ORIGINAL PAPER

Mineralogy, petrography, and biostratigraphy of the LowerEocene succession at Gebel El-Qurn, West Luxor,Southern Egypt

Hossam A. Tawfik & Esam K. Zahran &

Abdel T. Abdel-Hameed & Wataru Maejima

Received: 23 June 2009 /Accepted: 3 May 2010 /Published online: 28 May 2010# Saudi Society for Geosciences 2010

Abstract The widely exposed siliciclastic/carbonate suc-cession exposed at Gebel El-Qurn, west Luxor, has beeninvestigated from the mineralogical, petrographical andbiostratigraphical points of view. The succession belongs tothe lower Eocene, including the upper Esna Shale and theThebes Formations that have been deposited under variedmarine conditions and during alternating periods ofabundant and ceased clastic influx. They contain abundantand well-diversified planktonic foraminifera and calcareousnannoplankton, suggesting deposition in open marine innerto middle shelf environments. Mineralogical analysiscarried out by XRD revealed the presence of smectite,illite, kaolinite, sepiolite, palygorskite, and smectite–illite-mixed layer as the principal clay minerals, and calcite,dolomite, quartz, anhydrite, gypsum, hematite, and goethiteas non-clay minerals. The clay mineral distributions in thesediments reflect the climatic conditions and the weatheringprocesses at the source area as well as the differentialhydraulic sorting during transportation. Calcite is the mostabundant non-clay mineral, and this is consistent with highcalcareous fossil content of the sediments. Petrographicexamination of the carbonate lithologies within the succes-

sion enables to identify eight microfacies associations.These microfacies were affected by several diageneticprocesses including; micritization, compaction, cementa-tion, neomorphism, dissolution, dolomitization, and silici-fication. Dissolution of original test wall and replacementand infilling by iron oxides and recrystallized calcite werecommonly observed. Calcareous nannofossils are generallycommon to frequent, highly diversified, and moderately towell preserved. Two calcareous nannofossil biozones;Tribrachiatus contortus Zone (NP10) and Discoasterbinodosus (NP11) are recorded in the studied sedimentssuggesting lower Eocene age. Their associated nannofossiltaxa are characterized by the predominance of warm waterspecies. Sea-level fluctuations, basin physiography, climate,paleogeography, and sediment supply were the majorcontrols on the deposition of the lower Eocene sedimentsat Gebel El-Qurn.

Keywords Luxor . Gebel El-Qurn . Silicification .

Tribrachiatus contortus .Discoaster binodosus

Introduction

The stratigraphic succession of Gebel El-Qurn, nearLuxor, is one of the classical sections of the lower Libyan(Zittel 1883) and the type-section of the Thebes Formation(Said 1962). The succession belongs to the ThebesMountains that extend for several hundred kilometerstoward the west of the western side of the Nile River.These Mountains consist of rugged limestone hills whichhave been deeply incised by a series of dendritic wadisthat mainly controlled by gravity, structure, and/orlithology (Yehia 1973). They reach a maximum elevationof more than 500 m with a net relief of about 400 m from

H. A. Tawfik (*) :W. MaejimaDepartment of Geosciences, Graduate school of Science,Osaka City University,Sumiyoshi-ku, Osaka 558-8585, Japane-mail: hossamatawfik@yahoo.com

E. K. ZahranDepartment of Geology, Faculty of Science,Damanhour University,Damanhour 22514, Egypt

A. T. Abdel-HameedDepartment of Geology, Faculty of Science, Tanta University,Tanta 31527, Egypt

Arab J Geosci (2011) 4:517–534DOI 10.1007/s12517-010-0158-6

the Nile River to the top of Gebel El-Qurn, the highestpeak in the study area (Fig. 1).

The sedimentary rocks forming the Thebes Mountainsare composed mainly of shales, marls, chalk, and lime-stones, and are part of the three important formations (orgroups) that outcrop in and around the study area. Theyinclude, from base to top, the Paleocene Tarawan Chalk, thePaleocene/Eocene Esna Shale, and the Lower EoceneThebes Formation which were unconformably overlain bythe Plio-Pleistocene conglomerate. These rocks werelargely deposited during the Paleocene and Lower Eoceneepoch (∼52 Ma) in a pelagic and shallow environmentbelow and above the phototropic zone (Said 1990).

The first geologic investigations on the Thebes Moun-tains started during the middle of the nineteenth century.Further studies focused on various details of the ThebesMountains in Luxor, such as the reconstruction of thebiostratigraphy (Barakat and El-Dawoody 1973; Anan1974; Hamam 1975; Perch-Nielsen et al. 1978; Strougoand Hassaan 1984), sedimentology and structural geology(Aboul Fetouh 1973; Gindy et al. 1985; Yehia 1986; El-Kammar et al. 1991; Hafez and Hussein 2004) and recentlygeoenvironmental investigations (Wüst and McLane 2000;McLane et al. 2003; El-Bayomi 2007; Leisen et al. 2008).

The documentation of the mineralogical and petrograph-ical characteristics of the lower Eocene succession at GebelEl-Qurn, near Luxor is the main target of the present work.It is also intended to provide a biostratigraphic zonation inthe area under consideration by means of calcareousnannofossils in an attempt to define its correlation withother localities.

Materials and methods

In this study, mineralogical analysis was conducted on atotal of 26 samples collected from the upper Esna Shale andThebes Formations (Fig. 2). The X-ray diffraction (XRD)technique was applied on the oriented clay-size fraction (<2 μm) and bulk powdered samples following the methodsdescribed by Biscaye (1965) and Millot (1979). Theanalyses were carried out on a RIGAKU RAD-1 X-raydiffractometer fitted with CuKα-radiation at 30 KV and10 mA. The samples were scanned at a scan speed of 1°/min from 2–50 2θ. All minerals were identified by theircharacteristic reflections (Moore and Reynolds 1997), andtheir abundances were determined by the aid of Diffrac/ATsoftware (Table 1).

