The Molluscan Fauna of the Eocene Lillebælt Clay, Denmark
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Research, December Cainozoic 8(1-2), pp. 41-99, 2011 The molluscan fauna of the Eocene Lillebælt Clay, Denmark Kai+Ingemann Schnetler¹ &Claus Heilmann-Clausen² 1 Fuglebakken 14, Stevnstrup, DK-8870Langd, Denmark; [email protected] 2 Department ofGeoscience, Aarhus University, DK-8000 Aarhus C., Denmark; [email protected] Received 18 October 2011; revised version accepted 30 October 2011 For the first time the molluscan faunaofthe latestYpresian-early Lutetian LillebæltClay Formationof Denmark is treated monographi- cally. The assemblage yielded 75 species. Four new species and one new subspecies are introduced, viz. Astartefiligera treldensis n. subsp., Abra madseni n. sp., Thracia barsoei n. sp., Galeodeaanderseni n. sp. and Desorinassa tonneseni n. sp. The molluscan assem- blage contains species known from different stratigraphic levels of the British Eocene, where the Lillebaslt Clay correlates with the BrackleshamGroup, basedon micropalaeontologicaland magnetostratigraphical data. The molluscan fauna suggests water depths during deposition to have ranged from lower sublittoral to upper bathyal, probably 100-300 m. KEY-WORDS:: Mollusca, taxonomy, biostratigraphy, palaeontology, palaeoecology, LillebæltClay, Eocene, Denmark, North Sea Basin. Introduction In spite of the rarity of macrofossils, intensive collecting since the 1960s has resulted in much material of mainly Clays and marls were deposited during the Ypresian and washed out specimens, including molluscs. Bonde (1968) Lutetian in most or all ofthe present Danish land area. At gave an account of the material known at that time. The least in some places deposition ofsimilar clays continued bulk of it was collected on the SE coast of Trelde Naes by into the Late Eocene. The area was covered by the North the junior author, then a high-school student, and most of Sea, andthe environmentwas offshore with deep, probably his material has been included in the present study. The upper bathyal waters. The clays are of an unusual, ex- fauna is preserved as impressions in sideritic/phosphatic tremely fine-grained lithology and represent a hemipelagic concretions, or as pyrite moulds.However, manypyritized sediment type. They have been known since the 19th cen- specimens have been destroyed by disintegration during ‘ tury as the ‘Plastic Clay’ (locally also as valkeler fuller’s the years. Many macrofossils were also collected in these The in coastal cliffs the earth). clays are exposed along years by the late Henning Lange (Fredericia). Since the of strait shores northernLillebaelt, the between Jutland and 1990s a large number of specimens have been collected exceed in Funen (Fig. 1). They 100 m thickness, and are washed out on the SE coast ofTrelde Naes, mainly in side- now formally named, in ascending order, the Rosnaes Clay ritic/phosphatic concretions, especially by Mogens Mad- Lillebaelt Formation and Sovind Marl and the late Formation, Clay sen, Sten Bo Andersen, H. C. Hansen Soren Formation(Heilmann-Clausen etal, 1985). The thickest of Peter Andersen (all from Fredericia), Ole Barsoe Hansen these formations is the Lillebaelt Clay, which form nearly (Kolding) and Susan Schou Sorensen (Valby). all ofthe outcrops in the northern Lillebaelt area. Thus, the large material from Trelde Naes now available The Plastic Clay is very poor in macrofossils, which is why has justified a monographic treatment of the molluscan its age was not ascertained for many years. Molluscs are fauna for taxonomic reasons. Furthermore, a study of the the shell generally poorly preserved, as original aragonitic assemblage has yielded important information on the material in all has dissolved. nearly cases been Internal palaeoecology and has formed the basis for comparisons moulds and impressions are found in sideritic/phosphatic with other Eocene faunas. concretions, so in many cases casts of silicone latex must be made. Internal moulds may also be preserved as pyrite, Abbreviations sometimes with the original shell material preserved or replaced by pyrite. The pyritized molluscs can be found on MGUH Geological Museum, type collection, Copen- the foreshore together with pyrite-stems of crinoids. Such hagen, Denmark. specimens are difficultto store in collections, due to pyrite GM Geological Museum, Copenhagen, Denmark. disintegration. In the clays rare calcitic bivalves may be NHMUK- G The Natural History Museum,Department of found with parts ofthe shell preserved. Palaeontology, London, UK. -42- CHC Claus Heilmann-Clausen collection, Aarhus, mark. Denmark. SPAF the late Soren Peter Andersen collection, Frede- ISL Kai Ingemann Schnetlercollection, Langa, Den- ricia, Denmark. mark. GPF Geologisk-Palaeontologisk Museum collection, OBK