurbaut, 1998 p 116); also the Waaienberge (Wayenberghe) 945

railway section near Ronse ( described in King 1988 (1990) p 359 and figured 946

in Steurbaut & Nolf 1988 (1990) p 328) is a potential stratotype section. 947

948

31

Version juin 2016


The following borehole logs in the reference compendium have a Mons‐en‐950

Pévèle signature Zemst‐Hofstade – 073E0397, and are confirmed by analysis 951

in the Mol – 031W0237, Kester‐ 101W0079 wells. 952

Typical Roubaix Mbr log response confirmed by clay dominated lithology can 953

be observed in Kallo ‐ 014E0355, Knokke ‐ 011E0138. 954

The signature in the Merchtem–072E0229 borehole is somewhat 955

intermediate but according to the borehole description sand‐layers represent 956

only 24 % of the interval and therefore the interval is classified as Roubaix 957

Mbr base on the 50‐60% sand layer criterion (see above). 958

959

TieltFormation960

OnthepositionoftheEgemMember961

The Egem Member, traditionally ranked into the Tielt Formation (see a.o. also 962

1:50 000 map legend), has in this review been ranked in the Hyon Formation. 963

The Hyon Formation has been introduced in the literature by Steurbaut 964

(1998, p 115) and described in the review by Geets et al. (2000) but was not 965

retained in the official NCS stratigraphy by Laga et al., (2001). The grouping of 966

the Egem Member in the Hyon Formation has been suggested by Steurbaut 967

(2011) applying the logic to group the sandy deposits, like the Egem Member, 968

in the Hyon Formation and the clayey deposits like the Kortemark and 969

Egemkapel Members in the Tielt Formation. This definition is also practical 970

when no distinction can be made between the sand members (Egem and 971

Mont‐Panisel) of the Hyon Fm, as is the case in the subsurface occurrence in 972

northeast Belgium 973

TieltFormation974

Authors: Geets (1988b), Steurbaut (1998). 975

976

Description: this marine unit consists in general of a very fine sandy, coarse 977

silt and clay. 978

979

32

Version juin 2016

Stratotype: at its base the formation is defined by the top of the Aalbeke 980

Member (see Aalbeke Member). In the compendium this boundary is placed 981

at 48.5m in the Tielt 053E0061 borehole (see also Aalbeke Member). 982

Steurbaut (1998) defined a boundary in the Tielt 068E0169 borehole in which 983

the Aalbeke top was located at 71 m (see also Geets, 2000) which is different 984

from the present definition (see Aalbeke Member). Steurbaut (1998) 985

correlated the in‐the‐present‐text defined top of the Aalbeke Member with 986

the top of his unit D in the Kortemark silt Member (sensu Steurbaut 1998) of 987

the Tielt 068E0169 borehole located at 46.7 m ( Sheet 21/6 (Wakken). Co‐988

ordinates: x =76439, y = 187576, z = +48 m). 989

The upper boundary is placed at the base of the Egem Mbr in the "Ampe" 990

quarry ‐ 053W0060 (see Steurbaut, 1998 Figs 5,11). Sheet 21/1 (Wingene). 991

Co‐ordinates: x = 70.150, y = 190.150, z = +44 m. 992

993

Area: the western and northern part of Belgium. The formation outcrops in 994

the north of Hainaut, the south and the centre of East‐ and West‐Flanders 995

and the western and southwestern part of Brabant. Outliers occur in the 996

Mons Basin and south of the river Sambre. The regional distribution map of 997

the Tielt Formation is figured in Maréchal, R. (1993, p 222), Walstra et al. 998

(2014) and in https://dov.vlaanderen.be 999

1000

Thickness: maximum 25 m in the centre of the outcrop area. It decreases to 1001

the south and the east, and probably to the north. 1002

1003

Members: the formation is subdivided into the Kortemark Mbr and the 1004

Egemkapel Mbr. 1005

1006

Age: Middle to Late Ypresian. 1007

1008

Remarks: the formation is also discussed by De Coninck (1973), De Moor & 1009

Geets (1973), Geets (1979), Laga et al. (1980), Maréchal (1993), Steurbaut 1010

(1988), Steurbaut & Nolf (1986). 1011

1012

33

Version juin 2016

KortemarkMember1013

Authors: Steurbaut & Nolf (1986), Steurbaut (1998), Geets (1988), Geets et al. 1014

(2000). 1015

Description: 1016

A grey silty clay unit with sandy layers of several dm thickness have been 1017

observed near the base. The presence of silt and sand is distributed in layers 1018

of cm to dm. Several subunits can be distinguished as proposed by Jacobs et 1019

al. (1996a, b) and Steurbaut (1998). The Kortemark Member occurs between 1020

heavy clay units: the Aalbeke Member below and the Egemkapel Member 1021

above. The maximal thickness is about 25 m (Geets et al., 2000). 1022

1023

In the top of the underlying Aalbeke Member a gradual coarsening upwards 1024

occurs, ended by a sharp coarsening that marks the start of coarser sediments 1025

in the Kortemark Member (see analyses from Geets (1991) and interpreted in 1026

Steurbaut (1998)). In geophysical log patterns the start of the coarsening 1027

upwards interval in the Aalbeke Mbr above its very clay‐rich main part, as 1028

well as the sharp coarse shift at the top of the coarsening upwards part of the 1029

Aalbeke Mbr which marks the position of a fine sand layer, can be observed 1030

fairly consistently (e.g. Torhout 052E0195, Tielt 053E0061, Gent 55W1020, 1031

On‐Kallo 1 014E0355). The formal boundary between the Aalbeke and 1032

Kortemark Members is drawn at the position of the major grain‐size shift and 1033

the income of the first fine‐sand layer (correlation profiles in Welkenhuysen 1034

and De Ceukelaire, 2009). This boundary definition at the base of the lowest 1035

fine‐sand layer has the advantage to correspond to an observable horizon 1036

with water outflow in the upper part of the Desimpel clay pit in Kortemark 1037

(Steurbaut,1998, Fig.5). 1038

1039

Detailed lithological analyses of the Kortemark Member sections in the Tielt 1040

borehole 068E0169 and the Kortemark and Egem extraction pits, are figured 1041

in Geets (1991) and Steurbaut (1998, p 117). Details in the geophysical well 1042

log signature in the Kortemark Member can be correlated between wells, 1043

34

Version juin 2016

especially the significant higher values of the resistivity, standing for more 1044

sandy layers, can be correlated between the different logs. 1045

1046


The Kortemark Member occurs north of Kortrijk and in particular in the west 1048

of Flanders where it can reach 25 m thickness. It is also known towards the 1049

east and northeast of Flanders (Antwerp Province) where it becomes thinner. 1050

In the southeast of East Flanders and the neighbouring eastern Brabant 1051

provinces, the Mont‐Panisel Member overlies the Aalbeke clay Member in 1052

Kerksken (086E0340) and Kattem (087W0479) (Geological Service Company, 1053

2003), implying the disappearance towards the east of the Kortemark 1054

Member and the Egemkapel Member (see also Mont‐Panisel Member). Also 1055

on the map sheet 23 Mechelen, Buffel et al. (2009) note that the Kortemark 1056

Member disappears to the east and is only present in the western part of the 1057

map. 1058

1059

In the 1:40 000 geological map legend the Kortemark Member is included in 1060

the Yc unit and in the Stratigraphic Register of the Conseil Géologique(1929) 1061

and the Aardkundige Raad (1932) in the Y1a division. 1062

In the Bolle & Jacobs (1993) nomenclature the unit Yd1c unit is tentatively 1063

correlated to the Kortemark Member. In the present review the Yd2 unit of 1064

these authors, a 5 m densily packed fine glauconitic sand underlying the 1065

Egemkapel clay Member, is also included in the Kortemark Member, 1066

notwithstanding its resemblance to the Egem Sand above. (see also 1067

Egemkapel Member) 1068

1069

Stratotype: 1070

Steurbaut (1998) has proposed the level of about 71m below surface in the 1071

Tielt borehole (068E0169); map sheet 21/6 x=76.439; y=187.576; z=48) for the 1072

lower boundary with the underlying Aalbeke Member. However, in‐the‐1073

35

Version juin 2016

present‐text the base of Kortemark has been replaced at a level in the 1074

Kortemark Desimpel quarry corresponding to the level at 48 m depth in the 1075

Tielt borehole according to the log interpretation Tielt 053E0061 in the 1076

compendium. Indeed because the top of the Aalbeke Member gradually 1077

becomes siltier upwards (see analyses in Steurbaut, 1998 fig. 5) it has been 1078

argued in‐the‐present‐synthesis that the first marked sandy layer in the 1079

Desimpel clay pit in Kortemark (marked as ‘ sharp junction waterflow’ at the 1080

base of subunit C in Steurbaut 1998 p 117) (map sheet 20/3‐4 Kortemark‐1081

Torhout, x= 57.050,y= 190.400, z= +16m) is a more easily recognisable 1082

lithostratigraphical horizon to mark the base of the Kortemark Member. In 1083

the present review this level is chosen as the formal boundary between the 1084

Aalbeke and Kortemark Members (see discussion in Description above). 1085

1086

The top of the Kortemark Member has during many years (80’s and 90’s) 1087

been exposed in the classical Egem extraction pit – 053W0060 (map sheet 1088

21/1, x= 70.150, y= 190.150) as an erosive contact with the overlying 1089

Egemkapel (see Steurbaut, 1998, p 117). 1090


Reference boreholes with geophysical log pattern of the Kortemark Mbr 1092

between the Aalbeke and Egemkapel clay Mbrs are in the outcrop area of the 1093

unit : Tielt ‐ 053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Torhout 1094

