Facies and ichnology of an Upper Cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models

Journal of the Geological Society, London, Vol. 169, 2012, pp. 435 –447. doi: 10.1144/0016-76492011-136.

435

Modern and Late Neogene sedimentary deposits influenced by long-living mainly contour-parallel bottom currents, so-called con-tourites, have received much attention in recent years (see Rebesco & Camerlenghi 2008, and references therein). In contrast, the lit-erature contains only few descriptions of their ancient counterparts (Hünecke & Stow 2008). The architecture of recent and sub-recent contourite systems is well known from seismic reflection records, and a wide range of sea-floor topographic phenomena including drifts, moats and channels have been recognized (e.g. Hollister & Heezen 1972; Stow 1982; McCave & Tucholke 1986; Stow & Faugères 1998; Stow et al. 2002; Viana & Rebesco 2007). Many of these geomorphological elements have been cored, allowing analy-sis of lithology, sedimentary structures, and trace and body fossils (e.g. Stow et al. 1986, 1998).

Contourites are mainly formed in relatively deep water (>500 m) and most contourites are fine grained, but lithology, sedimentary structures, and body and trace fossils vary considerably, depending on type of sediment influx, sedimentation processes and rates, current velocity and oxygenation. If the term contour current is considered to be essentially synonymous with bottom current (see Rebesco et al. 2008), then silty–sandy contourites would show normal traction current structures, including parallel and cross-lamination, whereas mud-grade sediments would be characterized by subtle changes in grain size such as small-scale coarsening- or fining-upwards trends and commonly by pervasive bioturbation (for an overview see Hünecke & Stow 2008). These features allow distinction between true event beds such as turbidites and beds subsequently reworked by bottom currents. There is, however, no obvious reason why sediments influenced by bottom currents should contain particular sedimentary signals that would permit establishment of useful contourite facies models. The so-called ‘standard contourite model’ (see Stow & Faugères 2008) seems mainly to be of relevance for relatively coarse-grained siliciclastic contourites and essentially comprises a thin coarsening- or fining-upward succession subdivided into a number of divisions, where

certain divisions may be absent. Very fine-grained muddy contour-ites, such as may be developed in chalk, do not show any clear sedimentological signal and appear mostly homogeneous and are generally highly bioturbated (e.g. Hünecke & Stow 2008), and it is argued here that they do not yet justify erection of depositional models for muddy contourites.

Chalk has generally been considered as deposited by a gentle rain of coccolith debris under quiet sea-floor conditions except for areas of resedimentation mainly adjacent to active inversion zones, but new work has shown that this classical paradigm for chalk dep-osition cannot be upheld. Deposition of the Upper Cretaceous chalk of NW Europe was influenced by mainly contour-parallel bottom currents over long periods of time, reflected by the almost ubiqui-tous presence of all the main seismic-scale features characteristic of contourite systems such as mounded and sheeted drifts, valleys, moats and channels (Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007a,b; Surlyk & Lykke-Andersen 2007; Surlyk et al. 2008; Esmerode & Surlyk 2009; Bjerager et al. 2010; Surlyk et al. 2010a). The influence of bottom currents on the geometry of chalk deposits has, in addition, been demonstrated by Quine & Bosence (1991), Jarvis (1992, 2006), Evans & Hopson (2000), Evans et al. (2003) and Van der Molen (2004). Chalk contourite drift com-plexes are now known from the Baltic Sea, Øresund, the Kattegat Sea, the Danish, German and Dutch North Sea sectors, and the Paris Basin, and seem to be the rule rather than the exception.

Two fully cored scientific boreholes, Stevns-1 and Stevns-2, have been drilled through the upper part of the Chalk Group in the eastern part of the Danish Basin (Fig. 1) (Stemmerik et al. 2006; Schovsbo et al. 2008; this study). The boreholes penetrated up to 443.3 m of upper Campanian−Maastrichtian chalk, which, accord-ing to seismic evidence, was deposited under the influence of rela-tively powerful bottom currents (Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007b).

The aim of the present paper is to describe the main facies, and body and trace fossils of a fully cored contourite chalk succession

Facies and ichnology of an Upper Cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models

SUSANNE LIL RASMUSSEN1,2 & FINN SURLyK1*

1Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark2Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark

*Corresponding author (e-mail: [email protected])

Abstract: The Upper Cretaceous chalk of the Danish Basin has been interpreted as a major contourite com-plex on the basis of high-resolution seismic data. The sea floor had a pronounced topography with kilometre-wide ridges and valleys up to almost 200 m deep, interpreted to have been formed by contour-parallel bottom currents. Only few ancient contourite systems have been recognized, mainly based on sedimentary facies and only rarely on architecture and morphology. Two cored boreholes, 345 and 443.3 m deep through the Danish chalk contourite complex, offer a unique possibility to compare seismic and sedimentary facies. The contourite chalk is completely bioturbated except for thin intraclast conglomerates and a few thin levels, showing possible primary lamination. In terms of lithology and trace fossils the contourite chalk is similar to horizontally bedded pelagic chalk uninfluenced by bottom currents. Published contourite models cannot normally be used for the chalk because of the very fine grain size, generally complete bioturbation, and lack of any vertical trends in grain size on a millimetre to centimetre scale. It is thus only rarely possible to docu-ment the influence of bottom currents on the basis of facies analysis alone and this can be inferred only by architectural analysis of seismic-scale outcrops.