Ole Barsoe Hansen collection, Kolding, Den- Fredericia, Denmark. mark. SSV Susan Schou Sorensen collection, Valby, Den- JTK Jesper and Thomas Hansen collection, Kolding, mark. Denmark. KPJ Kolja Petersen collection, Jelling, Denmark. MMF Michael Tonnesen collection, Fredericia, Den- JFR Jytte Frederiksen collection, Risskov, Denmark. Figure 1. The Trelde Nass peninsula with localities mentioned in the text and in Fig. 2. The position ofthe LB 38 borehole (GEUS file is also shown. inserted no. 134.518) The map of Denmark and northern Germany shows the position ofTrelde Nass (arrow). In black: Eocene subcrop (below Quaternary). -43- Figure 2. Logged Eocene sections on Trelde Nass and in the LB 38 borehole. All sections drawn to scale. Lithostratigraphy ofLB 38 after Dinesen (1965) and Heihnann-Clausen et al. (1985). Kirstmebjerg East and Vesterskov after Heilmann-Clausen (1978). Kirstinebjerg Southafter Heilmann-Clausen etal. (1985); this section was not studied for dinocysts. Calcareous nannofossil dataafter E. Thomsen, Aarhus University (pers. comm. 2011). The dinocyst zones are defined in Heilmann-Clausen (1988). P. regalis Zone for (new name the P. geminatumZone) (Heilmann-Clausen & van Simaeys, 2005). Magnetostratigraphyafter Heilmann-Clausen& Beyer (in prep.). -44- Figure 3. The Kirstinebjerg East section exposing beds L2-L4 of the LillebaeltClay Formation. The reddish interval is Bed L3. The strata in form an anticline, here seenin cross section e. photographed the directionofthe fold axis), June2010. The person asymmetrical (/. , is standing at the fold axis, close to the level ofthe ‘mussel layer’ of Fig. 2. The Eocene outcrops on the Trelde Næs peninsula sented in the sliding masses ofL5-L6 is probably much in the thicker, most likely about 40 m, as recorded Late Ypresian and early Lutetian plastic clay crops out nearby LB 38 borehole (Figs 1 and 2). the of Trelde between the few small ofthe Sovind Marl Formationhave extensively on SE coast Nass, A outcrops Fredericia of been 0sterskov 1 and and the city of and the tip the peninsula (Fig. 1). logged at (Figs 2) near Most of the outcrops consist of strata of the Lillebaelt Kirstinebjerg East section (Fig. 1). At the latter locality Clay Formation, subdivided into 6 regionally distributed, small temporary exposures ofthe Late Oligocene Brej- formally described beds, numberedLI to L6 (Heilmann- ning Formation have also been observed (Schnetler, Clausen et al, 1985). On TreldeNaes only the beds L2 to 1985). L6 are present (Fig. 2). The oldest strata (beds L2-L4) Large quantities of sideritic and phosphatic concretions occur in a few semipermanent sea-cliffs up to c. 15m are continuously washed out of the landslides and sea of and accumulate the beach. The ofthe high with glacially intensively folded successions the cliffs, on majority thickness of concretions rich in fossils but devoid of strata (Fig. 3). A stratigraphical 17.5-18 m are trace macro- ofthis has been measured fossils. small numberofthe concretions include fossils part (Heilmann-Clausen et al, A 1985). The youngerbeds L5 and L6 are highly smectitic ofinvertebrates, mainly molluscs, and vertebrates. Small and, due to the strong water-binding capacity ofthis clay pyrite concretions, including rare moulds and replace- of mineral, these beds form no permanent cliff sections. ments of body fossils, are likewise washed out the landslides and accumulate residual fields the Instead they comprise large, spectacular along clay may as pyrite on most of the 6 km long SE'coast of TreldeNaes (Fig. 4). beach. Despite the rarity of macrofossils intensive col- Because of the disturbances the total thickness of beds lecting by numerous enthusiasts over many years have L5 and L6 is unknown. Heilmann-Clausenetal. (1985) brought to lightmore than a thousandmollusc specimens the logged a thickness of c.l 1 m in sections exposing beds which have provided basis for the present description the L5 and L6 (Fig. 2), but the totalthickness of strata repre- of assemblage. -45- Figure 4. Sliding masses ofbeds L5/L6 northeastof0sterskov section, 2009 Calcareous nannofossil and zonations northern dinocyst Germany and Belgium was established (Heil- mann-Clausen & Costa, 1989). The correlation with Bel- The L2-L4 beds non-calcareousand calcareous showed are nanno- gium that Bed LI and the lowerpart ofBed L2 are fossils are only in a few in Bed of latest present thin, marly layers Ypresian age and that the Ypresian/Lutetian tran- L5 and in more numerous in Bed L6. The sition marly layers (as then conceived) occurs within Bed L2. An inde- nannofossils that beds L5 and L6 Trelde Naes suggest at pendent study by de Coninck (1991) came to a similar con- in NP15 belong (Lutetian) (Fig. 2), and it is probable that clusion. However, the recently established GSSP for the Bed L5 in