052E0195, Pittem ‐053W0073 and also Knokke – 011E0138, Rijkevorsel – 1095

007E0200, Kallo – 014E0355. 1096

1097

1098

EgemkapelMember1099

1100

Authors: Steurbaut (1998), Geets et al. (2000). 1101

Description: 1102

36

Version juin 2016

A thin heavy clay unit of about 6m thick, contrasting with underlying silty to 1103

sandy clays of the Kortemark Member and the sandy overlying deposits of 1104

the Egem Member. The unit is thinner than the Aalbeke Member. In the Egem 1105

Quarry the unit has an erosive basis with a characteristic lag deposit of fossils, 1106

mainly fish remains but also snake vertebrae and bird bones and even a rare 1107

mammal tooth (Steurbaut, 1998; Smith & Smith, 2003,2013); also, a thin 1108

transgressive sandy layer, less than 1m thick, occurs just overlying the erosive 1109

basis and well expressed on some borehole logs. This thin basal lag sand is 1110

different from and should not be confused with the underlying sandy top of 1111

the Kortemark Member (the Yd2 unit, Jacobs et al., 1996a,b). Also the upper 1112

boundary with the Egem Member is erosive. The Egemkapel Member is a 1113

clay‐rich unit, contrasting sharply with the more silty and sandy unit below 1114

(Kortemark Member) and above (Egem Member) as shown in core 1115

descriptions (see e.g. unit Yd3 in Jacobs et al., 1996a fig. 9), grain‐size analysis 1116

(see Steurbaut, 1998 fig. 5;) and in the geophysical well pattern (see 1117

compendium). 1118

1119


In the legend of the 1:40 000 maps it was included in the top of the Yc unit. 1121

Steurbaut & Nolf (1986) included the Egemkapel clay in the top of the 1122

Kortemark silt unit and Jacobs et al. (1996 a, b) in the Egem Member. 1123

As a thin unit, the Egemkapel was only individualised as a separate Member 1124

when its consistent occurrence over the whole central Flanders north of the 1125

Mons area became obvious (see e.g. Walstra et al., 2014). The unit disappears 1126

towards the east of the East Flanders and Brabant but is still recognised in the 1127

Kallo wells 027E0148 & 014E0355 north of Antwerp and in the Rijkevorsel 1128

well – 007E0200. 1129

1130

Stratotype: 1131

The name Egemkapel refers to the hamlet where the Ampe – 053W0060 or 1132

Egem extraction pit is located (map sheet 21/1, x= 70.150, y= 190.150). The 1133

37

Version juin 2016

clay unit has been exposed in this pit during a long time in the 80’s and 90’s, 1134

occurring between two erosive horizons: at its base with the underlying 1135

Kortemark Member and at its top with the strongly erosive base of the Egem 1136

Member of the Hyon Formation. 1137

A detailed description of the Ampe extraction pit anno 1994‐1995, comprising 1138

the Egemkapel, Egem and Pittem Members can be found in Willems (1995) 1139

and Steurbaut, 2015). 1140


To define the Egemkapel Member on the geophysical logs the reference must 1142

be the Egem quarry section correlated by Steurbaut (1998,Fig.5) to the close‐1143

by Tielt borehole using grain size and log signature: the thickness is 4,5m 1144

with sharp boundaries and a very clay‐rich core of 2‐3 m; sharp boundaries 1145

are confirmed in geotechnical logs (Bolle & Jacobs, 1993;Jacobs et al., 1146

1996a,b) an by grain size data in the BGD‐ Kallo borehole (Geets, 1990). 1147

This logic was followed to identify the pattern in the reference boreholes, a 1148

thin marked GR and RES excursion, exemplary expressed in boreholes : Tielt ‐ 1149

053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Rijkevorsel – 1150

007E0200, Brugge – 023W0454, Torhout 052E0195, Pittem‐ 053W0073. 1151

Mohammad (2009) and Van Marcke et al. (2005) included additional clay 1152

layers from the underlying Kortemark Member in their identified Egemkapel 1153

Member. 1154

1155

1156

1157

1158

HyonFormation1159

1160

Authors: Steurbaut and King (1994), Steurbaut (1998, p 115), Geets et al. 1161

(2000) 1162

38

Version juin 2016

The Hyon Formation has been introduced in the literature by Steurbaut and 1163

King (1994) at the occasion of the study of the Mont‐Panisel research 1164

borehole (Dupuis et al., 1988) and formalised by Steurbaut (1998, p 115). The 1165

Hyon Formation was reported in the review by Geets et al. (2000) but not 1166

retained in the official NCS stratigraphy by Laga et al. (2001). In addition to 1167

the original descriptions in the literature, also the Egem sand unit has been 1168

included now as a Member in the Hyon Formation to make a lithological 1169

distinction more practical between a sandy Hyon Formation and a clayey Tielt 1170

Formation in which the Egem Member was traditionally included. 1171

1172

Description1173

Fine sand, with dispersed clay or layers of clay, rich in galuconite and 1174

including sandstone layers and concretions. 1175

Members 1176

Egem Member, Mont‐Panisel Member , Bois‐la‐Haut Member 1177


The Egem Member of the Hyon Formation occurs over most of the provinces 1179

West and East Flanders and part of the Antwerp Campine whilst the Mont‐1180

Panisel Member of the Hyon Formation occurs in the Brabant and Hainaut 1181

area where its thickness reaches maximum 25m (Steurbaut, 2006); further 1182

northwards the Mont‐Panisel Member is only locally preserved from erosion 1183

(Steurbaut, 2006). The lateral geometrical relationship between the two 1184

sandy Members had already been noticed in the classical lithostratigraphic 1185

paper by Steurbaut & Nolf (1986), in which the Mont‐Panisel Member was 1186

indicated as ‘Panisel Sand’. The relationship between the sand members has 1187

been interpreted in sequence stratigraphic reconstructions (Vandenberghe et 1188

al., 1998, 2004; Steurbaut, 2011). 1189

The introduction of the Hyon Formation arranges the position of the strata in 1190

the hills of Bois‐la‐Haut and Mont‐Panisel, located in the village of Hyon 1191

southeast of Mons (map figure 1 in Steurbaut & King, 1994), and which are at 1192

the origin of the former classic but now obsolete ‘Paniselian’ stage 1193

(Steurbaut, 2006). The problematic geometric position of the ‘Panisel sand’ in 1194

39

Version juin 2016

Brabant and in outliers in the Hainaut area, as it was demonstrated in 1195

Steurbaut& Nolf (1986), has been solved by the introduction of the Hyon 1196

Formation. 1197

1198

Stratotype: 1199

The section between 0 and 21,85m depth in the Mont‐Panisel borehole 1200

(151E0340) on the topographic sheet 45/7‐8 (Mons‐Givry) (x=122.300, y= 1201

125.375, z= +102m). This location is an outlier and at this location the Egem 1202

Member does not occur. 1203

Biostratigraphy : upper part of nannoplankton NP12 (Steurbaut,2006). 1204

1205

EgemMember1206

Authors: Laga et al. (1980); Steurbaut & Nolf (1986); Steurbaut (1988, 1998) ; 1207

Geets (1979). 1208

Description: 1209

The sediment is a finely laminated, well sorted, mica and glauconite 1210

containing and generally fossiliferous fine sand. Lamination is mainly 1211

horizontal with in addition cross lamination, hummocky stratification and 1212

infilling of broad shallow gullies. Heavy clay layers occur of cm and dm scale 1213

often cut by erosive sand‐filled channels. The base of the Egem Member is a 1214

strongly erosive level with active channelling above the Egemkapel Member. 1215

A marked paleoseismite horizon occurs in the middle of the Egem Member 1216

exposed in the Ampe pit (Crepin et al. ,2004). Towards the top, the sediment 1217

becomes coarser and more homogeneous with numerous nummulites. A 1218

detailed section of the Egem Member in the Ampe quarry and corresponding 1219

grain‐size data (Geets, 1991) in the Tielt borehole 068E0169, are shown in 1220

Steurbaut (1998, p 117). Subdivisions of the Egem Member can be regionally 1221

followed in CPT logs and borehole descriptions (Jacobs et al., 1996a, b). On 1222

geophysical logs the base of the Egem Member can generally be recognised 1223

by a sharp increase in resistivity as it generally overlies the clay‐rich 1224

Egemkapel Member. 1225

40

Version juin 2016

1226


The Egem Member occurs over most of the provinces of West and East 1228

Flanders (Steurbaut & Nolf, 1986; King, 1991 p370) and its extension 1229

northeastwards into the Antwerp province has been interpreted in many 1230

publications although an identification as Hyon Formation is probably more 1231

prudent . On regional profiles the base of the Egem Member is clearly erosive 1232

into underlying units (Vandenberghe et al., 1998; King,1991). 1233

The Ledeberg sand and Evergem sand are synonymous for the Egem Member 1234

(Geets et al., 2000). 1235

In the legend of the 1:40 000 maps the Egem Member is representing the Yd 1236

and the P1b units and in the stratigraphic register (1929, 1932) the Y1b 1237

division (Geets et al., 2000). 1238

1239

Stratotype: 1240

The Ampe extraction pit ‐ 053W0060 in Egem (Pittem) (mapsheet 21/1 1241

Wingene x= 70.150, y= 190.150, z= +44m) between +39.5 m to +19 m T.A.W, 1242

between the Egemkapel Member below and the X‐sandstone (in this review 1243

named the Hooglede Bed) bed underlying the Pittem Member above (Geets 1244

et al., 2000). 1245

A detailed description of the Ampe extraction pit anno 1994‐1995, comprising 1246

the Egemkapel, Egem and Pittem Members can be found in Willems (1995) 1247

and Steurbaut (2015). 1248

1249


In the central West Flanders type area of the Egem Mbr several boreholes can 1251

be used as reference for the Egem Mbr and its Yd4, 5, 6 subdivisions: Tielt 1252

053E0061, Gent – 055W1020, Brugge – 023W0454, Torhout – 052E0195, 1253

Oosterzele – 070E0237, Kruishoutem – 084E1412. 1254

1255

41

Version juin 2016

Mont‐PaniselMemberandtheBois‐la‐HautMember. 1256

Authors: d’Omalius d’Halloy (1862, p 536 & 625), Steurbaut & Nolf (1986), 1257

Steurbaut & King (1994), Steurbaut (1998), Geets et al. (2000). 1258

Name : The Mont‐Panisel hill is the twin hill of Bois‐la‐Haut in the village of 1259