research-articleResearch ArticleXXX10.1144/0016-76492011-136S. L. Rasmussen & F. SurlykChalk Contourite Drift Complex2012

S. L. RASMUSSEN & F. SURLyK436

in the Stevns-1 borehole. This succession was deposited in the east-ern part of the Danish Basin under the influence of long-lived, uni-directional, and apparently contour-parallel bottom currents of varying strength (Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007b; Surlyk & Lykke-Andersen 2007; Surlyk et al. 2010a). It is demonstrated that this contourite chalk is lithologically similar to, and hosts the same body and trace fossil assemblages as horizon-tally bedded pelagic chalk uninfluenced by bottom currents. The validity and relevance of contourite facies models for chalk depos-its are subsequently evaluated in this light.

Material and methods

The present study is based on the cored 443.3 m thick chalk succes-sion from the Stevns-1 borehole drilled close to the coastal cliff of Stevns Klint, eastern Denmark (Figs 1 and 2). This borehole is tied to the Stevns-2 borehole situated 8 km south of Stevns-1 by means of seismic reflection and refraction sections, and several long paral-lel seismic reflection sections have been recorded immediately off-shore (Figs 3 and 4; Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007b). These data allow correlation between sedimentary facies, seismic facies and palaeo-sea-floor topography. Stevns-1 has been logged in detail for lithology, structures, cyclicity, trace fossil

assemblages, degree of bioturbation, body fossils, skeletal debris, pyrite, porcellanite, flint, stylolites and fractures, and the sedimento-logical log is accompanied by a spectral γ-ray log (Fig. 2).

The Stevns-1 core is lithologically rather monotonous in overall appearance but in detail a number of characteristic facies can be recognized (Fig. 2a−d). The low colour contrasts in the dry core of the overall white to slightly greyish chalk do not allow detailed observations, but they were enhanced by adding water or light oil to the clean and flat surface of the split core. In this way trace fos-sils, intraclast conglomerates and other subtle features can be observed and scanned for illustrations.

The Upper Cretaceous contourite complex

The Stevns peninsula is located over the northeastern margin of the Ringkøbing–Fyn High, which limits the Danish Basin to the south, whereas the northern basin margin is formed by the inverted Sorgenfrei–Tornquist Zone (Figs 1 and 5). The contourite chalk in the Stevns area is part of a huge Upper Cretaceous contourite com-plex in the Danish Basin. An example, with a wide and deep moat situated adjacent to the Sorgenfrei–Tornquist Zone, and flanked by a thick, elongate, detached mounded drift several hundred kilome-tres long, is shown in Figure 5. It was interpreted as formed by a

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Fig. 1. Left, map showing the main structural features, and isopachs of the Upper Cretaceous–Danian Chalk Group. Modified after Vejbæk et al. (2003). Study area is indicated by red star. Right, map of the Stevns peninsula, showing location of the coastal cliff, Stevns Klint and of the seismic line shown in Figure 3, and of the two fully cored boreholes Stevns-1 and Stevns-2. Modified from Lykke-Andersen & Surlyk (2004).

CHALK CONTOURITE DRIFT COMPLEX 437

1 Chondrites

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Fig. 2. Sedimentological log of the 456.1 m long core from the Stevns-1 borehole (position indicated in Fig. 1). The log is simplified from a detailed log measured at a scale of 1:10 and shown by the enlarged inset logs to the right showing typical examples of characteristic facies developments. Lithostratigraphy, biostratigraphy, trace fossil assemblages and spectral γ-ray log are indicated. The lithostratigraphic boundaries broadly coincide with marked changes in seismic velocities as recorded by seismic refraction data of Nielsen et al. (2011) and with changes in water temperature as recorded by stable isotope and coccolith data (Thibault et al. 2011a,b). The detailed log in (a) is characterized by intraformational conglomerates, in (b) by alternating beds of marly chalk and pure chalk, in (c) by skeletal laminae, and in (d) by almost pure chalk with flint nodules.


bottom current flowing parallel to the fault zone towards the NW. It is close to identical to numerous modern moat–drift systems such as the mounded and separated Faro–Albufeira drift and the Alvarez Cabral moat (Stow et al. 2002, and references therein; see also Hernández-Molina et al. 2010a,b).

Seismic reflection sections up to 50 km long collected immedi-ately offshore and parallel to the coastal cliff (Stevns Klint) show that the Base Chalk reflection is flat, undisturbed and dips about 0.6° towards the north (Fig. 3; Lykke-Andersen & Surlyk 2004). The Upper Cretaceous Chalk Group was subdivided into six seis-mic sequences (1–6, from the base) on the basis of reflection ter-minations, seismic facies, and recognition of particularly prominent reflections (Lykke-Andersen & Surlyk 2004). The Stevns-1 borehole penetrated sequences 6, 5, 4 and 3, and reached down to a depth just below the sequence 3–2 boundary (Figs 3 and 4).