Thin sections were prepared from 23 indurated carbonatesamples for detailed petrographic description of the studiedrocks and proper identification of their mineral composi-tion. The samples were later examined under the polarizingmicroscope both in plane and cross nicols for identification

of grain constituent and microfacies associations. Theidentification of minerals and examination of diageneticfeatures are also supplemented by the aid of the scanningelectron microscope (SEM).

The analysis of calcareous nannofossils was carried outon the raw sediment samples using standard smear slidepreparation as described by Bown (1998). All samples wereprepared in the same manner to insure uniformity in thedistribution of material and to minimize bias. Smear slideswere examined using a Carl Zeiss microscope under cross-polarized light at 1250X magnification. Nannofossils wereidentified following the standard taxonomic schemes ofPerch-Nielsen (1985), Bybell and Shelf-Trial (1997), andWise et al. (2004). Paleontological investigations werepreceded at the University of Tanta, Egypt, whereas themineralogical and petrographical analyses were carried outat the University of Osaka City in Japan.

Stratigraphy

The stratigraphic succession at Gebel El-Qurn, west Luxor,comprising the Esna Shale and Thebes Formations, reachesabout 400 m thick (Fig. 2) and span from the upperPaleocene to lower Eocene. The Esna Formation which hasa total thickness of about 50 m can be divided into twoparts. The lower part is about 30 m thick and is exclusivelycomposed of olive-grey fissile shale with abundant anhy-drite nodules (dehydrated gypsum; Fig. 3a) and few verythin (5-25 mm) silt and fine sand layers. This part containsthe Paleocene/Eocene boundary with the coccolith zonesNP9 and NP10 (Perch-Nielsen et al. 1978). The top of thelower part also yields a broad assortment of fossil types, alllimonitic and commonly of small sizes, including bivalves,gastropods, nautiloids, brachiopods, crinoid stems, and fishteeth and bones (Faris and Strougo 1998).

The upper part is about 20 m thick and consists ofinterbedded reddish brown shale and relatively harder buffto yellow marls (Fig. 3b). The marly bands lie closer toeach other at the top and become enriched in lime upward,making the transition into the Thebes Formation. The EsnaFormation is overlain by the thick-bedded limestone of theThebes Formation (Said 1960). According to Said (1990)the Esna was deposited in an unstable shelf setting probablyduring a high sea-level stand.

The Thebes Formation is about 350 m thick andcommences with fine-grained micritic limestone andmarl rich in bands and nodules of chert, probablydeposited at bathyal depths according to Snavely et al.(1978), and ends with repetitive skeletal limestones andoyster banks formed in a shallow water setting. Said(1962) and Curtis (unpublished data) divided the ThebesFormation into four members (Mb I to Mb IV) according

518 Arab J Geosci (2011) 4:517–534

to variations in lithology and weathering characteristics(Fig. 2).

Member I of the Thebes Formation (∼ 90 m thick)consists of alternations of thinly laminated pinkish marl(Fig. 3c), nodular micritic limestone and thinly beddedargillaceous limestones. The rocks of this member togetherwith the shales and marls of the Esna Formation showabundant planktonic and few benthic foraminifers. Theywere deposited on a wide carbonate-rich platform with ahigh plankton production rate (nannoplankton ooze, Globi-gerina sp., Morozovella sp., and Chilogümbilina sp.). Thewater depth was about 300 m, followed by a shallowingupward cycle towards the top of Member I and waterdepths of approximately 50 m (Wüst 1995). This memberends with a silicified dolomitic limestone band of approx-imately 20 cm thick (Fig. 2).

The overlying geological units, Members II to IV, arecomposed of more than 260 m of an alternating series ofdolomitic limestones, marls, shales, and nodular limestones(Fig. 3d) with interbedded chert nodules and chert bands.They are regarded as individual shallowing upward cyclesof deposition.

Furthermore, they are richly fossiliferous, with cephalo-pods (Nautilus sp.), gastropods (Turritella sp.), pelecypods(Ostrea sp.), and anodont bivalves (Lucina thebaica), andvarious nummulites and Operculina sp. (Fig. 3e, f). Theywere deposited in shallow water (10–100 m) on anextensive carbonate platform (Wüst 1995).

Results and discussion

Mineralogy

X-ray diffraction analyses of the clay-size fraction (<2 μm)and bulk rock samples have indicated the presence ofvarious clay and non-clay minerals (Table 1).

Clay minerals

The analysis of the clay-size fraction of the sediments (EsnaShale and Thebes Formation) shows that they are composedof different varieties of clay minerals; smectite, illite,kaolinite, sepiolite, palygorskite, and smectite/illite-mixed

Fig. 1 Location and geological map of Gebel El-Qurn, West Luxor, Southern Egypt

Arab J Geosci (2011) 4:517–534 519

layer (Table 1). The relative proportions of these minerals,which are calculated as peak areas (Biscaye 1965), aregraphically illustrated (Fig. 4). The obtained clay mineralsare:-

Smectite minerals are the most common clay minerals inthe studied rocks and dominate both the lower and upperparts of the Esna Shale as well as the lower members (Mb Iand Mb II) of the Thebes Formation (Fig. 4). Its averageconcentration decreases upward in the succession, reachingabout 19.3% in the Esna Shale and 5.0% in the ThebesFormation (Table 1). The basal reflections of the detectedsmectites are broadly diffracted and of low intensity,suggesting a poorly crystalline type (Millot 1979).

Illite is restricted only to the lower and upper parts of theEsna Shale (Fig. 4), and its concentration increases upward

(av. 6.7%; Table 1). It is probably formed in soils with littlechemical weathering in cold/or dry climate, and in areas ofhigh relief where physical erosion is predominant.