Hyon, near Mons (map figure 1 Steurbaut & King, 1994). 1260

1261

Description: 1262

Poorly sorted, faintly laminated, prominently glauconitic and highly 1263

bioturbated clayey fine sand occur in the reference borehole section of this 1264

unit at the Mont‐Panisel (151E0340) (Steubaut & King, 1994). A separate 3,6 1265

m thick layer at the base of the section in the Mont‐Panisel borehole 1266

(151E0340, between 18 and 21,58m), is highly glauconitic, highly bioturbated, 1267

rather well‐sorted fine to medium sand with clayey patches in contrast to the 1268

finer and less‐sorted sand above (see section in Steurbaut and King, 1994 1269

fig.3). The lower part is named the Bois‐la‐Haut Member and the main upper 1270

part is called the Mont‐Panisel Member. The latter contains also numerous 1271

irregularly shaped siliceous sandstone concretions and locally poorly 1272

cemented nummulite‐bearing sandstones occur (Steurbaut, 2006). Maximal 1273

thickness is 20 m. 1274

Geets et al. (2000) report that somewhat coarser glauconite‐rich sand in 1275

boreholes between Aalst and Brussels could correspond to the Bois‐la‐Haut 1276

Member. The X‐stone bed , named Hooglede Bed in this review, underlying 1277

the Pittem Member in the Ampe quarry has also been tentatively suggested 1278

to be a lateral equivalent of the Bois‐la‐Haut Member by Steurbaut (1998) 1279

although in Steurbaut (2011, fig.8 p 255) the X‐stone bed is again included in 1280

the base of the Pittem Member. Although the Bois‐la‐Haut Member is until 1281

now only clearly identified in the Mont Panisel borehole, it is ranked as a 1282

member seen its thickness of several meter ,more than the normal thickness 1283

for a lithostratigraphic bed. 1284

1285


42

Version juin 2016

These deposits were originally described by d’Omalius d’Halloy (1862) as 1287

‘psammites, sables et argiles du Mont‐Panisel’ at the Mont‐Panisel near 1288

Mons. The Mont‐Panisel Member occurs in the area Gent‐Brussel‐Mons‐1289

Kortrijk. The Mont‐Panisel Member overlies the Aalbeke Clay in clay pits 1290

around Kortrijk (e.g. Mulier clay pit) (Steurbaut 2006). 1291

The sands correspond to the previously used unit ‘Panisel sand’ in Steurbaut 1292

& Nolf (1986) and this Member corresponds to the ‘Unnamed Sand member’ 1293

in the top of the Mouscron borehole and the Kortrijk outcrops of King (1991 p 1294

365). It also occurs in the hills of North France. It corresponds to the term 1295

‘Paniselien’ used by Gulinck in his profiles around Brussels ( Archives Belgian 1296

Geological Survey,MG/00/250‐329‐547;MG/53/327;MG/55/335; MG/56/176‐1297

177‐313‐316;.MG/58/249). 1298

Whereas in the Gent (055W1020) area the Egem Member subdivisions Yd4, 1299

Yd5, Yd6 (sensu Bolle & Jacobs, 1993) can be recognised between the 1300

Egemkapel (Yd3 unit sensu Bolle & Jacobs,1993) and the Merelbeke Clay, such 1301

identification becomes difficult to the east near the boundary with the 1302

Brabant province. It seems that in this latter area and more to the east, the 1303

Mont‐Panisel Member replaces the Egem Member. Jacobs et al. (1996a p 28) 1304

have reported that the Egem Member becomes more clayey to the south. 1305

In the southeast of East Flanders and the neighbouring eastern Brabant 1306

province, about 6 to 11 m of glauconitic sand occurs containing sandstone 1307

layers and overlies the Aalbeke clay Member in Kerksken (086E0340) and 1308

Kattem (087W0479) boreholes; its description corresponds to the Mont‐1309

Panisel Member (Geological Service Company, 2003). The typical Mont‐1310

Panisel sand in the borehole is overlain by a clayey sand of about 11 m which 1311

in its turn is capped by the Merelbeke clay Member. The lithostratigraphic 1312

position of this clayey sand unit overlying the Mont‐Panisel sand is further 1313

discussed under the Kwatrecht Member. 1314

The implication of this succession is also that towards the east, the Kortemark 1315

Member and the Egemkapel Member have disappeared. Also on the map 1316

sheet 23 Mechelen, Buffel et al. (2009) note that the Kortemark Member 1317

disappears to the east and is only present in the western part of the map. 1318

43

Version juin 2016

More northwards in the Brabant province (east of Aalst), the Merchtem 1319

borehole (072E0229) (Buffel et al., 2009) shows above the Aalbeke Member 1320

and below the Merelbeke Member, the same twofold borehole geophysical 1321

log signature and thickness as the Kerksken – 086E0340 and Kattem – 1322

087W0479 boreholes, with the lower part being the typical Mont‐Panisel 1323

Member below a more clayey glauconitic sand without sandstones (see 1324

Kwatrecht Member). This pattern can also be observed further west and 1325

north‐westwards in geophysical logs (Meise borehole 073W0394 in 1326

Welkenhuysen & De Ceukelaire, 2009) and in grain‐size analysis of the Zemst‐1327

Weerde borehole (073E0359) (Buffel et al. 2009). The presence of Merelbeke 1328

clay in the Zemst‐Weerde ‐ 073E0359 borehole was confirmed by 1329

micropaleontological data (Buffel et al., 2009). Note that in the Zemst‐1330

Weerde‐ 073E0359 interpretation by Buffel et al. (2009) these two units 1331

together were named Egem Member, an interpretation not followed in the 1332

present review. Also, just north of Brussels in Vilvoorde, Gulinck described in 1333

his profile MG 00/504 ‘Paniselien’ above a clay rich top of the ‘Ypresian’ and 1334

below the Brussel and Lede Formations; this ‘Paniselien’ is characterised by 1335

stone layers in its lower part. 1336

Over a short distance to the east, between Zemst‐Weerde (073E0359) and 1337

Zemst‐Hofstade (073E0397) the Mont‐Panisel sand and the overlying clayey 1338

sand have disappeared, except maybe for a very thin remnant, and it appears 1339

that the Aalbeke and the Merelbeke clay Members are almost superposed 1340

(interpretation Johan Matthijs), although this needs micropaleontological 1341

confirmation. This superposition would imply the wedging out of the Mont‐1342

Panisel and Kwatrecht units by erosion before the deposition of the 1343

Merelbeke Member rather than their later erosion before deposition of the 1344

overlying Zenne Group as would be the case if only Aalbeke clay is present 1345

(see also Merelbeke Member). 1346

1347

On the other hand in the Kallo wells (027E0148/014E0355), more north‐1348

westwards, the Kortemark and Egemkapel Members can be recognised and 1349

between the Egemkapel and the Merelbeke clay Members the sandy unit is 1350

often interpreted as Egem Member, although identification as Hyon 1351

Formation is probably more prudent. The same log signature of the Hyon 1352

44

Version juin 2016

Formation interval in the Kallo well 014E0355 is also recognised in the 1353

Rijkevorsel 007E0200 borehole. 1354

Stratotype: 1355

The section between 0 and about 21.58m depth in the borehole of the Mont‐1356

Panisel (151E0340) (topographic map sheet 45/7‐8 Mons‐Givry, (x=122.300, 1357

y= 125.375, z= +102m). 1358

As the Mont‐Panisel borehole is located in an outlier area of the Mont‐Panisel 1359

Member, the interval 46‐54 m in the borehole Zemst‐Weerde (073E0359) can 1360

be considered a parastratotype of the Mont‐Panisel Member (verslag Zemst, 1361

FV Matthijs‐Buffel, 2000; Steurbaut et al., 2015). 1362

1363


Typical log signature of the Mont‐Panisel Mbr can be observed in the 1365

reference borehole logs: Merchtem ‐ 072E0229, Zemst‐Weerde ‐ 073E0359, 1366

Wieze ‐ 072W0159, Wortegem – 084W1475, Kerksken – 086E0340, Kester – 1367

101W0079. 1368

Often it is not possible to distinguish Egem and Mont‐Panisel Mbrs. In that 1369

case the signatures are best described as Hyon Fm such as a prudent 1370

interpreter could do in the case in the reference logs Rijkevorsel – 007E0200, 1371

Mol – 031W0237, Kallo – 014E0355,... 1372

In the Kallo well – 014E0355, and eventually the Rijkevorsel borehole – 1373

007E0200, the subdivisions Yd4,5,6 are still recognisable and could be 1374

assigned to the Egem Mbr as has commonly be done in the Mol well – 1375

031W0237 (M. Gulinck) although in this Mol well, Steurbaut (1988) has 1376

differentiated Kortemark and Egem above the Aalbeke Mbr. 1377

The Egem Member in the Knokke well 011E0138 is intriguing as the Yd4,5,6 1378

subdivisions on the geophysical logs are apparently identifiable although only 1379

Yd6 is a sandy deposit but Yd4,5 are reliably described as clay deposits in the 1380

cores; only the Yd6 interval is therefore considered as the Egem Mbr in the 1381

Knokke Memoir (Laga & Vandenberghe, 1990; King, 1990; Welkenhuysen & 1382

De Ceukelaire, 2009 p. 72). The profile designed by Van Burm and Bolle (in 1383

45

Version juin 2016

Jacobs et al, 1996a) indicates the appearance of a clay‐layer above the 1384

Egemkapel Member; this clay layer called Yd5 is clearly increasing to the 1385

west. Apparently in the Brugge‐Knokke area the lower part of the Egem 1386

Member is developed as a clay whereas it was a sand, subdivided in Yd4‐Yd5‐1387

Yd6, in the Tielt‐Gent area (Jacobs et al, 1996a‐b). A provisional informal 1388

name could be used for this clay unit in the Brugge‐Knokke area, the 1389

Hazegraspolder unit referring to the location of the cored Knokke borehole. 1390

1391

DifferentiationbetweentheMont‐PaniselandtheEgemMembers:1392

1393

The Mont‐Panisel Formation consists of poorly sorted clayey glauconitic 1394

sands in which the glauconite can make up to 15%. The Egem Member 1395

consists of well sorted laminated fine sands in which also somewhat thicker 1396

clay layers can occur. In this sense, the Mont‐Panisel Formation is more 1397

homogeneous than the Egem Member. 1398

The Mont‐Panisel Member, in contrast to the Egem Member, contains 1399

numerous irregularly shaped siliceous sandstone concretions whilst 1400

sandstones in Egem are rare. 1401

In the practice of the 1:50 000 mapping, the Egem Member was identified 1402

whenever it could be subdivided in the subunits Yd4,5,6 introduced by Jacobs 1403