A marked sea-floor topography rapidly developed after the onset of chalk deposition in sequence 1, and the top of the sequence shows a relief of 30–40 m, comprising non-erosional channels and intervening ridges, especially in the southern part of the seismic section (Fig. 3). This relief became exaggerated during deposition of sequence 2 with marked internal mound-like features, showing pronounced southwards onlap onto each other. The top of the sequence has a relief up to 80 m to the south, where a major depo-sitional valley system was formed, bounded by ridges and about 70 m deep. The northern channel-bounding ridge developed a marked convex-upwards shape and attained a width of about 20 km during deposition of sequence 3. Stevns-1 was placed over the crest of this ridge (Fig. 3). Contemporaneously, an up to 30 km wide val-ley was formed north of the ridge. The part of sequence 3 pene-trated by Stevns-1 shows a wavy internal reflection pattern, but erosional features are not identified (Figs 3 and 4). Sequence 4 is thin and has almost wedged out at the Stevns-1 site (Fig. 3). The succession comprising sequences 1–4 was gently tilted towards the north at this time, as indicated by the wedge-like shape of sequence 5. The tops of sequences 5 and 6 are essentially horizontal but still show a pronounced superimposed sea-floor topography.

The large-scale sea-floor topography, formed at the end of depo-sition of sequence 3 with two major valleys separated by ridges, persisted into the Danian. This is outlined by the wavy topography of the K–T boundary interval topping sequence 6, which has a relief of about 40 m in Stevns Klint (Fig. 3). The irregular, wavy nature on a number of scales, in this and other seismic sections from the Danish Basin, have been interpreted as the result of dif-ferential deposition and erosion by contour-parallel bottom cur-rents (Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007b; Surlyk & Lykke-Andersen 2007; Bjerager et al. 2010; Surlyk et al. 2010a). The contourite chalk offshore Stevns Klint forms part of a huge drift complex, comprising several related large drifts and cov-ering an area of at least 30000 km2 and a minimum volume of 2 × 10 km3 (F. Surlyk & H. Lykke-Andersen, unpublished data).

Stratigraphy

Stevns-1 reached a depth of 456.1 m. In the following all depths refer to metres below core tops. The interval from the base at 456.1 m to 325 m is upper Campanian, and the Campanian–Maastrichtian boundary interval occurs at 325−319 m (Thibault et al. 2011a,b). The rest of the core up to 12.8 m is Maastrichtian, and the lower−upper Maastrichtian boundary is situated at about 170 m. The interval above 12.8 m comprises lower Danian bryo-zoan limestone of the Stevns Klint Formation (Surlyk et al. 2006) topped by about 1 m of Quaternary deposits (Fig. 2).

The Upper Cretaceous chalk of the Danish Basin has not been lithostratigraphically subdivided into formations and members except for the uppermost part. The Maastrichtian part was tenta-tively referred to the Tor Formation of the North Sea but it was noted that a new formation probably had to be erected for this part of the succession based on the study of cored boreholes (Surlyk et al. 2006).

The upper Campanian–Maastrichtian part of the cored succes-sion is subdivided (from below) into three informal members (1−3) and the upper part belongs to the Rørdal Member of Surlyk et al. (2010b), overlain by the Sigerslev and Højerup Members of Surlyk

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Fig. 3. North–south-oriented seismic profile about 50 km long, recorded about 1 km offshore east of the coastal cliff of Stevns Klint. The Base Chalk reflection (red) is flat, undeformed and dips 0.6° towards the north. The Chalk Group overlying this reflection is up to about 1 km thick and is subdivided into six seismic sequences (1–6). All units show irregular and wavy reflection patterns, representing broad valleys and ridges, smaller channels and drifts. A large-scale system comprising two wide valleys (marked by V) separated by equally wide ridges (marked by R) started to develop during deposition of sequence 2, reached a maximum development at the end of deposition of sequence 3, and persisted into the Danian as seen by the marked relief of the K–T boundary exposed in Stevns Klint. New processing of a migrated version of the section presented by Lykke-Andersen & Surlyk (2004) with new depth conversion based on Nielsen et al. (2011).


et al. (2006) (Fig. 2). The overlying Danian bryozoan limestone belongs to the Stevns Klint Formation of Surlyk et al. (2006) and is not dealt with further here.

Sedimentology

Chalk

Pure chalk occurs, both as units up to 20 m thick and as decime-tre- to metre-thick beds, alternating with marly chalk beds. The chalk is completely burrow mottled. Well-defined trace fossils occur in varying intensity and are easiest to observe in marly chalk facies and at chalk–marl transitions. The ichnofauna includes Thalassinoides, Planolites, Zoophycos, Chondrites, and rare Taenidium and Phycosiphon (Fig. 2). Body fossils are relatively scarce, but inoceramid bivalve fragments are common at certain levels, and fragments of echinoids, asteroids, other bivalves and sponges occur in places. Bryozoans are particularly common in the lower Sigerslev Member and the Højerup Member. Fractures and horizontal stylolites and are common throughout members 1 and 3.