Smectite–illite-mixed layer is recorded abundantly in thelower part of the Esna Shale (QR 1, QR 2 and QR 3), andits average concentration reaches about 8.8% (Table 1).El-Kammar and Basta (1983) and (El-Kammar et al. 1991)pointed out that the prevailing weathering in the aridEgyptian desert is sufficient to convert illite to smectitepassing through illite–smectite-mixed layer. This conver-sion is accompanied with significant release of silica(Morris and Shepperd 1982; Carrels 1984). The releasedsilica is authigenetically deposited in the intergranular poresand concentrated between the upper and lower parts of theEsna Shale.

Fig. 2 Lithostratigraphic suc-cession of Gebel El-Qurn, WestLuxor, Southern Egypt

520 Arab J Geosci (2011) 4:517–534

Tab

le1

X-ray

diffractiondata

show

ingmineral

compo

sitio

nof

thestud

iedsamples

Fm.

Sam

ple

%Clayminerals

%Non

-clayminerals

Smectite

Sepiolite

Palyg

roskite

Kaolin

iteIllite

Mixed

Calcite

Dolom

iteQuartz

Anh

ydrite

Gyp

sum

Hem

atite

Goethite

Illite-

Smc

THEBES

FORMATIO

NMem

ber

IVQR26

–5.1

––

––

75.9

15.8

3.2

––

––

QR25

–4.6

––

––

77.5

15.9

2.0

––

––

QR24

–4.5

––

––

78.9

13.5

3.1

––

––

QR23

–3.9

––

––

76.2

17.3

2.6

––

––

QR22

–5.2

3.0

––

–71

.317

.23.3

––

––

Mem

ber

III

QR21

–4.8

4.0

––

–70

.117

.83.3

––

––

QR20

–5.3

4.7

––

–73

.67.8

5.6

–3.0

––

QR19

–3.4

2.3

––

–78

.811.1

4.5

––

––

QR18

–5.8

4.4

––

–74

.210

.82.3

–2.6

––

QR17

2.1

4.9

3.2

––

–75

.710

.43.7

––

––

Mem

ber

IIQR16

3.0

–2.2

––

–77

.412

.15.3

––

––

QR15

6.4

–4.2

––

–76

.07.1

6.3

––

––

QR14

7.1

–3.6

––

–73

.411.0

5.1

––

––

QR13

12.7

–7.4

––

–60

.38.7

11.0

––

––

QR12

B10

.94.2

11.9

––

–49

.214

.49.3

––

––

Mem

ber

IQR11

10.8

12.3

8.7

––

–28

.824

.113

.4–

–1.9

–

QR10

12.7

10.8

7.3

––

–30

.620

.615

.0–

–2.9

–

QR09

11.3

11.7

8.6

––

–28

.921

.716

.1–

–1.8

–

QR08

12.0

11.2

9.5

––

–29

.620

.617

.2–

–2.0

–

QR07

10.7

12.1

9.7

––

–28

.322

.414

.4–

–2.4

–

Av.

Thebes

5.0

5.5

4.7

––

–61

.715

.07.3

–0.3

0.5

–

ESNA

SHALE

Upp

erQR06

14.8

––

2.7

9.0

–26

.017

.516

.910

.4–

2.8

–

QR05

15.8

––

2.5

12.2

–22

.714

.315

.914

.0–

2.6

–

QR04

17.5

––

4.6

8.2

–23

.613

.417

.913

.2–

1.5

–

Low

erQR03

21.3

––

6.1

4.6

18.2

12.2

5.5

14.1

10.1

–3.4

4.6

QR02

22.1

––

7.0

4.1

16.2

10.0

3.8

16.2

14.1

–4.3

2.2

QR01

24.1

––

6.2

2.4

18.4

11.6

3.2

16.1

10.2

–4.7

3.2

Av.

Esna

19.3

––

4.9

6.7

8.8

17.7

9.6

16.2

12.0

–3.2

1.7

Arab J Geosci (2011) 4:517–534 521

Kaolinite is also, confined to the lower and upper partsof the Esna Shale (Fig. 4), reaching an average concentra-tion of 4.9% (Table 1). Kaolinite is relatively enriched inthe lower part of the Esna Shale together with smectite–illite-mixed layer and is remarkably poor in the upper, morecalcareous, part of the Esna Shale. The XRD pattern andpeak sharpness of kaolinite suggests poor crystallinity(Weaver and Pollard 1973), and this disordered naturewas confirmed by the SEM.

Sepiolite is almost present in all the samples of theThebes Formation, except at Member (II) and particularlyin rocks displaying strong to cryptic spheroidal weathering.It is present in impure limestone and marl beds of theThebes Formation exhibiting planar, papery exfoliation.The average concentration of sepiolite in the ThebesFormation reaches about 5.5% (Table 1), and it is almostlyassociated with palygorskite and smectite. In this study,

sepiolite might have formed diagenetically through trans-formation of smectite to palygorskite and then of palygor-skite to sepiolite (Millot 1970). SEM study shows fibrousor hairy growth of sepiolite with an obvious micro-intercrystalline porosity (Fig. 5a, b).

Palygorskite is detected in the first three members (MbI–Mb III) of the Thebes Formation where its averageconcentration reaches about 4.7% (Table 1). It is alwayspresent and increases with increasing concentrations ofsmectite and sepiolite. Palygorskite is also present in chalkswith cryptic spheroidal weathering (Gindy et al. 1985).Cases of neoformation of diagenetic growth of palygorskitethrough absorption of moisture in calcareous soils of aridregions are well documented (e.g., Hegab 1974; Al-Sayeghet al. 1976; Yaalon and Wieder 1976). However, it issuggested that palygorskites (like sepiolites) may be formeddiagenetically after smectite.