& Bolle (1993) and Jacobs et al.(1996a,b); towards the east, in the 1404

neighbourhood of Aalst, the sediment became more clay‐enriched and the 1405

traditional Egem sand subdivisions could no longer be followed in the 1406

mapping; consequently, this more clayey unit in the east, which also 1407

contained sandstones, was mapped as the Mont‐Panisel Member (see Jacobs 1408

et al., 1996, a /Fig.15 showing this transition). 1409

1410

GentbruggeFormation1411

Author: see also Geets (1988) and Steurbaut (1998). 1412

The formation is also discussed by de Heinzelin & Glibert (1957), De Moor & 1413

Geets (1973), De Moor & Germis (1971), Fobe (1986), Geets (1979), Gulinck 1414

46

Version juin 2016

(1967), Gulinck & Hacquaert (1954), Kaasschieter (1961), Maréchal (1993), 1415

Steurbaut & Nolf (1986) and Wouters & Vandenberghe (1994). 1416

1417

Description: this formation of marine origin consists at the base of a very fine 1418

silty clay or clayey, very fine silt. To the south and upwards, it is followed by 1419

an alternation of layers of glauconiferous, clayey silty, very fine sand and 1420

clayey sandy, coarse silt, disturbed by bioturbation. The clayey members are 1421

covered by fine sand, clearly horizontally bedded or cross bedded. The 1422

sediments contain different layers of sandstones. 1423

1424

Stratotype: stratotypes have only been designated for the members. 1425

1426

Area: the formation mainly outcrops in the centre of East‐ and West‐Flanders 1427

and on the hills in the southern part of East‐ and West‐Flanders. It occurs also 1428

in the subsoil of the province of Antwerp and northwest Belgium. Some 1429

outliers can be observed to the south till northern Hainaut and eastwards 1430

from the Senne River. 1431

The regional distribution map of the Gentbrugge Formation is figured in 1432

Maréchal (1993, p 222) as understood at that time; the extension mapped on 1433

the 1:50 000 geological maps can be consulted on 1434

https://dov.vlaanderen.be/dovweb/html/geologie.html. 1435

1436

Thickness: maximum 50 m in the north and decreasing to the south and the 1437

east. 1438

1439

Members: the formation is subdivided into the Kwatrecht, Merelbeke, Pittem 1440

and Vlierzele Members. Note that in the present review the now more 1441

generally recognised, Kwatrecht Member is ranked in the Gentbrugge 1442

Formation because of its more clayey nature compared to the sediments in 1443

the Hyon Formation. 1444

1445

The Vlierzele Member has been traditionally included in the Gentbrugge 1446

Formatie of the Ieper Group. It could be argued that the Vlierzele unit as a 1447

sand unit would be better ranged in the sandy Zenne Group. However it is 1448

also argued that the Vlierzele unit also contains clayey parts and therefore 1449

47

Version juin 2016

should remain in the Ieper Group. However, taking into account the full 1450

significance of the clayey parts of the Vlierzele unit led Fobe (1995, 1997) to 1451

differentiate different members in the Vlierzele unit and to rank the Vlierzele 1452

unit as a Formation (see further/ to be discussed). In the present review the 1453

Vlierzele is kept as a Member and ranked in the Gentbrugge Formation (see 1454

below). 1455

1456

1457

Age: late Ypresian. 1458

1459

Remark: the Gentbrugge Fm is called Gent Fm on the 1:50 000 geological 1460

maps. The name Gent Fm was changed to Gentbrugge Fm since it was 1461

already in use for Quaternary eolian cover‐sand deposits in Flanders (Paepe & 1462

Vanhoorne 1976, see website NCS Quaternary subcommission). 1463

1464

KwatrechtMember1465

Authors: De Moor & Geets (1973) 1466

The occurrence described in the Gent area (Merelbeke) as reported by De 1467

Moor & Geets (1973) is only documented in this one section (boreholes 1468

DB11&12) as alternating clayey and sandy sediments in contrast to the more 1469

well‐calibrated fine sand of the Egem Member below. No further 1470

sedimentological details can be derived from this description and the 1471

suggestion for a fill of erosive channels as figured on the De Moor and Geets 1472

(1973) profile is not considered justified by the existing borehole data. An 1473

analogue profile in the same area around Gent (Jacobs et al. 1996a fig. 9) 1474

omitted the Kwatrecht unit below the Merelbeke Member. Vandenberghe et 1475

al. (1998 & 2004) have suggested this Kwatrecht unit could be an erosion 1476

remnant as a consequence of intense erosion phases in the late Ypresian and 1477

early Lutetian. 1478

However a review of the original documentation and biostratigraphic 1479

information (dinoflaggellate data) suggests that this clayey sand unit can be 1480

maintained as a separate unit. Furthermore a detailed analysis of the Zemst‐1481

Weerde borehole (Steurbaut et al. , 2015) seems to confirm a more regional 1482

extension of this lithological unit at the same biostratigraphic level. 1483

48

Version juin 2016

Therefore it is justified to introduce the Kwatrecht Member as aformal 1484

lithostratigraphic unit. 1485

1486

Description: 1487

A layered complex of greenish glauconitic and micaceous bioturbated sand 1488

and sandy clays, without stone beds, originally indicated as the Kwatrecht 1489

Complex, has been described underlying the Merelbeke Member and 1490

overlying the Egem Member in the Gent area near Merelbeke by De Moor 1491

and Geets (1973, see 2.2.3.3). 1492

In regional sections, the Kwatrecht Member is geometrically positioned 1493

between the Egem and Merelbeke Members by Steurbaut & Nolf (1986), 1494

Steurbaut (1991) and Willems & Moorkens (1991). Based on geometry and 1495

biostratigraphy the Kwatrecht Member has been related to the Hyon 1496

Formation by Vandenberghe et al. (2004). However more recently the 1497

Kwatrecht Complex is ranged in the Gentbrugge Formation by Steurbaut 1498

(2006, 2011). 1499

Regionaloccurrenceandstratigraphicposition: 1500

Original description by De Moor and Geets (1973) in the Gent area. Steurbaut 1501

(2006, p 79) has reported the presence of the Kwatrecht Member in the 1502

Zemst‐Weerde borehole (073E0359, Buffel at al., 2009); according to the 1503

description of this borehole in the present review (see Mont‐Panisel 1504

Member), the about 5 m clayey sand between the Mont‐Panisel Member and 1505

the Merelbeke Member, are meant as Kwatrecht Member by Steurbaut 1506

(2006). Consequently this Kwatrecht Member is now also recognised in the 1507

east of the Brabant province (boreholes Kerksken – 086E0340, Kattem – 1508

087W0479, Meise – 073W0394, Merchtem ‐ 072E0229.see Mont‐Panisel 1509

Member). 1510

1511

Stratotype: 1512

The Gent area (Merelbeke) section as described by De Moor & Geets (1973). 1513

As data from this stratotype are not easily accessible, the 41‐46 m interval in 1514

49

Version juin 2016

the Zemst‐Weerde (073E0359) borehole could be considered as the 1515

parastratotype. 1516

1517


A twofold subdivision of a sand layer between the Aalbeke and Merelbeke 1519

Mbrs allows to distinguish an upper Kwatrecht Mbr signature above a Mont‐1520

Panisel Mbr signature in the reference boreholes Merchtem – 072E0229, 1521

Zemst‐Weerde – 073E0359, Kerksken – 086E0340, Wortegem – 084W1475, 1522

(Wieze – 072W0159?) and in the analysis of the Kattem borehole – 087W0479 1523

(Geological Service Company, 2003) and the published Meise borehole 1524

(073W0394) (Welkenhuysen & De Ceukelaire, 2009 Fig. 32). 1525

1526

1527

MerelbekeMember1528

Authors: De Moor & Germis (1971,p 57), Steurbaut & Nolf (1986, p 128), 1529

Geets et al. (2000). 1530

Description: 1531

The Merelbeke Member is a compact heavy to silty clay; it is a marine 1532

deposit, although some periods with fresh water algae influx have also been 1533

observed. Thin sand laminae with organic matter and small pyritic 1534

concretions have been described by De Moor & Geets (1974). The Merelbeke 1535

Member thickness is generally limited to about 6 to 7 m but exceptionally up 1536

to 14 m near Melle in the profile 3 by De Moor & Geets (1973). The thickness 1537

variations as figured in the profile by DeMoor and Geets (1973) are most 1538

probably due to the lack of borehole sample quality although they may also 1539

be the result of strong erosion at that stratigraphic interval (Vandenberghe et 1540

al. 2004). 1541

1542


The Merelbeke Member occurs in the western part of the Brabant province 1544

and in the north of the provinces of East and West Flanders. Its distribution is 1545

50

Version juin 2016

irregular because of erosion by later Eocene deposits (Vandenberghe et al., 1546

1998, 2004). 1547

Where the Merelbeke Member occurs, it overlies either the Egem Member or 1548

the Mont‐Panisel Member as in the Ronse‐Aalst‐Brussel area or the 1549

Kwatrecht Member in the east. The Member is overlain by the Pittem 1550

Member. 1551

On the 1:40 000 maps the Merelbeke Member is mapped as P1m, a code also 1552

often used in borehole descriptions. In the stratigraphic register (1929, 1932) 1553

it is part of the Y2 division. In the 1:40 000 mapping, Merelbeke and Aalbeke 1554