Marly chalk

Marly chalk occurs as decimetre- to metre-thick beds, alternating with chalk beds, and is most prominently displayed in the Rørdal

Member, whereas the facies is rare in the Sigerslev and Højerup Members. The marly chalk is only slightly greyer than the pure chalk and is difficult to recognize in a dry core. The boundaries between chalk and marl beds are mainly gradational, but sharp boundaries also occur. A faint primary lamination with millimetre-thick laminae outlined by different shades of grey occurs at a few levels. The lamination is similar to that described from the Maastrichtian chalk of the North Sea Central Graben (Damholt & Surlyk 2004). The fine laminae were interpreted as deposited from dilute small-volume turbidity flows and suspension clouds, alter-nating with deposition directly from pelagic rain of coccolith debris in periods with low oxygenation at the sea floor. This interpretation is followed here but a detailed study of the laminae has not been undertaken for the marly chalk of the Stevns-1 core because of the rarity of the structure. The marly chalk beds contain the same body fossils as the chalk.

Marl

This facies comprises distinct and mainly well-defined dark grey millimetre- to centimetre-thick laminae and beds occurring preferen-tially in the middle part of the marly chalk beds in an apparently cyclic manner, especially in members 1 and 2. They are rare in the overlying members. Most layers are horizontal and they typically have sharp, but in some cases irregular boundaries, although both top

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Fig. 4. (a) Detail of the seismic section shown in Figure 3 with Stevns-1 projected onto the section, which is located about 3 km to the east of the borehole. (b) Interpreted version of (a). The whole succession dips slightly to the west and the bottom of the borehole, which is located a few kilometres west of the seismic line is thus stratigraphically somewhat higher than shown in the figure. Examination of the part of the succession penetrated by Stevns-1 in this vertically strongly exaggerated version highlights the wavy and mound-like nature of the reflections around the borehole. The brown reflection at the base of the core thus has a relief of about 60 m, whereas the yellow and green reflections have a relief of about 40 m. In sequence 5 there is a major valley cutting down with at least 60 m to the south below Stevns-2 (Fig. 3). The part of sequence 5 immediately adjacent to the Stevns-1 shows a pronounced small-scale mound-like reflection pattern with amplitudes of about 20 m superimposed on the large-scale structures best seen in Figure 2.


and bottom of the layers may be gradational (Fig. 6). The sharp nature of most boundaries was probably enhanced by dissolution at the boundary between marl and chalk (Fig. 7). Some marl layers con-tain thin, flat lenses of white chalk, which are likewise interpreted as reflecting pressure solution at marl−chalk boundaries and remnants of burrows. A few marl layers show parallel lamination, interpreted as primary in origin (Fig. 5) and interpreted as deposited in periods with low oxygenation at the sea floor by the same processes as the laminated North Sea chalk described by Damholt & Surlyk (2004).

Conglomerate

Beds of chalk intraclast conglomerates up to 1 m thick occur at many levels but are particularly common in member 2 (Figs 2, 8 and 9). The clasts are millimetres to a few centimetres in diameter and are subrounded to angular. Most beds have transitional bound-aries, and upwards transitions from fractured chalk to brecciated chalk, to matrix-supported conglomerate and back again to brecci-ated and then fractured chalk at the upper boundary are characteris-tic (Figs 8 and 9). This type of conglomerate is not of primary depositional nature but may reflect hydrofracturing, brecciation and subsequent matrix fluidization under burial. This subfacies is at present undergoing special study. The boundaries of a few very thin beds are sharp and these conglomerates have a debrite-like appearance but they were probably formed in the same way as the rest of the conglomerates.

Skeletal laminae

Mainly millimetre-thick laminae of fragmented shells, notably bryozoans, bivalves and relatively large benthic foraminifers, are common throughout the core, in particular associated with the uppermost conglomerates of member 2 and with laminated marls of member 3 in the 225–199 m interval (Figs 2 and 10). Laminae may be shorter or longer than the core diameter. The short laminae, which appear as thin, highly elongate lenses, have somewhat dif-fuse outlines and may be slightly inclined with respect to overall bedding. They are interpreted as representing concentrations in small burrow fills, a feature that is relatively common in chalk (Kennedy 1967, 1970; Ekdale & Bromley 1983; Lauridsen et al. 2011). The long laminae commonly have sharp lower and upper boundaries, and may also represent burrow concentrations but it cannot be excluded that they represent thin and very fine-grained lags after current winnowing.