Fig. 3 Photographs showing aabundant anhydrite (dehydratedgypsum) veinlets within theEsna Shale, b interbedded marland shale layers at the upper partof the Esna Shale, c thinlylaminated pinkish marl ofMember I, that entirely shearedbetween two competent lime-stone beds. Note the bed weath-ers like a schist in stronglydisturbed faulted area, d nodularlimestone bed at the upper partof Member II of the ThebesFormation, e, f highly fossilifer-ous limestone of Member IV ofthe Thebes Formation

522 Arab J Geosci (2011) 4:517–534

Non-clay minerals

X-ray diffraction analyses of the bulk rock samples haveindicated that calcite, dolomite, quartz, anhydrite, gypsum,hematite and goethite are the main non-clay mineralsconstituents (Table 1). In the following paragraphs, eachmineral or group of minerals will be discussed separately:

Calcite minerals are the most common carbonateminerals in the studied rocks, and their average concentra-tion reaches 17.7% and 61.7% in the Esna Shale andThebes Formation, respectively (Table 1). Calcite andphyllosilicates show opposite behavior (Fig. 3). Thestaining technique, using Alzarin Red S and PotassiumFerricyanide (Friedman 1971), has shown that they arerepresented mainly be ferroan type. Different forms ofcalcite are recognized in the present carbonate samplesincluding fine-grained micrite, microsparite, and sparrycalcite cements. Calcite minerals also occur in the form ofvarious skeletal microarchitecture such as foraminifera,pelecypods, gastropods, nummulites, and calcareous nan-nofossils (Fig. 5c). All these types are preserved with theirprimary fabric and original composition, or may berecrystallized to more stable form, or may be replaced byother minerals such as dolomite, silica, etc.

Dolomite is the second abundant carbonate mineral in allsamples and its average concentration reaches about 9.6%and 15.0% in the Esna Shale and Thebes Formation,respectively (Table 1). In addition to optical properties, itsidentification was aided by the staining technique andsupplemented by XRD and SEM studies (Fig. 5d). Theunstained nonferroan variety is abundant in the dolomicriticmarls of the upper part of the Esna Shale and the lower partof the Thebes Formation (Member I). It is characterized byvery fine anhedral crystals, closed packing and a well-defined intercrystalline porosity (Fig. 5e, f). Dolomites areusually associated with detrital quartz, evaporites andFe-oxide materials.

Quartz is the most common detrital non-clay mineral inthe studied rocks, averaging 16.2% in the Esna Shale and7.3% in the Thebes Formation (Table 1). Detrital quartz isusually present as individual grains (silt to fine sand sized)scattered in the carbonate groundmass. The verticaldistribution of quartz indicates a negative relationshipbetween this mineral and carbonate minerals (Fig. 4).Meanwhile, the increased amount of detrital quartz incarbonate rocks reflects elevated intervals of terrigenoussupply from continental sources.

Gypsum and anhydrite minerals are recognized in thepresent samples and their presence is confirmed by theXRD. Anhydrite veinlets (dehydrated gypsum) are recordedabundantly in the Esna Shale and their average concentra-tion reaches about 12.0% (Table 1). Gypsum is detectedsporadically in few samples with small amounts (av. 0.3%)in the limestones and dolomitic limestones of the ThebesFormation (Fig. 4). The development of gypsum in theEsna Shale seems to be occurred by two episodes of waterinjection at different times. The first one consisting of up to10 cm long bladed gypsum related in origin to theshrinkage of the host shale layers and the second one,consisting of fibrous satin spar, is related to hydration ofanhydrite to secondary gypsum.

Hematite and goethite minerals are found in the studiedEsna Shale and lower member of the Thebes Formation.They are present as reddish to yellowish brown nodulesand streaks that are principally due to dissemination ofsubmicroscopic crystallites of hematite and goethite.Although hematite gives weak resolution when examinedby XRD, however, it is detected in the dolomitic shalesand marls of the Esna Shale (Table 1). Frank (1981) andEl-Kammar (1984) attributed such association colour-ation to weathering of ferroan dolomite and clayminerals. The iron oxides of the Thebes Formationmainly came from an outside source, either landmass orintrabasin highs. This iron was deposited in shallow

Fig. 4 Vertical distribution chart showing the abundances of clay and non-clay minerals

Arab J Geosci (2011) 4:517–534 523

water due to oxidation and early coagulation. The smallproportion of iron in true solution may have beenresponsible for replacement in the vicinity of thecavities and of the matrix.

Petrography

Petrographic studies were done on the carbonatesamples collected from the upper marl beds of the EsnaShale and the four members (Mb I–Mb IV) of theThebes Formation. Microfacies analysis was carried outon the samples following the classification schemes ofFolk (1959), Dunham (1962), and Flugel (1982),and theassociated diagenetic features are detected petrographi-cally, these microfacies associations are described,illustrated, and evaluated environmentally (Figs. 6 and7).

Micrite Most of the Thebes limestones in the study area,are texturally considered as a micrite (mudstone) being amatrix supported carbonate. Diagenetically, the depositedmicritic sediments were lithified by micritic cement.Micrites, occasionally, were replaced by coarse mosaics ofmicrospars through aggrading neomorphism. This associa-tion is represented in the lower part of Member I of theThebes Formation (QR 7). The lime-mudstone faciesreflects deposition in shallow subtidal setting under lowenergy-protected conditions.

Dolomicrite This microfacies is present in both the Esnaand Thebes Formations (QR 6 and QR 8, respectively), andthis association is texturally considered as a micrite(mudstone) (Dunham 1962 and Embry and Klovan 1972).Diagenetically, the deposited sediments were first lithifiedby precipitation of micritic cement, followed by dolomiti-

Fig. 5 SEM images showing afibrous sepiolite clay mineral,Sample QR 7, Mb. I, ThebesFm., b micro-intercrystallineporosity in sepiolite clay miner-al, QR 7, Mb. I, Thebes Fm., ccalcareous nannofossil ooze thatforming the main constituents ofmicritic limestone, QR 10, Mb.I, Thebes Fm., d)rhombohedralcrystal of dolomite, QR 23, Mb.IV, Thebes Fm., e closed pack-ing mosaic in dolosparite, QR23, Mb IV., Thebes Fm., fintercrystalline porosity in dolo-micritic matrix, QR 23, Mb. IV,Thebes Fm

524 Arab J Geosci (2011) 4:517–534

zation in which the original calcite matrix was replaced byfine crystalline dolomite. The presence of visible euhedaldolomite crystals supports the incomplete replacement,since in complete replacement, crystals grow together andthe euhedral shape is lost (Faris et al. 1961). According to

Badiozamani (1973), dolimitization has been predicted forseawater–meteoric water mixing zone, where salinitiesdecrease but Mg/Ca ratio is maintained. The presence ofthis association indicates submarine diagenesis at sediment-seawater interface.