Members have been confused in the southwest of Flanders. 1555

In the area west of Mechelen (Hombeek, Zemst...), the Merelbeke Member 1556

has been confused in some borehole descriptions with the P1n clay (1:40.000 1557

map legend), which is a unit occurring above or in the top of the Vlierzele 1558

Member (Buffel et al., 2009). This confusion in North Belgium was already 1559

pointed out by Fobe (1995). 1560

Stratotype:1561

The section described between +5,6 and ‐4,9 m T.A.W. in the borehole Melle 1562

(055E0783) (222/E3/SWK/F/DB11), topographic map sheet 22/1‐2,Gent‐Melle 1563

(X= 109.125, Y = 188.775, Z = + 12.6 m) (Geets et al., 2000). 1564

1565


The Merelbeke Member signature in the reference borehole logs can be 1567

observed in many boreholes: Merchtem – 072E0229, Zemst‐Weerde – 1568

073E0359(confirmation by biostratigraphy in Buffel et al., 2009), Kerksken – 1569

086E0340, Brugge – 023W0454, Knokke – 011E0138, Kallo – 014E0355, 1570

Rijkevorsel – 007E0200, Oosterzele – 070E0237, Kruishoutem – 084E1412, 1571

Merksplas – 017W0280. 1572

The Zemst‐Hofstade – 073E0397 borehole presents an interesting case. The 1573

top clay unit, consisting of two parts on the log, is either entirely the Aalbeke 1574

Clay or it might be composed of the Aalbeke Clay overlain directly by the 1575

Merelbeke Clay, or with only a very thin remnant about 1m Mont‐Panisel 1576

Sand in between (interpretation Johan Matthijs); the latter case implies the 1577

51

Version juin 2016

erosion of the Mont‐Panisel, and probably Kwatrecht, units before the 1578

deposition of the Merelbeke Member rather than their erosion before 1579

deposition of the overlying Zenne Group as would be the case if only Aalbeke 1580

clay is present. 1581

It should be noted that in the reference borehole Knokke – 011E0138, and 1582

also Mol‐SCK15 – 031W0237, also a similar two fold Aalbeke Mbr signature is 1583

observed. 1584

1585

PittemMember1586

Authors: Geets (1979), Geets et al. (2000), Steurbaut et al. (2003) 1587

Description: 1588

The Pittem Member consists of a bedded alternation of thin, dm scale, layers 1589

of silty clay and clayey fine glauconitic sand, locally cemented into thin 1590

sandstone and siltstone beds which can be microporous after dissolution of 1591

sponge spiculae and fossils. Bioturbation is common. Tidal gullies have been 1592

reported by Geets et al. (2000). The thickness of the Pittem Member is about 1593

15 to 20 m. Traces of lignite have been reported in the Pittem Member 1594

occurring between Knokke and Kruibeke in the north of West and East 1595

Flanders by Fobe (1993). 1596

The lower boundary is easily distinguished from either the underlying 1597

Merelbeke Member, the Egem Sandstone Bed or the Egem Member. The 1598

often reported gradual transition between Pittem Member and the overlying 1599

Vlierzele unit in boreholes, is erroneous and due to a confusion between the 1600

Pittem Member and clayey parts of the Vlierzele unit sensu Fobe (1995) 1601

(Fobe, 1995 p 143). 1602

Also, in typical cases, the limit between the clayey sediment of the Pittem 1603

Member and the overlying Vlierzele Sand can be traced with reasonable 1604

confidence in the geophysical log correlation profiles by Welkenhuysen and 1605

De Ceukelaire (2009). 1606

52

Version juin 2016

Fobe (1997) reports that in the subsurface of northwest Belgium the upper 1607

part of the Pittem Member is a conspicuous horizon, brown coloured by 1608

lignitic material. 1609


The Pittem Member occurs almost continuously in a small zone north of a line 1611

Torhout‐Tielt‐Oudenaarde‐Ninove and in West Brabant but subcrops over a 1612

larger area north of this line. South of this line it occurs only in the South 1613

Flemish hills. Towards the south the Pittem Member becomes more sandy. 1614

On the geological maps 1:40 000 the Pittem Member is represented by the 1615

P1c unit and in the stratigraphic register (1929, 1932) as part of the Y2 1616

division. On the 1: 40 000 maps of the Kortrijk area, clayey deposits of the 1617

Tielt Formation have been incorrectly interpreted as P1c. The name ‘sandy 1618

clay of Anderlecht’ is a synonym. 1619

Stratotype : 1620

Geets (1979) considered the now abandoned Claerhout extraction pit in 1621

Pittem (topographic map 21/5‐6, Izegem‐Wakken, X = 74.250, Y = 187.540, Z = 1622

+ 46 m) as the reference section for the Pittem Member. An identical section 1623

is exposed in the Ampe pit – 053W0060 between +43.5 and +40 m T.A.W 1624

(topographic mapsheet 21/1 Wingene x= 70.150, y= 190.150, z= +44m) above 1625

the X‐stone Bed. 1626

1627


The Pittem log signature can be observed in the reference boreholes of the 1629

type area of central Flanders such as Tielt 053E0061, Brugge – 023W0454, 1630

Knokke – 011E0138, Oosterzele – 070E0237, Kruishoutem – 084E1412 but also 1631

in the borehole logs of Merchtem – 072E0229, ON‐Kallo‐1 – 014E0355, 1632

Rijkevorsel – 007E0200 and Merksplas – 017W0280. 1633

1634

HoogledeSandstoneBed1635

Authors: Bolle & Jacobs (1993), Fobe (1997b), Steurbaut et al. (2003, p 33, 34) 1636

53

Version juin 2016

Description: 1637

A pale yellowish brown, limonite stained, about 40 cm thick, cemented and 1638

originally shelly coarse‐grained sandstone layer; most shells have been 1639

dissolved and left large voids. Typically, numerous very coarse glauconite 1640

grains are dispersed across the sandstone and sometimes glauconite staining 1641

occurs in the dissolved shell voids. The fossils in the layer are bivalves, 1642

oysters, nautiloids and shark teeth; also phosphatic nodules are reported 1643

(Steurbaut, 2006). The sandstone bed overlies the Egem Member and 1644

underlies the Pittem Member. 1645

Because of its characteristic aspect, it is preferred to attribute a bed status to 1646

the stone bed. It has been named bed X in Steurbaut (1998, fig. 5), bed 22 in 1647

Steurbaut (1998) and Steurbaut et al. (2003, p34). It is proposed in this review 1648

to name the bed the Hooglede Sandstone Bed of the Pittem Member after 1649

Fobe (1997b). 1650

1651


The bed occurs in the classical Ampe extraction pit – 053W0060 at Egem and 1653

is named bed 22 in the classical section of the pit published by Steurbaut 1654

(1998). Lithotratigraphically, the Egem Sandstone Bed has generally been 1655

considered as the base of the Pittem Member (a.o. Steurbaut, 2003) as 1656

several thin and fine‐grained sandstone beds also occur in the Pittem 1657

Member ; it was tentatively suggested to be a lateral equivalent of the Bois‐1658

la‐Haut Member by Steurbaut (1998) (reported also in Geets et al., 2000), 1659

although in Steurbaut (2011, fig.8 p 255) the Egem Sandstone Bed (X‐stone 1660

bed) is again included in the base of the Pittem Member. 1661

Stratotype : 1662

The Ampe extraction pit – 053W0060 in Egem (Pittem) (mapsheet 21/1 1663

Wingene x= 70.150, y= 190.150, z= +44m) between the Egem Member and the 1664

Pittem Member. 1665

VlierzeleMember1666

Commentonthestratigraphicranking: 1667

54

Version juin 2016

In the recent literature, the Vlierzele Member has been included in the 1668

Gentbrugge Formation of the Ieper Group. However it could be argued that 1669

the Vlierzele Member as a sand unit better fits in the Zenne Group overlying 1670

the clay dominated Ieper Group. This would be partly in line with Fobe (1995) 1671

who argues that the Merelbeke and Pittem Members as clayey units should 1672

be united in the Gentbrugge Formation and distinguished from the sandy 1673

Egem and Vlierzele units, respectively below and above. 1674

As the lower part of the Vlierzele unit can also be clayey (see Jacobs et al., 1675

1996a Fig. 13; Lochristi unit sensu Fobe, 1995 p 142), and also for continuity 1676

reasons, the present review keeps the Vlierzele unit in the Gentbrugge 1677

Formation as a Member. 1678

1679

Authors: Kaasschieter (1961), Geets et al. (2000), Fobe (1995, 1997) 1680

Description: 1681

In the recent literature, based on outcrop observations, the Vlierzele Member 1682

is described as consisting of a lower part of mostly bioturbated, slightly 1683

clayey, glauconitic sand and an upper section of alternating units of tidal 1684

cross‐bedding with mud drapes and structureless intercalations; the upper 1685

section may contain lignite lumps (Houthuys, 1990). 1686

In the recent literature, based on outcrops, the Vlierzele Member is described 1687

as consisting of fine glauconitic green‐grey mostly bioturbated sand, finely 1688

laminated horizontally and in cross stratification. Towards the base the sands 1689

becomes clayey and more homogeneous. Towards the top individualised clay 1690

layers occur together with humic intercalations. Macrofossils are very rare. 1691

Thin cemented siliceous sandstone beds commonly occur (Geets et al., 2000); 1692

irregularly shaped siliceous sandstone concretions are also common. The 1693

maximal thickness is about 20 m; in the type locality the cross bedded sand 1694

above is 7m thick and the lower homogeneous sand at least 5 m (see sections 1695

in Houthuys & Gullentops, 1988 p 142). 1696

Fobe (1995), after reviewing information available from more than 25 1697

localities, considers the ‘traditional Vlierzele sand sensu stricto’ as only one of 1698

5 members in a formation between the Pittem Member and the Aalter Sand 1699

55

Version juin 2016

in the Zenne Group. Steurbaut (2006) reports erroneous correlations in Fobe’s 1700

(op.cit.) subdivisions ; the Beernem sand, traditionally a member of the Aalter 1701

Formation of the Zenne Group (Maréchal & Laga,1988 p 120‐121; Geets et al., 1702

2000), is included in the Vlierzele unit by Fobe and the existence of a distinct 1703