Ichnology

Virtually all facies types in the Stevns-1 core are completely burrow mottled (Figs 2, 11, 12, 13 and 14), but exceptions are rep-resented by the rare laminated marl beds and most of the conglom-erates. Four well-defined ichnogenera are common: Thalassinoides, Planolites, Zoophycos and Chondrites, whereas Phycosiphon and Taenidium are relatively rare. It is difficult to differentiate between

Danish Basin Sorgenfrei-Tornquist Zone5 km

SW NE

Fig. 5. Moat and elongate detached mounded drift in Maastrichtian chalk in the Danish Basin. Alternation between sheet-like and wedging drifts with clinoforms facing the moat. After Surlyk & Lykke-Andersen (2007).


Thalassinoides and Planolites in the core, and poorly defined bur-rows that may represent Planolites are therefore grouped with Thalassinoides. The trace fossils occur in eight recurrent assem-blages, comprising one to four ichnogenera: 1, Chondrites alone; 2, Zoophycos alone; 3, Thalassinoides alone; 4, Chondrites and Zoophycos; 5, Chondrites and Thalassinoides; 6, Zoophycos and Thalassinoides; 7, Chondrites, Zoophycos and Thalassinoides; 8, Chondrites, Zoophycos, Thalassinoides and Taenidium.

The distribution of all identified trace fossils is indicated on the measured log and the vertical extents of the eight assemblages are recorded (Figs 2 and 15). Intervals assigned to a specific trace fossil assemblage may include thin units where one of the diagnostic ich-nogenera is missing. The cumulative thickness of units containing assemblages 1–8, respectively, is measured and plotted as percent-

ages of the total core length in a histogram (Fig. 15). Assemblage 1 is the least common, as it occurs in less than 2 vol.% of the core. Assemblage 8 has the highest diversity, but occurs in only about 2%

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Fig. 6. (a) Faintly laminated marl seam, 2.5 cm thick with sharp base and top, possibly enhanced by dissolution at the chalk–marl–chalk boundaries. The hole at the chalk–marl boundary is from a belemnite guard that has been removed; the hole to the right is due to sampling. The thicker internal dark laminae, probably representing Zoophycos burrows (Z), and the underlying primary laminae are bent over and drape the belemnite, indicating that the marl seam was compacted after burrowing, and that the seam does not represent a residuum after dissolution. Sub-millimetre thick chalk lenses in the top part of the seam, especially to the upper right, were formed by small-scale dissolution at the marl–chalk boundary and may represent remains of Chondrites burrow fills. The curved dark line to the right of the belemnite hole is a shell fragment. The chalk is completely bioturbated, and the chalk above the marl seam shows indistinct Thalassinoides burrows with small Chondrites in the burrow fills. Visibility was enhanced by oil staining. Member 2, uppermost Campanian, 311 m (Fig. 2). (b) Interpreted version of (a).

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Fig. 7. (a) Top of faintly laminated marl seam, showing evidence for dissolution in the form of so-called flaser structures. (b) Interpretation of (a). The fill of the Thalassinoides (Th) burrow to the left is truncated at the chalk–marl boundary and contains Chondrites (C). Concentrations of sand-grade fossil debris in lenses (arrows) are interpreted as burrow fills. Visibility was enhanced by oil staining. Member 1, upper Campanian, 419.2 m (Fig. 2).

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Fig. 8. (a) Intraformational conglomerate; total thickness is 50 cm but only the lower part is shown here. The top part is shown in Figure 9. (b) Interpretation of (a). Slightly fractured chalk passes upwards into chalk pieces separated by broader fractures and further upwards into chalk clasts, floating in a non-bioturbated matrix. Visibility was enhanced by oil staining. Member 2, lowermost Maastrichtian, 317.95 m (Fig. 2). The two horizontal furrows in (a) and (b) are marks after sampling.


of the core; it is closely followed by assemblage 3 with an only marginally higher abundance of 2.2%. Assemblage 6 is represented in about 8% of the core, and assemblages 2 and 4 occur in about 12% and 15%, respectively. Assemblages 5 and 7 dominate and are represented by about 28% and 30%, respectively. The variations in trace fossil assemblage are interpreted as reflecting changes in oxy-genation at the sea floor, nutrient influx, changes in bottom current velocity and water temperature. The cored succession thus shows marked variations in relatively cooler and warmer water tempera-tures and nutrient influx interpreted on the basis of nannofossil con-tent, δ18O and δ13C (Thibault et al. 2011a,b). There is no clear correlation between trace fossil assemblages and water temperature, but it is remarkable that the boundaries of the independently defined lithostratigraphic units correlate with changes between warm and cool periods. Members 1 and 2 thus correspond to a warm period with maximum warming just above the member boundary. The lower part of member 3 corresponds to a cool period and the upper

part to a warm period. The Rørdal Member and the bulk of the Sigerslev Member correspond to a cool period, and the top Sigerslev Member and the Højerup Members to a warm interval.