Fig. 6 Optical photomicro-graphs showing a foraminiferalbiomicrite (wackestone/ pack-stone) that composed of plank-tonic foraminiferal testsembedded in micritic matrix,plane light, QR 5, Esna Shale, bforaminiferal biosparite (wacke-stone), with large foraminiferaltest (Morozovella velascoensis),plane light, QR 11, Mb. I,Thebes Fm., c fossiliferous bio-micrite (wackestone/packstone)with various macro- and micro-fossils embedded in micriticmatrix, crossed polars, QR 15,Mb. II, Thebes Fm., d nummu-litic biomicrite (packstone).Note, small nummulite shellsthat embedded in micritic car-bonate matrix, plane light, QR17, Mb. III, Thebes Fm., enummulitic biosparite (grain-stone) welded by sparry calcitecement, crossed polars, QR 22,Mb. IV, f pelecypod biomicrite(packstone). Note, mega sizedpelecypod shells that embeddedin micritic carbonate matrix,plane light, QR 25, Mb. IV,Thebes Fm., g pelecypod bio-micrite (wackstone). Note, dis-solution of original test wall andinfilling by recrystallized sparrycalcite, crossed polars, QR 26,Mb. IV, Thebes Fm., h dolospar-ite (grainstone) with a coarse-grained dolomite rhombs,crossed polars, QR 12A, Mb. I,Thebes Fm

Arab J Geosci (2011) 4:517–534 525

Dolosparite This microfacies association is recorded in thesilicified dolomitic limestone band on top of Member I ofthe Thebes Formation and is texturally considered as asparite (grainstone). Neomorphism is represented by thepresence of irregular crystal boundaries, and irregularity ofcrystal size and shape throughout the mosaic. This wasfollowed by dolomitization in which the original limestoneswere totally replaced by anhedral dolomite mosaic (Fig. 6h).According to Longman (1977), this association was formedunder mixed zone (marine and freshwater) phreatic envi-ronment where neomorphism and dolomitization for sparitetook place.

Foraminiferal biomicrite This is present in the upper partof the Esna Shale and the lower member (Mb I) of theThebes Formation (QR 5 and QR 11, respectively). Theallochems are mainly represented by micritic walled forams(23%), algal filaments (2%) with less significant dolomite

rhombs (Fig. 6a). The foraminifera are represented byplanktonic forams e.g. Morozovella velascoensis thatoccasionally embedded in microsparite matrix (Fig. 6b).This association is texturally considered as a biomicritic(wackestone) being a matrix supported carbonates. Thesediment is loosely packed; the cement of the fine sparitefills the chambers of forams. Cementation was followed bydolomitization in which the original calcite matrix waspartially replaced by fine euhedral dolomite. Veinlets ofsparry calcite and calcitic mosaic filling fissures, cracks andvugs are also present. Some fauna are nearly obscured dueto recrystallization. Fe-oxide (2%) is present inside somepores as well as with cement. Selective destruction of faunais due to solubility of the shells in undersaturated porewaters.

Fossiliferous biomicrite This type of microfacies isrecorded in the middle part of the Thebes Formation (QR

Fig. 7 Optical photomicro-graphs showing a pore-liningisopachous cement appearing asa rim of crystals of equal thick-ness around the periphery ofpores and characteristics ofphreatic zone, crossed polars,QR 19, Mb. III, Thebes Fm., bmeteoric water neomorphoses inpelecypod shells where skeletalgrains are replaced by coarsermosaics of ferroan calcite,crossed polars, QR 25, Mb. IV,Thebes Fm., c replacement ofsiliceous radiolarian grains (tri-angles) by coarse ferroan calciteduring late diagenetic stage,crossed polars, QR 8, Mb. I,Thebes Fm., d selective de-struction of pelecypod shells dueto late dissolution effect andformation of moldic pores,crossed polars, QR 26, Mb. IV,Thebes Fm., e complete silicifi-cation of long pelecypod shellsthat probably occurred duringlate burial diagenesis, crossedpolars, QR 24, Mb. IV, ThebesFm., f globular foraminiferal testpartially filled with reddishbrown iron-oxide material, planelight, QR 8, Mb. I, Thebes Fm

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14, QR 15, and QR 16) and varies between fossiliferouslime-mudstone, wackestone and rarely packstone/wacke-stone, depending on the abundance of allochems (Dunham1962). Petrographic examination reveals that the frameworkis dominated by bivalve shell fragments (18%), gastropods(5%), algae (8%), echinoid spines (2%), and forams (10%).The non-bioclastic grains are rare. Subrounded to roundedquartz grains of coarse silt to fine sand are present. Thesediment loosely packed being a matrix supported lime-stone (Fig. 6c). Cementation was followed by neomorphism(calcitization) whereas the allochems are partially orcompletely filled by an irregular mosaic of crystals ofvarying sizes and shape, which is neither the originalmicrostructure nor a drusy mosaic, but a product ofneomorphism. Furthermore, the dissolution of the sedimentand shells produced secondary porosity as vuggy, moldicand channel one. This rock type indicates shelf margin orsubtidal environment under marine phreatic precipitation.