Aalterbrugge unit is refuted by this author. Therefore the present review is 1704

not following the interpretations by Fobe (1995, 1997) but recognizes that the 1705

clayey basis, a 3‐10m very fine clayey sand with mm‐thick clay layers, 1706

(Lochristi layer sensu Fobe) and locally a thin coarser basal layer (Hijfte layer 1707

sensu Fobe) merit a separate mention aside the traditional Vlierzele sand 1708

sensu stricto (which according to Fobe 1995,1997 could be named Oosterzele 1709

unit). 1710

1711


The Vlierzele Member outcrops in the northern and central parts of the 1713

provinces East and West Flanders and in the western part of the Flemish 1714

Brabant province. It also occurs as outliers in the top zones of the South‐1715

Flemish hills. On a regional scale the base of the Vlierzele is erosive into 1716

underlying strata (see also Fobe, 1989b, 1995). In northern Flanders the grain‐1717

size properties of the Vlierzele Member seem to be more variable (Laga & 1718

Vandenberghe, 1990 p 1; Fobe, 1993, 1995). The boundary between the 1719

clayey sediment of the Pittem Member and the overlying Vlierzele Sand can 1720

mostly be traced with reasonable confidence in the geophysical log 1721

correlation profiles by Welkenhuysen and De Ceukelaire (2009). 1722

1723

On the legend of the 1:40 000 maps the Vlierzele Member is coded P1d and 1724

P1n for the upper clayey facies. In the stratigraphic register (1929, 1932) the 1725

Vlierzele Member is included in the Y2 division. 1726

The P1n‐clay, defined by Rutot (1890) and described as a local top clay in the 1727

Vlierzele Sand (Gulinck & Hacqaert, 1954) is believed to correspond in fact to 1728

the Merelbeke Clay (Fobe, 1995; Buffel et al., 2009). 1729

1730

56

Version juin 2016

Stratotype: The Vlierzele locality is part of the Sint‐Lievens‐Houtem 1731

municipality in the East Flanders province where several extractions have 1732

been active in the past. The sand pit, formerly known as the Verlee or 1733

Balegem sand pit (at present Balegro sand pit)– 070E0050, is the stratotype; it 1734

is located on topographic map sheet 22/7‐8, Oordegem‐Aalst (X = 116.650, Y 1735

= 181.725, Z = + 45 m) (Geets et al., 2000). 1736

However this stratotype is limited to the Vlierzele sand sensu stricto. 1737

Following Fobe (1995), as far as the Vlierzele sensu stricto, the Lochristi and 1738

Hijfte layers are concerned, the Ursel borehole (039W0212 x= 87.910, y= 1739

204.260, z= + 29 m TAW) shows the Vlierzele Member between 58 and 69,3 m 1740

with the Vlierzele sand sensu stricto between 58‐63 m, Lochristi layer 1741

between 63‐66 m and the Hijfte layer between 66‐69,3 m. 1742

1743


The Vlierzele Mbr has been identified on top of the Pittem clay Mbr in the 1745

following reference borehole logs: Brugge, Merchtem, Knokke (comprising 1746

the Hijfte, Lochristi and Oosterzele units sensu Fobe 1995,1997), ON‐Kallo‐1, 1747

Rijkevorsel. 1748

1749

AalterbruggeBed1750

Authors: Hacquaert (1939); Gulinck & Hacquaert (1954); De Moor & Geets 1751

(1973); Fobe (1995); Van Simaeys (1999). 1752

Description: 1753

The Bed consists of clays and sand occurring in a complex geometrical 1754

relationship like usually encountered in continental deposits; also lignite beds 1755

and drift wood, sometimes silicified, occur. It occurs between the Vlierzele 1756

Member and the Aalter Formation (Zenne Formation) (Maréchal & Laga, 1757

1988; Steurbaut, 2006), both more homogeneous, glauconitic, marine 1758

sediments. 1759

In the synthesis on Belgian geology (P.Fourmarier, 1954, Prodrome d'une 1760

description géologique de la Belgique, Soc. Géol. Belg.) Gulinck & Hacquaert 1761

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(1954) describe in the chapter XIV the Complexe d'Aalterbrugge occurring in 1762

the top of the Vlierzele sand unit as follows : " Ces sables prennent souvent 1763

dans les zones supérieures, quelquefois aussi dans les parties moyennes, un 1764

facies plus grossier, pauvre en galuconie, parfois ligniteux, avec bois flottés 1765

percés de tarets et souvent silicifiés. On y rencontre également des niveaux 1766

de galets de glaise, spécialement dans la région de Torhout (Rutot 1767

[explic.carte géologique 1:40 000]) et d'Aalter (Hacquaert, [1939]). 1768

Les bois flottés sont parfois très volumineux. Leur nature fragmentaire 1769

permet rarement une détermination précise, mais on a pu y distinguer une 1770

dizaine d'espèces (F. Stockmans [région de Bruxelles])". 1771

The Aalterbrugge unit as represented on the section in De Moor & Geets 1772

(1973, fig.4) attains 10m thickness. The temporary exposure described by 1773

Jacobs (2015 p 137) was at maximum 3m thick (Steurbaut or. com.). Van 1774

Simaeys (1999) shows the presence of the Aalterbrugge Complex in the Hijfte 1775

borehole (040E0373) section between 46.1 and 53.1 m depth. 1776

No Aalterbrugge unit is reported in Geets et al. (2000) and also in the 1777

explanatory notes of the 1:50 000 map sheet 22 Gent, the Aalterbrugge unit is 1778

not reported (Jacobs et al., 1996). 1779

From his extensive data review, Fobe (1995) even concludes that the 1780

Aalterbrugge layer does not exist as a separate facies and has been confused 1781

with lignitic rich zones occurring at different levels in the Vlierzele unit. 1782

The Aalterbrugge unit is well documented and analysed by Van Simaeys 1783

(1999) in the Hijfte borehole 040E0373 in which it is 7m thick. Because of this 1784

thickness the member rank is justified for the Aalterbrugge Member. A 1785

several meter thick similar unit was also exposed during the construction of 1786

the pedestrian bridge over the E40 motorway in Wetteren. 1787

Boundaries: 1788

The section in De Moor & Geets (1973, fig.4) suggest an erosive base into the 1789

underlying Vlierzele Sand. Also Hacquaert (1939) reports intraformational 1790

clasts at the lignite level, suggesting erosion during the complex formation. 1791

Also Steurbaut (2015 p132) suggests that with the regression after the 1792

Vlierzele Sand deposition gullies were formed in the area of Aalterbrugge. 1793

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Maréchal & Laga (1988, p 119) attribute a bed status to the Aalterbrugge 1794

layer between the Gent Fm and the Aalter (at that time named Knesselare) 1795

Formation and note that the transition between the Aalterbrugge bed and 1796

the overlying marine Aalter Sand is continuous. Jacobs (2015, fig.3.21 p 137) 1797

shows the Aalterbrugge unit in Wetteren as a continuous transition between 1798

the Vlierzele and Aalterbrugge unit, but reports that the top of the 1799

Aalterbrugge unit is eroded. 1800

RegionalOccurrence: 1801

The Aalterbrugge Bed is most often reported between Aalterbrugge and 1802

Beernem (Jacobs, 2015). It was also described in the Hijfte borehole ‐ 1803

040E0373 northeast of Gent. A recent outcrop along the E40 in Wetteren also 1804

showed the presence of the Aalterbrugge Bed. 1805

Roche (1988‐1991, p 375) and DeConinck (1988‐1991, fig. 9 p 304) report the 1806

presence of the Aalterbrugge Complex in the boreholes Kallo ‐ 027E0148 1807

(level 203 m) and Woensdrecht (NL) (level 385 m). 1808

Stratotype: 1809

sections along the Gent‐Brugge canal (Hacquaert 1939 section). 1810

Parastratotype in the Hijfte borehole (040E0373) section between 46,1 and 1811

53,1 m depth (Van Simaeys, 1999). 1812

Remark : The ‘Aalterbrugge Member’ of the Hijfte borehole (Van Simaeys, 1813

1999) contains isolated records of the freshwater fern Azolla sp. which occurs 1814

massively in the North Sea and even the Atlantic Ocean at the base of chron 1815

C21r (Vandenberghe et al., 2004). 1816

1817

1818

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COMPENDIUMOFREFERENCELOGSWITHCOMMENTS1819

1820

GR and RES geophysical logs from a selection of boreholes are interpreted in the 1821

lithostratigraphic terms outlined in the present review, dated May 2016. The selection of the 1822

boreholes is made to illustrate the geographical variations observed. The location of the 1823

reference boreholes in this compendium is shown in the map. More borehole data than 1824

those represented in the compendium are available and have been used in the discussions 1825

by the working group. However the selection does represent the present state of knowledge 1826

and also illustrates the still remaining correlation issues. It is not yet well understood how 1827

the Kortemark, Egemkapel, Egem (Hyon Fm), units … a succession so typical for the west and 1828

central Flanders area is replaced laterally over a fairly short distance to the south, east and 1829

the northeast by the undifferentiated Hyon Formation and Mont‐Panisel and Kwatrecht 1830

Members. It is suspected that tectonic tilting rearrangements in the area play a more 1831

important role than previously estimated. 1832

To make progress in these issues, it is imperative to have more combined sedimentological 1833

and biostratigraphic analyses on representative borehole samples from carefully chosen 1834

localities selected after the study of the geophysical borehole patterns. 1835

In the comment section of each borehole only those issues are addressed that make the 1836

proposed interpretation debatable, eventually referring to other boreholes. If the given 1837

interpretation straightforwardly follows from the definitions explained in the text no further 1838

comments are given. 1839

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1840

1841

61

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Brugge (023W0454) 1842

1843

The threefold subdivision of the geophysical log pattern in the Egem Mbr interval 1844

(‘Yd4’,’Yd5’,’Yd6’) terminology for respectively the sandy lower part, a clayey middle part 1845

and a sandy upper part) can be recognized as determined in the area of the reference Egem 1846

sand and clay pit in central Flanders where the unit is about 20m thick (....14m in Tielt 1847

borehole). However on the Brugge borehole log, only the upper sandy part is expressed as a 1848

manifest sand layer which is the main lithology defining the Egem Mbr. This log pattern is 1849

similar to what is interpreted in the present review as the Egem Mbr interval in the Knokke 1850

borehole (011E0138) (see Knokke borehole) although the Yd4 and Yd5 parts are about 10m 1851

thick compared to almost double in Brugge while the sandy Yd6 part has comparable 1852

thickness of about 10m in both the Knokke and Brugge boreholes. In previous studies of the 1853