The common trace fossils of the Chalk Group show a pro-nounced tiering in the sense of Bromley & Ekdale (1986) and Lauridsen et al. (2011), comprising an uppermost tier with back-ground mottling, an upper tier with Thalassinoides, a middle tier with Taenidium, Planolites and Phycosiphon, and a lower tier with Zoophycos and Chondrites. The exclusive presence of Chondrites has been proposed as an indicator of dysoxic condi-tions in the upper sediment layer (Bromley & Ekdale 1984). The rare assemblage 1 (Chondrites only) may thus reflect relatively poor oxygenation below the sediment–water interface at the time of burrowing by the Chondrites-producing animal. The two most prominent assemblages, 5 and 7, are dominated by Thalassinoides and are similar to the main assemblages known from onshore

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Fig. 9. (a) Top part of the conglomerate shown in Figure 7. CT scan. (b) Interpretation of (a). Chalk clasts floating in a non-bioturbated matrix pass upwards into chalk pieces separated by broader fractures and further upwards into slightly fractured chalk. The clasts are subrounded to subangular but rather angular in detail. Visibility and colour difference between matrix and clasts were enhanced by oil staining. Member 2, lowermost Maastrichtian, 317.6 m (Fig. 2).

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Fig. 10. (a) Laminae of skeletal debris (arrows in (b), which shows an interpretation of (a)), up to a few millimetres thick, interpreted to have been concentrated in thin, elongate burrows set in a background of totally bioturbated chalk with a few indistinct cross-sections of cylindrical or oval burrows. Two thin stylolite seams occur at the top of core piece. Visibility was enhanced by oil staining. Member 2, uppermost Campanian, 325.55 m (Fig. 2).


outcrops of chalk (Ekdale & Bromley 1983, 1991). The composi-tion of the trace fossil assemblages may, however, also broadly reflect water depth. Thalassinoides is the dominant trace fossil in relatively shallow water chalk with high densities of shells of minute suspension feeders, and showing omission surfaces, hardgrounds, and bands of nodular flint. The presence of Zoophycos and the absence of Thalassinoides characterize deeper water chalk poor in benthic shelly fossils, lacking omission sur-faces, and with only few flint nodules (Ekdale & Bromley 1984). This is probably the case in the upper part of the upper Maastrichtian Sigerslev Member, which is dominated by assem-blage 2 (Zoophycos only), has a very low content of benthic shelly fossils, and displays a monotonous pure chalk lithology

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Fig. 12. (a) Branched Thalassinoides burrow of the middle tier with small and large deep-tier Chondrites, having reburrowed the fill. (b) Interpretation of (a). Background is completely bioturbated. Visibility was enhanced by oil staining. Member 3, upper Maastrichtian, 116 m (Fig. 2).

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Fig. 13. (a) Zoophycos (Zoo) and Chondrites (Ch) of the deepest tier are well defined and superimposed on a totally bioturbated background. (b) Interpretation of (a). Vertical shaft of Zoophycos is marked by an arrow. (Note minor oblique fractures, dipping towards the right.) Phycosiphon (Ph) also occurs. Visibility was enhanced by oil staining. Member 3, lower Maastrichtian, 283.6 m (Fig. 2).

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1 cm

B

Fig. 11. (a) Two Thalassinoides burrows of the middle tier with small and large deep tier Chondrites, having reburrowed the fill. (b) Interpretation of (a). Background is completely bioturbated. Visibility was enhanced by oil staining. Basal Sigerslev Member, upper Maastrichtian, 69.1 m (Fig. 2).


(Surlyk et al. 2006; Anderskouv et al. 2007; this study). The marl-rich levels, showing high values on the γ-ray log, are in gen-eral characterized by the most diverse trace fossil assemblages (member 2 and the Rørdal Member). The visibility of trace fos-sils in chalk is enhanced by lithological colour contrasts, notably in chalk–marl cycles, and they are more difficult to observe in pure chalk. Differences in trace fossil diversity between pure chalk and chalk–marl cycles may thus represent a kind of tapho-nomic effect, and not the true diversities (Lauridsen et al. 2011). However, the marl-rich levels are also richer in fossils of minute, mainly suspension feeding invertebrate fossils, so the higher trace fossil diversity is considered real.

Discussion

The seismic reflection data show that the sea floor in the Stevns Klint area had a pronounced relief during deposition of the Chalk Group (Figs 3 and 4) and was clearly deposited under the influence of bottom currents, flowing roughly parallel to the NW–SE-oriented bathymetric contours and to the axis of the Danish Basin (Lykke-Andersen & Surlyk 2004; Esmerode et al. 2007b; Surlyk & Lykke-Andersen 2007). There are marked lateral differences in thicknesses of seismic stratigraphic sequences and units seen in sections running roughly at a right angle to the orientation of the basin axis. However, there are only a few levels characterized by erosion and most of the thickness differences are interpreted as due to varying sedimentation rates governed by different bottom cur-rent velocities. This is shown not only by the seismic data (Figs 3 and 4) but also by correlation between the Stevns-1 and Stevns-2 cores (Figs 1 and 3).