Nummulitic biomicrite This association is recorded in themiddle and upper parts of the Thebes Formation (QR 17,QR 18, QR 19 and QR 23). Petrographically, scatterednummulite shells and shell fragments (37%) are recorded(Fig. 6d). The Nummulites with thick wall and very large tomedium shells are seen. Their walls have radial fibrousstructure aligned at right angles to the shell wall. Theframework is supported by lime mud filling the intergran-ular pores. This association is texturally considered asbiomicrite (wackestone/packstone) being a matrix sup-ported carbonate. The sediment is loosely packed inferringthat cementation occurred before significant compaction.Micrite envelope outlined the skeletal grains of nummu-lites. Diagenetically, the deposited biomicritic sedimentswere lithified by micritic cement. Micrites occasionallywere replaced by coarser mosaics of microspars throughaggrading neomorphism. Large neomorphic ferroan andnonferroan sparry calcite is observed filling some fractures.Porosity ranges from low to moderate as evidenced by thepresence of small irregular vuggy pores.

Nummulitic biosparite This microfacies is recorded inMembers III and IV in the upper part of the ThebesFormation (QR 20 and QR 22). Petrographic examinationreveals that the framework is totally dominated bynummulite shells (57%) that intensively micritized orsometimes leached. This type is texturally considered as abiosparite (grainstone) being a grain-supported limestone(Fig. 6e). The framework is supported by neomorphicsparry calcite cement. The original micrites were occasion-ally replaced by coarser mosaics of microspar throughaggrading neomorphism. This cement is early diageneticformed in the phreatic zone where the Mg calcite recrystal-lized into calcite. Sometimes, Nummulites are recrystallized

into calcite spar. The sediment is tightly packed andnummulite shells are aligned parallel to bedding planes.Some shells were dissolved and their molds are either filledwith calcite spar or remain as unfilled moldic pores. Thedissolution processes and secondary porosity indicate earlydiagenetic process. Selective destruction and void forma-tion are due to the solubility of the shells. Generally, thisrock type indicates shallow marine subtidal environment.

Pelecypod biomicrite This microfacies is present in theupper part of the Thebes Formation (QR 24, QR 25, andQR 26). The carbonate allochems are represented mainly bypelecypod shells that partly or completely silicified (32%)and few algae and micritic peloids are present. Thisassociation is texturally considered as a biomicrite (wacketo packstone) being a matrix supported carbonate (Fig. 6f,g). The sediment is loosely packed inferring that cementa-tion occurred before significant compaction. Micrite enve-lope outlined the skeletal grains of pelecypods, withmicrosparite pore-filling. The following stages are sug-gested for diagenesis. (1) Micritization of allochems(pelecypod shells), and the shape of grains being main-tained by the micrite and microsparite cement, (2) dissolu-tion of skeletal grains and development of moldic pores, (3)Reprecipitation of CaCO3 as coarse mosaics of ferroancalcite filling the pore spaces. In addition, diagenetic silica,in the form of chalcedony, is observed replacing partially orcompletely some shell fragments.

Diagenetic overprint

The main diagenetic processes affecting the studiedcarbonate rocks are; Micritization, compaction, cementa-tion, neomorphism, dissolution, dolomitization, and silici-fication.

Micritization In general, this process seems to be lessimportant in the studied rocks compared to other processes.Herein, the margin of shell fragments becomes partially orcompletely micritized forming micrite envelope, as dis-cussed by Winland (1971) and Bathurst (1975).

Compaction The mechanical and chemical compactionsrefer to those processes reducing the bulk volume of therock (Moore 1989). The effect of mechanical compactionon carbonate sediments decreased by early cementationeither in the marine environment or during meteoricdiagenesis (Purser 1978). Once the carbonate sedimenthas been compacted, continued burial will increase theelastic strain at individual grain contacts. This may lead toincreased chemical potential that increases the solubility at

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grain contact resulting in point contact dissolution (Bathurst1975). The present samples showed that compactionprocess has a little effect, apart from some fracturedpelecypod shells.

Cementation The precipitation of cements in carbonatesediments is a major diagenetic process and it takes placewhen the pore-fluid becomes supersaturated with respect tocement phase (Tucker et al. 1990). In the present study, twodistinct types of cement are recorded in the limestones, onemicrosparite cement as pore-filling and pore-lining isopa-chous cement characteristics of phreatic zone appearing as arim of crystals of equal thickness around the periphery ofpores (Fig. 7). This cement was mostly early and beforeburial. Inside fossils sparite in the Thebes Formationdecreases the intraparticle porosity. Silica cements are alsorecognized as pore-filling crystals occluding pores of someforaminiferal chambers (intragranular cement). Reddishbrown iron-oxide materials are also observed fillingglobular chambers of foraminiferal tests (Fig. 7f).

Neomorphism The term neomorphism includes all transfor-mation between one mineral and itself or a polymorph(Folk 1965). In the studied carbonates, micrites aresusceptible to diagenetic alterations and occasionallyreplaced by coarser mosaics of microspar throughaggrading neomorphism. Neomorphism was found to beinversely related to intensity of aragonite dissolution. Inthe present study, it is believed to be form meteoric waterdiagenesis. Skeletal grains originally composed of high-Mg calcite, may be preferentially replaced by coarseferroan calcite (Fig. 7b) during late diagenetic stage(Richter and Fuchrbauer 1978). Similarly, siliceous radio-larian shells may also be replaced by coarse ferroan calciteduring late diagenetic processes (Fig. 7c). However,silicification of grains or matrix precludes recrystallizationprocess suggesting that recrystallization is a late diageneticprocess.

Silicification Silicification event is represented here inwhere several types of calcareous grains and matrix weresubjected to partial silicification. Long pelecypods andnummulites are partially or completely silicified (Fig. 7e).Such silica replacement suggests that silicification hasoccurred during late burial diagenesis. Non-selectivesilicification is also recorded in some samples where thetextural elements of the rock (both grains and matrix) wereattacked.