Knokke borehole (011E0138) (see a.o. King, 1990 ; Welkenhuysen & De Ceukelaire, 2009 p. 1854

72) only this upper about 10m thick sandy part is identified as the Egem Sand Mbr. 1855

1856

Gent (055W1020) 1857

The presence in the top of the Kortemark Member of a sandy package identified as Yd2 on 1858

the log pattern seems to be a common feature of the Kortemark Member (compare to Tielt, 1859

Kruishoutem, Knokke boreholes...). The Egem Member section is comparable to the lower 1860

subdivisions of the threefold subdivision in the Brugge (023W0454) borehole but the upper 1861

most sandy part, and genuine Egem Sand Mbr (see comment Brugge, Knokke boreholes) is 1862

missing under the erosive base of the Quaternary deposits. 1863

1864

Hijfte (040E0373) 1865

1866

The stratigraphic position of the succession of the upper part of the Vlierzele Member, the 1867

Aalterbrugge Member and also the Aalter unit, already part of the Zenne Group, is taken 1868

from the work of Van Simaeys (1999, fig.2.2). This borehole is shown in the compendium 1869

because it is the only available geophysical log signature associated with a reliably identified 1870

Aalterbrugge Member ( see text Aalterbrugge Member). The interpretation of the boundary 1871

position between the Pittem and Vlierzele Members is according to the lithological 1872

interpretations of Van Simaeys (1999) and Fobe (1997,fig.3) located at about 68m depth; a 1873

marked geophysically change is also observed at about 62m depth the meaning of which is 1874

not clear. Fobe (1997) has identified between 58 and 59m depth a lithological boundary 1875

between the Lochristi and Oosterzele units within the Vlierzele unit of this author. 1876

The top of the Aalter unit is also taken from Van Simaeys (1999); this author interprets the 1877

sand unit above the Aalter unit as Lede Sand in comparison with the Lede Sand in the 1878

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Woensdrecht (Nl) borehole whilst Fobe (1999) interprets this sand as a few meter of lede 1879

Sand overlain by Wemmel Sand . 1880

ON‐Kallo 1 (014E0355) 1881

The Mont Héribu Mbr is recognised by the thin silty basal sediment also containing 1882

glauconite recognised in the cores of the borehole. 1883

The boundary levels between Orchies and Roubaix Members a proposed by Steurbaut (1988, 1884

1998) in the BGD Kallo 027E0148 borehole, with cores but without geophysiscal logs, can be 1885

approximately transferred to the ON‐Kallo‐1 borehole(014E0355), with cores and 1886

geophysical logs, using the depth conversion formula: m(ON‐Kallo)=1.0016(Kallo BGD) + 1887

23,638 (courtesy Peter Stassen) although it must be warned that comparing depth values 1888

from core descriptions and geophysical well logs has some inherent inaccuracies. 1889

The top Aalbeke Mbr is following the silting‐up trend till the first sandy interval (RES curve) 1890

marking the start of the overlying Kortemark Mbr. 1891

The top and base of the Egemkapel Mbr, could be interpreted with some degree of freedom 1892

if solely based on GR,RES; however the boundaries are chosen based on the published grain‐1893

size data by Geets (1988) and the grain‐size and geophysical‐log response in the reference 1894

area of the Egem clay and sand pit and the Tielt borehole (Steurbaut, 1998). 1895

The core description of the Hyon interval corresponds to a carbonate containing sand in 1896

which stone layers are absent. A threefold subdivision can be made in the Res pattern in the 1897

Hyon Fm interval, somewhat comparable to the threefold subdivision of the Egem Mbr 1898

interval in the Knokke borehole (011E0138); however, in the Knokke well only the upper 1899

subdivision consists of fine sand and is considered Egem Sand (Laga & Vandenberghe, 1990) 1900

(see comment Brugge and Knokke boreholes). In the ON‐Kallo‐1, additionally a subdivision 1901

could be made based on the GR curve reflecting the lithology observed in the cores: the 1902

lower part being a fine sand and the upper part a very fine sand and glauconitic fine sand. 1903

However it is not possible to unequivocally identify this unit as the Egem Member and 1904

therefore it is more prudent to describe this unit as the Hyon Fm of which the Egem Sand is a 1905

Member. 1906

Based on the log readings,the sediments above the Merelbeke Mbr are clayey over about 1907

5m followed by sand. Fobe (1995, Fig.5) describes in the nearby Kruibeke borehole 1908

(042E0314) 5m Pittem Mbr sediment overlying 12m Merelbeke Mbr, and overlain by at least 1909

10m Vlierzele Sand Mbr (top of the ‘sensu Fobe Oosterzele facies ‘). 1910

1911

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Kerksken (086E0340) 1912

The log pattern of the Mont‐Panisel and Kwatrecht Members is very comparable with the 1913

interval in the Weerde‐Zemst (073E0359) borehole in which the interpretation of the Mont‐1914

Panisel and Kwatrecht Members is discussed by Steurbaut et al. (2015). 1915

1916

Kester (101W0079) 1917

Different stratigraphic interpretations exist of this borehole(see Houthuys 2014; see also a 1918

reinterpretation discussion of the borehole by Houthys and Matthijs , 2016 posted on DOV ) 1919

and available on http://mars.naturalsciences.be/geology/boreholes/110w0079‐1920

kesterberg/view 1921

The sandy clay base of the Ieper Group (105‐111m) is unusual. The low GR at the base is 1922

maybe comparable to the lower GR at the base of the Merchtem borehole (072E0229) log. 1923

The Lower Orchies unit is about 15m thick. 1924

The interpretation of the Aalbeke Mbr overlain by the Mont‐Panisel Mbr is based on the 1925

similar pattern observed in the Merchtem borehole (072E0229). 1926

The lithology between the Aalbeke and the top of the Upper Orchies Member seems more 1927

clayey at its base and more sandy towards the top. Based on the expected vertical 1928

succession of lithostratigraphic intervals, this interval could be interpreted as the Roubaix 1929

Mbr or as the Mons‐en‐Pévèle Fm. The Roubaix Mbr interpretation could be supported by 1930

the GR signal which is not so different from the signal in the Orchies section below. The 1931

lithological description of the section in the Kester borehole however (Archives Belgian 1932

Geological Survey) reports a very dominantly sand lithology, confirmed by grain‐size analyses 1933

in Geets (2001, p68) and hence the interval corresponds to the Mons‐en‐Pévèle Fm 1934

according to the criterion outline above in the text (>50‐60% sand layers). The geophysical‐1935

log pattern events in that interval can be interpreted as similar to those observed in the 1936

boreholes Merchtem (072E0229) and Zemst‐Hofstade (073E0397) : in the former the pattern 1937

is considered transitional between Roubaix and Mons‐en‐Pévèle (see Merchtem log in the 1938

compendium) and in the latter the pattern is interpreted as Mons‐en‐Pévèle ( see log in 1939

compendium). 1940

1941

Knokke (011E0138) 1942

The Zoute Mbr is defined (Steurbaut, 1988) between 283,4‐288m. 1943

The top of the Aalbeke Mbr is consistent with a clay mineralogy boundary as published by 1944

Mercier‐Castiaux & Dupuis (1988). 1945

The GR and RES log signatures between 154 and 157,5m are interpreted as the level 1946

corresponding to the Egemkapel clay Member. In 1947

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Plates 2 and 3 in Laga & Vandenberghe(1990) illustrate brecciation in the intervals 153.20‐1948

153.40m, at 154.20m and also at 170‐171m and 180‐180,90m which could represent 1949

submarine erosion levels associated with low sea‐level positions. 1950

1951

Unfortunately biostratigraphic data are missing in this interval (nannoplankton of the Knokke 1952

borehole in Steurbaut (1990,p48); dinoflagellates (see De Coninck, 1991), and the few 1953

dinoflagellate marker horizons by Hochuli (in Vandenberghe et al. (1998, fig.7)) are probably 1954

lacking precision). 1955

The practice of considering the sediment interval between the Egemkapel and the 1956

Merelbeke units as the Egem Member is the basis of the present interpretation which seems 1957

supported by the consistent correlation of subunits in the Egem Member Yd4‐Yd5‐Yd6 which 1958

is exemplary demonstrated in the Tielt‐Gent area (legends geological maps Tielt and Gent, 1959

Jacobs et al, 1996a‐b); however it seems that towards the north‐west in the Brugge‐Knokke 1960

area, the lower part of the Egem Member is developed as a clay. This is also observed in the 1961

Brugge borehole. In previous studies of the Knokke borehole (011E0138) (see e.g. King, 1962

1990; Welkenhuysen & De Ceukelaire, 2009 p. 72), the identification of the Egem sand Mbr 1963

is logically limited to the upper about 10m thick sand layer (‘Yd6’) in the interval (see also 1964

Comment, Brugge borehole). 1965

The profile designed by Van Burm and Bolle (Jacobs et al, 1996a), reproduced below, is 1966

already an indication of the appearance of a clay‐layer above the Egemkapel Member. The 1967

layer called Yd5 is clearly increasing to the west. It is proposed to call informally the clayey 1968

lower Yd4 and Yd5 part the Hazegraspolder unit, referring to the location of the deep cored 1969

Knokke borehole (Laga & Vandenberghe (1990). 1970

The intervals of the Merelbeke, Pittem, and Vlierzele Mbrs are in accordance with core 1971

descriptions (see Laga & Vandenberghe, 1990) and with the Fobe (1995,1997 fig.3) 1972

interpretation, if this last authors’ Beernem unit ‘ with base at 116m, is considered part of 1973

the Aalter Fm of the Zenne Group as discussed in the text under Vlierzele Member. 1974

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1975

1976

Kruishoutem (084E1412) 1977

In the Hyon Fm interval (8‐18,2m), absence of stone layers and a log pattern having a good 1978

resemblance to the Tielt, Brugge .. boreholes, suggests that the Egem Sand Mbr is present. 1979