The presence of a pronounced chalk–marl cyclicity throughout members 1, 2 and 3 also indicates that the influence of bottom cur-rents only rarely led to erosion or reworking of the sea floor. Hardgrounds are notably absent in the cores. The contourite chalk is lithologically similar to and hosts the same body and trace fossil assemblages as horizontally bedded pelagic chalk uninfluenced by bottom currents (Ekdale & Bromley 1983, 1984, 1991; Bromley & Ekdale 1986; Lauridsen et al. 2011). The only possible evidence in the core for very minor current winnowing is the presence of the long, thin sharp-bounded laminae of shell fragments or enriched in foraminifers, but they may, like the short diffuse laminae, represent concentrations of coarse particles in burrow fills.

Extensive bioturbation dominates most modern contourites and cross-lamination is only rarely described (Stow & Faugères 2008; Wetzel et al. 2008). The chalk is far too fine grained for cross-lamination to be developed and the Stevns-1 core is characterized by pervasive bioturbation except for a few thin intervals. The thickness and extent of the contourite chalks of the Baltic Sea, North Sea, and Paris Basin indicate that the bottom currents were long-lived and persistent in some areas for millions of years. The currents appear to have flowed roughly parallel to contours and to have been unidirectional subsurface currents that were in con-tact with a sidewall, such as a faulted inversion zone similar to conditions today (Zenk 2008).

It is difficult to estimate the palaeocurrent velocities of the bot-tom currents influencing chalk deposition and very little experi-mental work has been undertaken on the threshold values for resuspension and erosion of calcareous ooze (Southard et al. 1971; Black et al. 2003). Suspension of the finest sediment grains would be maintained at current velocities of about 5–9 cm s−1 measured 20 m above the sea bed. Significant erosion does not commence before the current velocity reaches 23–27 cm s−1 measured 20 m above the sea bed (Anderskouv et al. 2007). The actual velocities of the bottom currents influencing the chalk contourites were prob-ably higher. The chalk ooze contained large amounts of siliceous sponge spicules, which stabilized the sea floor, and in addition the currents were able to erode tens of metres down into the chalk. The seismic reflection data (Fig. 3) thus show an example of erosional downcutting of about 70 m downwards from a depth of about 600 m below the Stevns-1 borehole (Lykke-Andersen & Surlyk 2004). Erosional channel margins have also been described on the basis of seismic data from the Paris Basin and the German North Sea (Surlyk et al. 2008; Esmerode & Surlyk 2009) and from outcrops on the Normandy coast (Quine & Bosence 1991). This means that the strongest bottom currents were able to erode not only chalk ooze but also consolidated chalk. It should be noted, however, that most of the sea-floor topographies in the Chalk Group are of a

A B

1 cm 1 cm

Fig. 14. (a) Subhorizontal parts of deep-tier Zoophycos burrows on a background of completely bioturbated chalk. (b) Interpretation of (a). Visibility was enhanced by oil staining. Sigerslev Member, upper Maastrichtian, 17.1 m (Fig. 2).


depositional nature. In most cases the sea-floor relief thus reflects differences in sedimentation rates, and the current velocities were much lower or currents were even absent over the high-relief areas.

The desire to establish a general contourite model is understand-able but seems to us to represent a step backward. A contourite facies model was introduced by Gonthier et al. (1984), comprising a basal inversely graded unit overlain by a normally graded unit (Stow et al. 1998; Stow & Faugères 2008); bioturbation was also suggested to be a diagnostic feature. Those workers also introduced five divisions, C1–5, for their facies model, which was claimed to be analogous to the Ta–c divisions of the turbidite Bouma sequence, but they noted that contourite sequences are not products of single events but may represent long time spans. Even so, the analogy appears unjustified, as the Bouma sequence represents deposition by a single decelerating turbidity flow event. In contrast, there is no inherent reason why sedimentary structures and lithologies in con-tourite deposits should occur in a predictable vertical sequence. The general features of bottom current reworked deposits were listed by Shanmugam (2008) based on a literature search. The list includes only features characterizing fine-grained sand, and silt and mud were included only as a background facies or as drapes and mud-offshoots in ripples. Several of the criteria are contradictory and Shanmugam (2008) stated that none of the criteria are by them-selves unique to bottom current reworked sands but that associa-tions of several of the criteria together with knowledge of depositional setting would strengthen a contourite interpretation.

A wide range of different facies has been observed in contourites (Stow & Faugères 2008). This is mainly a reflection of the nature of the sediment sources and in itself says nothing about the pro-cesses active during transport and deposition. Contourite systems are essentially identified on the basis of seismic data and many con-tourite drifts have subsequently been cored to study the facies and interpret processes active during transport and deposition. A major goal has clearly been to identify particular facies, structures or ver-tical trends characteristic or even diagnostic of contourites. This would in particular be of great interest in studies of outcrops that are normally too small to allow identification of contourite archi-tectures and morphologies.