Dolomitization The origin of dolomite has been muchdebated among carbonate sedimentologists (e.g., Pettijohn1975; Sibley 1991; Tucker and Wright 1990; Purser et al.1994) and different models namely, reflux, mixed zone and

deep burial were proposed by different authors. In thepresent study, dolomite present in all the studied samples(Table 1) but with different signatures. A primary precip-itate of very fine-grained varieties (dolomicrite) andsecondary replacement dolomite are recognized. Theformer is sometimes found partially or totally recrystallizedto coarse dolomite (dolosparite). Dolomite replacementoccurs most commonly in two types; selective and non-selective. The second type is almost accompanied by partialdissolution and hence intercrystalline porosity is developed(Fig. 5f). In the studied carbonates, secondary dolomitiza-tion took place after deposition and cementation, and isconfined to the fine sediments indicating marine waterdiagenetic environments. Dolomite seems to have beenprecipitated from the passage of sulfate enriched fluids asindicated by the presence of anhydrite.

Dissolution The dissolution event in carbonate rocksgenerally occurs in response to significant change inchemistry of pore fluids, such as a change in salinity,temperature, or partial pressure of CO2 (Moore 1989). Boththe unstable mineral suite consisting of aragonite andmagnesian calcite are unstable under the influence ofmeteoric waters (James and Choquette 1984). The dissolu-tion (metastable phases) and precipitation (stable phase)were the main processes being more significant forporosity-permeability development. The CaCO3 dissolvedfrom aragonite is generally transported away from the siteof solution. If water flux in the diagenetic system is large,and the water is strongly undersaturated with respect toaragonite, aragonite grains will undergo total dissolution,moldic voids are formed, and all internal structure withinthe aragonitic grains is destroyed. The CaCO3 from thisgrain dissolution may be carried a significant distance downbefore calcite is precipitated as a void-fill (macroscaledissolution). Also, selective destruction and void formationare due to the solubility of the shells. The undersaturationof pore water was responsible for the dissolution producingvuggy and moldic porosity.

In the present study, most of the studied limestone facieswere badly suffered from late dissolution effect as theycharacterized by a large number of intergranular and moldicpores (Fig. 7d). The presence of detrital quartz grains inpore spaces of the limestones suggests meteoric watereffect.

Nannofossil biostratigraphy

Over the last two decades, calcareous nannofossil biostra-tigraphy has shown great improvement through the inten-sive studied on many deep-sea sediments and on-landsedimentary sections (Aubry et al. 1996). The biostrati-

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graphic zonation followed here is based on the zonalschemes proposed by Martini (1971) and Okada and Bukry(1980) for the low latitudes.

Calcareous nannofossils are generally common to fre-quent and well diversified throughout the studied interval,which facilitates detailed quantitative and semi-quantitativeanalyses of the assemblages. The reservation varies frommoderate to good and the dissolution and/or overgrowthseems to have variably affect the assemblages. Quantitativeanalysis has been preformed across the studied interval ofGebel El-Qurn section following the counting techniquedescribed by Jiang and Gartner (1986). The relative speciesabundances were determined by counting a group of about300 specimens along a random traverse.

Semi-quantitative analysis was carried out on all samplesand the relative abundances of calcareous nannofossils wereestimated following the criteria used by Bralower andMutterlose (1995); A=Abundant >10 specimens/field ofview (fov); C=common 9–1 specimens/fov; F=few 9−1specimens/1–9 fov; R=rare 1 specimen/>10 fov. Thespecies richness is given as the total number of speciesrecorded in each sample. The relative abundance of allspecies is given on the distribution chart (Fig. 8) and somerepresentative taxa are shown in Fig. 9.

The present study details the lower Eocene nannofossilbiostratigraphy (Zone NP10 and NP11; Fig. 9) at Gebel El-Qurn, near Luxor, based essentially on the standard zonalschemes of Martini (1971) and Okada and Bukry (1980),and modifications proposed by Aubry (1996; 1999).

Tribrachiatus contortus Zone (NP10) This zone spans theinterval from the First Occurrence (FO) of Tribrachiatusbramlettei to the Last Occurrence (LO) of T. contortus, andis recorded in the Esna Shale (QR 1–QR5). Aubry (1996)subdivided the NP10 Zone into four subzones: NP10a(biostratigraphic interval from FO of T. bramlettei to the FOof Tribrachiatus digitalis); NP 10b (Total range ofTribrachiatus digitalis); NP 10c (from LO of Tribrachiatusdigitalis to the FO of T. contortus) and NP 10d (Total rangeof T. contortus). Unfortunately, none of these subdivisionscan be differentiated in the current study which may beattributed to widely spaced sampling and/or small hiatus.

Discoaster binodosus Zone (NP11) The top of the NP10 isgenerally defined by the LO of T. contortus (Martini 1971;Romein 1979; Perch-Nielsen 1985) but this species was notobserved in the present samples. Alternatively, the FO ofTribrachiatus orthostylus takes place shortly before the LO

Fig. 8 Distribution chart of identified calcareous nannofossils in sediments of Gebel El-Qurn, West Luxor

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of T. contortus and “can be used as an approximation of theNP10/NP11 boundary” (Perch-Nielsen 1985). We havetentatively placed that boundary at the FO of T. orthostylus,thus our Zone NP11 may be a bit more reduced in reality thanit appears. In this context, Sphenolithus radians records itsFO at the base of NP11. In the present section, othernannofossil species that appear at the base of NP11, such asZygrhablithus bijugatus, Discoaster barbadiensis, and Chias-molithus solitus, are rare in the lower Eocene (Zone NP 11).

Many workers have investigated the calcareous nannofos-sils biostratigraphy of the lower Paleogene sequences in Egyptover the last three decades (e.g., Perch-Nielsen et al. 1978;Faris 1984; Aboul Ela 1989; Tantawy et al. 2000; Aubry et al.2002; Dupuis et al. 2003). In several sections in the NileValley (G. Gurnah: Faris and Strougo 1998; Wadi El-Dakhl:Strougo and Faris 1993) and in Farafra (Northern Gunna:Faris and Strougo 1998). It has been shown that T. orthostylussometimes co-occurs with T. contortus at the extreme top partof Zone NP 10 or appears directly above the last occurrenceof the latter, i.e., at the very base of Zone NP 11. T.orthostylus was originally described from the lower Eocenerocks of California and Austria (Bronnimann and Stradner1960). In Egypt, this species predominates the lower Eocenesediments and its stratigraphic value as a guide fossil hasalready been proved (Romein 1979; Tantawy 2006).