The top of the Aalbeke Mbr is based on the GR pattern but the base of the unit is more 1980

arbitrary and could be a few m lower. 1981

1982

Merchtem (072E0229) 1983

An inspection of the original lithological description of the borehole (DOV) shows less than 1984

25% of the interval between about 76‐118m is described as sandy and therefore, according 1985

to the criterion discussed in the text, the interval has to be considered as the Roubaix Mbr 1986

rather than the Mons‐en‐Pévèle Formation. However referring to the discussion about this 1987

criterion in the text and the relatively high values both GR and RES in the interval 76‐118m, 1988

make a distinction between Roubaix Mbr and Mons‐en‐Pévèle Fm doubtful. From the 1989

traditional events in the Roubaix Mbr only the 4,5,6 labeled events can be identified. 1990

The 2 units between the Aalbeke and Merelbeke clay Members have an almost identical log 1991

signature to a pattern recognized in the Kerksken (086E0340), Weerde‐Zemst (073E0359) 1992

(see discussion in Steurbaut et al. , 2015) and even maybe Wieze (072W0159) borehole logs 1993

and are therefore interpreted as the Mont Panisel and Kwatrecht Mbrs. 1994

Yd5

clay

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The Gentbrugge Fm above the Merelbeke unit is interpreted as the Pittem Mbr below and 1995

Vlierzele Mbr above, and the log seems similar to the Kallo (014E0355) and maybe even 1996

Rijkevorsel (007E0200) borehole logs. In the Kallo (014E0355) borehole the Pittem Member 1997

identification is in line with the about 5m Pittem Member identified in the nearby Kruibeke 1998

borehole by Fobe ( 1995,fig.5). 1999

2000

2001

Merksplas (017W0280) 2002

The presence of just a few meter of the Mont Héribu Mbr cannot be excluded as it is present 2003

in the nearby Kallo (014E0355) and Rijkevorsel (007E0200) wells; however it is estimated too 2004

thin to be positively identified on the Merksplas log. 2005

The Kortemark Mbr is identified as the unit between the Aalbeke and Egemkapel clay 2006

Members, although it has become thin in this borehole and the log pattern is not very 2007

charactersitic.The pattern is comparable to the nearby Rijkevorsel borehole. 2008

No further specification of the Hyon Fm is possible; the GR pattern is quite comparable to 2009

the Kallo borehole. 2010

Mol SCK 15 (031W0237) 2011

This well is included in the compendium because it is cored, reliably described (Gulinck & 2012

Laga, 1975) and many biostratigraphic work was done on samples from this well. More 2013

recent cored boreholes with geophysical wells were drilled and cored in the Mol‐Dessel 2014

area in the framework of the ONDRAF‐NIRAS research related to radioactive waste. 2015

The top of the Orchies Member corresponds with the base of the Mons‐en‐Pévèle Sand Fm 2016

above according to a detailed core description by Gulinck and Laga(1975) showing at that 2017

level the boundary between fine sand and clay. 2018

The fine sand above the Aalbeke Mbr has clay laminations in the middle as observed in the 2019

cores. It is interpreted as the Hyon Fm because of its sandy nature while the Kortemark Mbr 2020

is more clayey. No stone layers have been reported in the interval. 2021

Steurbaut (1988) has interpreted this interval as consisting of the Egem Mbr above 355m 2022

and the Kortemark Member below 355m; this could be supported by the somewhat similar 2023

borehole pattern as in the Merksplas borehole where the pattern is interpreted as the 2024

Kortemark, Egemkapel , Hyon succession. The clay laminations could then be considered the 2025

equivalent of the Egemkapel unit. 2026

Steurbaut has reviewed the nanoplankton data of this interval for the purpose of the present 2027

review; he concludes that the top of the Hyon Fm interval above 354,2m corresponds to the 2028

67

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Mont‐Panisel Mbr whilst the lower part cannot be attributed with certainty to any known 2029

unit, and neither to the Kortemark nor the Egem Member. 2030

Oosterzele (070E0237) 2031

The Hyon Fm between the Egemkapel and Merelbeke Mbrs lacks stone layers and its log 2032

pattern is comparable the pattern of the Egem Mbr in the nearby Gent borehole. 2033

The Gentbrugge Formation consists entirely of the Vlierzele Member which is consistent 2034

with the observation from field and borehole data of the absence of the Pittem Member in 2035

the area (see text). 2036

2037

Pittem (053W0073) 2038

The borehole is close to the nearby Tielt borehole (053E0061) and the Egem clay and sand 2039

pit (see Steurbaut, 1988, Fig. 5). The more clayey log pattern at the base of the Kortemark 2040

Member compared to the Kortemark Member log pattern of the Tielt borehole may be an 2041

indication of the lithological variability within the Kortemark Member. 2042

2043

Rijkevorsel (007E0200) 2044

Log patterns in the Rijkevorsel and Kallo (014E0355) wells are very similar for the Kortemark 2045

Mbr, Hyon Fm, Gentbrugge Fm... Subdivisions within the Orchies Member have been 2046

interpreted by Steurbaut (1998, Fig.10). The base of the Gentbrugge Fm probably consists of 2047

a few meter of the Pittem Member. 2048

Tielt (053E0061) 2049

Note that the borehole represented in the Compendium is a different borehole than the 2050

classical 068E0169 borehole from which grain‐size data are available (Geets 1988; and used 2051

in the interpretations by Steurbaut, 1998, Fig16). Distance between the two boreholes is 2052

more or less 2 km. Tielt is located close to the Egem clay and sand pit. 2053

The top of the Aalbeke Mbr is interpreted at 49,5m below which heavy clay was described in 2054

the borehole (De Geyter, Archives Belgian Geological Survey); on the GR and RES log pattern 2055

this level corresponds to the coarsening of the sediment in the top of the Aalbeke Mbr (see 2056

discussion in text). 2057

Torhout (052E0195) 2058

Although the geophysical logs are not of very good quality, a reasonable interpretation with 2059

typical log signatures can be made, based on a comparison with the Tielt borehole 2060

(053E0061). 2061

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Wieze (072W0159) 2062

The identification of the 3 sub‐members of the Orchies Member is straightforward, as is the 2063

labeling in the Roubaix Member and the identification of the Aalbeke Mbr. 2064

Above the Aalbeke Mbr, the RES and GR log pattern resembles the twofold subdivision 2065

pattern as in Kerksken (086E0340), Zemst‐Weerde (073E0359)…. and consequently can be 2066

interpreted as Hyon Fm/MontPanisel Mbr overlain by the Kwatrecht Member. However, the 2067

clay intervals at about 8 and 20 m depth could equally well be interpreted as respectively the 2068

Merelbeke and Egemkapel Members. In the latter interpretation the unit above the Aalbeke 2069

Clay would be the Kortemark Member instead of the Mont‐Panisel Member and above the 2070

Egemkapel Clay would occur the Egem Member instead of Kwatrecht Member. The 2071

signature of the possibly Kortemark Member is comparable to the Kortemark Member 2072

signature more to the west but the supposed Egem Member on the other hand would then 2073

appear to be more clay rich according to the logs than the Kortemark Member below it. It is 2074

hard to make a final choice without more sedimentological and biostratigraphic analyses in 2075

the area and in the reference areas. 2076

Both possible interpretations are shown on the log while in the log correlation figure by J. 2077

Matthijs, the Kortemark to Egem interpretation is represented. 2078

Wortegem (084W1475) 2079

The top Orchies Mbr corresponds to the systematic description by De Geyter (1990) as the 2080

limit between overlying silty clay and heavy clay below. The Roubaix Member log pattern is 2081

analoguous to the Roubaix section in the Oosterzele borehole. 2082

The Aalbeke Mbr GR, and also RES, pattern is comparable with its equivalent in the 2083

Kruishoutem borehole. 2084

The Mont‐Panisel and Kwatrecht Mbrs patterns are comparable to the patterns of these 2085

units in the Kerksken (086E0340) and Wieze boreholes 2086

2087

Zemst‐Hofstade (073E0397) 2088

The pattern of the Orchies and Mons‐en Pévèle section is comparable to the pattern of the 2089

Orchies‐Roubaix interval in the Merchtem log. The identification as Mons‐en Pévèle Fm is 2090

preferred over the Roubaix Mbr as the RES values are nearly double the ones in the 2091

Merchtem borehole, in which borehole the interval is considered transitional Roubaix to 2092

Mons‐en Pévèle (see Merchtem in the compendium) . In the Zemst‐Hofstade borehole the 2093

amount of sand layers, apparent from the RES log, are estimated to be about 50% of the 2094

total, which according the specified criterion (see text) is characteristic for the Mons‐en 2095

Pévèle Mbr. 2096

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The top clay unit consists either entirely of the Aalbeke Clay (see comparable signal in the 2097

Knokke and Mol‐SCK15 boreholes) or it might be composed of the Aalbeke clay overlain 2098

directly by the Merelbeke clay , an interpretation suggested by Johan Matthijs; in the latter 2099

case the Mont‐Panisel and Kwatrecht units wedged out or were eroded before the 2100

deposition of the Merelbeke Mbr, while in the former case, if only Aalbeke clay is present, 2101

the Mont‐Panisel and Kwatrecht units could have been eroded in a later were stage. 2102

Zemst‐Weerde (073E0359) 2103

The log pattern below the Merelbeke Mbr is very comparable with the pattern of the Mont‐2104

Panisel and Kwatrecht Mbrs in the Kerksken (086E0340) borehole. 2105

The stratigraphy of this borehole, including biostratigraphy and grain‐size analyses, has been 2106

elaborated in detail by Steurbaut et al. (2015). Below the Mont‐Panisel Fm occur a few 2107

meter of the Tielt Fm: from the log correlation by J. Matthijs , the Aalbeke Mbr is expected 2108

to occur below the Mont‐Panisel Mbr. 2109

2110

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2111

Referenties: 2112

2113

Ali, J., King, C., Hailwood, E.A., 1993. Magnetostratigraphic calibration of early Eocene 2114

depositional sequences in the southern North Sea Basin. In: Hailwood, E.A. & Kidd, R.B. 2115

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