According to Stow & Faugères (2008, and references therein) an ideal contourite sequence comprises a coarsening-upward succes-sion from muddy to silty to sandy sediment and back again through silty to muddy sediment. Well-defined sedimentary structures are

said to be rare, mainly as a result of bioturbation, and partial sequences are as common as or more common than complete sequences. Rebesco et al. (2008) emphasized that contourite facies are in general difficult to recognize because of the lack of unam-biguous diagnostic criteria, but two main criteria have been put forward: presence of traction structures and pervasive bioturba-tion. Of course, neither of these features is indicative of contour currents. The presence of traction structures only shows the pres-ence of currents strong enough to move silty and sandy sediment, whereas strong bioturbation is characteristic of most fine-grained sediments deposited slowly under oxygenated conditions. Currents were slow to absent over sea-floor highs, to allow deposition, and this may cause reduction in oxygenation, resulting in reduced bio-turbation (Lucchi & Rebesco 2007).

Muddy contourites are labelled C1 for the lower muddy division and C5 for the upper muddy division in the so-called ‘standard con-tourite facies model’ of Stow et al. (2002) and Stow & Faugères (2008, fig. 13.10). The C1−5 cycles were interpreted as caused by long-term fluctuations in current velocity or variations in sediment supply, and as having durations within the Milankovitch frequency band (Stow & Faugères 2008).

The chalk was deposited as aggregates such as faecal pellets or ‘marine snow’, comprising loose aggregates bound together by mucous or gelatinous structures made by zooplankton. At the time of deposition there was a significant component of opaline silica mainly represented by sponge spicules and probably also of fine aragonitic skeletal debris. Both of these components were dis-solved on the sea floor or during shallow burial. The rest of the chalk is composed mainly of coccolithic debris in the 1−5 µm range and, in addition, foraminifers and calcispheres at some levels. The sediment is thus far too fine grained for traction structures to be developed. Chalk is in general completely bioturbated and well-defined trace fossils belong to the middle and deep tiers. This is also the case for the chalk in the Stevns-1 core and primary sedi-mentary structures are absent except for a few thin laminated lev-els. The contourite chalk of Stevns-1 is thus similar to pelagic chalk in terms of lithology, structures, trace fossils and general bioturba-tion. It contains little if any direct sedimentological or trace fossil evidence for bottom current influence on deposition in spite of the marked sea-floor topography and significant thickness variations over short distances, which clearly reflect the action of bottom cur-rents. The interpretation of the chalk described here as a contourite

Trace fossil assemblages

0

5

10

15

20

25

30

35

31 2 4 5 6 7 8

Thalassinoides

Chondrites

Chondrites Zoophycos

Zoophycos

Chondrites ThalassinoidesZoophycos ThalassinoidesChondrites Zoophycos ThalassinoidesTaenidium Chondrites Zoophycos Thalassinoides

Fig. 15. Histogram showing the vol.% of the core occupied by each of the eight trace fossil assemblages recognized in the Stevns-1 core.


system is thus solely based on seismic reflection data, not on sedi-mentology, and contourite facies models are inapplicable.

Conclusions

(1) The 443.3 m thick upper Campanian–Maastrichtian chalk succession in the fully cored Stevns-1 borehole was deposited under the influence of bottom currents as revealed by seismic sec-tions, showing irregular wavy, mound- and channel-like features on a number of scales, and by marked thickness variations over short distances.

(2) The core is composed of chalk with varying contents of clay, mainly as thin marl layers and in more diffuse alternating thicker chalk–marly chalk cycles.

(3) The sediment is completely bioturbated except for intraclast conglomerates and a few thin levels showing possible primary lamination.

(4) The pervasive background burrow mottling is superimposed by well-defined trace fossils of the deeper tiers and eight trace fos-sil assemblages are recognized characterized by one to four ichno-genera.

(5) The different trace fossil assemblages are interpreted to reflect variations in oxygenation at the sea floor, water temperature, nutrient influx, bottom current velocity and water depth.

(6) Millimetre-thick laminae of small fragmented fossils, nota-bly bryozoans, bivalves, crinoids and benthic foraminifers, are common and are mainly interpreted as concentrations of fossil debris in the fills of small burrows.

(7) The only possible evidence for very minor current winnow-ing is long, thin, sharp-bounded laminae of fossil debris, but these may equally represent burrow fills.

(8) The contourite chalk of Stevns-1 is lithologically similar to and hosts the same body and trace fossil assemblages as horizon-tally bedded pelagic chalk uninfluenced by bottom currents.

(9) Published contourite models involving small-scale coarsen-ing- to fining-upwards units essentially reflect only minor changes in bottom current velocity and are not diagnostic of the deposits of contour currents.

(10) The muddy end-members of such models cannot normally be used for the chalk owing to the very fine grain size, limited grain-size distribution, generally complete bioturbation, and lack of any cyclicity on a millimetre to centimetre scale.

This study was funded by the Danish Natural Science Research Foundation. We thank N. Thibault for discussion and information on biostratigraphy, water temperatures and palaeoecology; K. Anderskouv and T. Nielsen for constructive reading of an early version of the paper; and journal review-ers J. Ineson and M. Rebesco for useful suggestions and comments, which improved the text.

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Received 7 November 2011; revised typescript accepted 27 March 2012.Scientific editing by John Marshall.

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Facies and ichnology of an Upper Cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models