Two different types of T. orthostylus were recorded inthe study section, which are characterized by small sizesand thin arms. The change from type A to type B isgradational. Type A is characterized by large size speci-mens and thick arms, whereas Type B is defined by itssmall size and relatively thin arms. However, more detailedstudies are necessary to subdivide the lower Eocenesediments in Egypt using the Rhomboaster and Tribrachia-tus and their related species.

Paleoclimates

Quantitative and semi-quantitative analyses of nannofossilassemblages from deep-sea and continental shelf sediments

in different parts of the world have revealed several bioticevents, a change in species richness, and fluctuation in theabundance of important paleoecological index species.Calcareous nannofossils are known to be sensitive toenvironmental changes in their surface water habitats, andtheir changes in the abundance of fossil assemblages reflecta response to paleobiogeographic perturbations (Tremoladaand Bralower 2004).

The studied nannofossil taxa of the lower Eocenesediments at Gebel El-Qurn are characterized by thepredominance of the warm water species (e.g., Rhamboas-ter spp., Discoaster barbediensis, Discoaster diastypus,Pontosphaera multipora, Z. bijugatus, ThoracosphaeraOperculata, Shenolithus spp., Ericsonia subpertusa, andT. bramlettei).

Summary and conclusions

The lower Eocene succession at Gebel El-Qurn, near Luxor,including the upper Esna Shale and the Thebes Formations,have been investigated from the mineralogical, petrograph-ical and biostratigraphical points of view. Mineralogically,the studied sediments are dominated by carbonate minerals(calcite and dolomite) and phyllosilicates (smectite, illite,kaolinite, sepiolite, palygorskite and smectite–illite-mixedlayer), followed by quartz, anhydrite, hematite, gypsum,and goethite. Calcite and phyllosilicates show oppositebehavior. This may explain the carbonate dilution by theland derived terrigenous materials (Ghandour et al. 2004).The dominance of calcite over other minerals in the bulkfraction is consistent with the high abundance of calcareousfossils.

The clay mineral distributions in the sediments reflectthe climatic conditions and the weathering processes at thesource area as well as the differential hydraulic sortingduring transportation and deposition. Smectite dominatesthe clay mineral assemblages followed by the other clayminerals. The dominance of smectite indicates that thesource area had experienced a warm climate with alternat-ing pronounced dry and less pronounced wet seasons.Kaolinite has resulted from the chemical weathering ofacidic igneous and metamorphic rocks or their detritalweathering products under tropical to subtropical humidclimatic conditions (Chamley 1989).

Illite typically forms in soils with little chemicalweathering in cold/or dry climates, and in areas of highrelief where physical erosion is predominant. However, theprevailing weathering in the arid Egyptian desert issufficient to convert illite into smectite passing throughillite–smectite-mixed layer (El-Kammar et al. 1991). Paly-gorskite and sepiolite clay minerals occur with considerableamount at specific horizons in the Thebes Formation.

Fig. 9 Early Eocene calcareous nannofossils from Gebel El-Qurnsection. Scale bar represents about 5 μm (1, 2) Pontosphaerapulchra, QR 7, Discoaster binodosus Zone (NP11), 3–6 Neo-chiastozygus junctus, QR 7, D. binodosus Zone (NP11), 7–11Zygrhablithus bijugatus, QR 9, D. binodosus Zone (NP11), 12Sphenolithus radians, QR 14, D. binodosus Zone (NP11), 13–15Chiasmolithus solitus, QR 10, D. binodosus Zone (NP11), 16Braarudosphaera bigelowii, QR 10, D. binodosus Zone (NP11),17–20 Tribrachiatus orthostylus, QR 7, D. binodosus Zone(NP11), 21, 22 Discoaster barbadiensis, QR 10, D. binodosusZone (NP11), 23 Tribrachiatus bramlettei, QR 1, T. bramletteiZone (NP10), 24 Discoaster diastypus, QR 10, D. binodosus Zone(NP11)

R

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Sepiolite is almost always associated with palygorskite andsmectite. It is suggested that sepiolite (like palygorskite)might have formed diagenetically through transformation ofsmectite to palygorskite and then of palygorskite tosepiolite (Millot 1970).

Apart from the climatic controls, which affect claymineral formation, the relative abundance of smectitecomparing to illite and kaolinite is clearly influenced byhydraulic sorting and relative sea-level changes. Smectitetends to settle as finer particles in deeper offshore settings,whereas kaolinite and illite tend to concentrate in relativelynear-shore shallow water settings (Raucsik and Merényi2000). Moreover, detrital smectite in marine sedimentsincreases during sea-level highstand periods, whereaskaolinite and mica increase during lowstand periods. Thepresence of abundant smectite in the Esna Shale sedimentsis generally linked to transgressive seas (Tantawy et al.2001).

Petrographic examination of the carbonate rocks withinthe succession enables to identify eight microfacies associ-ations. These are: micrite, dolomicrite, dolosparite, forami-niferal biomicrite, fossiliferous biomicrite, nummuluticbiomicrite, nummulitic biosparite, and pelecypod biomi-crite. These microfacies were controlled by several diage-netic processes including; micritization, compaction,cementation, neomorphism, dissolution, dolomitization,and silicification. Dissolution of original test wall andreplacement or infilling by iron oxides and recrystallizedcalcite were the main diagenetic features.

The lower Eocene sediments of Gebel El-Qurn containabundant and highly diversified planktonic foraminifera andcalcareous nannoplankton, suggesting deposition in openmarine inner to middle shelf environments. Two calcareousnannofossil biozones are identified; Tribrachiatus controtusZone (NP10) and D. binodosus (NP11), suggesting lowerEocene age. Their associated nannoplankton taxa arecharacterized by the predominance of warm water species.The sediment supply, sea-level changes, paleogeography,and carbonate productivity have significant signatures onthe lower Eocene sediments in the study area.

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