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Sedimentology and Ichnology of the Robbedale Formation (Lower Cretaceous), Bomholm, Denmark

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Sedimentology and Ichnology of the Robbedale Formation (Lower Cretaceous), Bomholm, Denmark Sedimentology and ichnology of the Robbedale Formation (Lower Cretaceous), Bomholm, Denmark JAN KRESTEN NIELSEN, KIM STEN HANSEN & LARS SIMONSEN Nielsen, J. K., Hansen, K. S. & Simonsen, L.: Sedimentology and ichnology of the Robbedale Formation (Lower Cretaceous), Bomholm, Denmark. Bulletin of the Geological Society of Denmark, Vol. 43, pp. 115-131. Copenhagen, 1996-12---05. https://doi.org/10.37570/bgsd-1996-43-12 An exposure of the Lower Cretaceous Robbedale Formation adjacent to the Rpnne Golf Course, Bomholm, has permitted a more detailed study of the trace fossils than has been made hitherto. A lithofacies analysis indicates that the sediments were deposited on the lower to upper shore face in a wave dominated barred coastal environment. The following trace fossils are present: Conichnus conosinus isp. n., Cylindrichnus isp., Ophiomorpha nodosa, Skolithos linearis, Teichichnus rectus, Thalassinoides suevicus forms 1 and 2. These seven trace fossilsare con­ tained in nine ichnofabrics. The ichnofabrics are closely related to sedimentary facies and suggest a shallow marine sedimentary environment. The abiotic fac­ tors, sediment stability and grain size, probably affected the distribution of ichnofabricsby controlling the occurrence of trace makers and their behaviour. Associated with Ophiomorpha nodosa at one horizon are a series of trace fossils that may be interpreted as fodinichnialstruc tures. They somewhat resemble "ray­ holes". The available nomenclature for such structures is discussed and the name Conichnus conosinus isp. n. is proposed. Key words: Sedimentology, ichnology, Robbedale Formation, Lower Cretaceous, Bomholm, Denmark. J. K.Nielsen and K. S. Hansen, Department ofHistor ical Geology & Palaeontol­ ogy, Geological Institute, University of Copenhagen, ())ster Voldgade JO, 1350 Copenhagen K, Denmark; L. Simonsen, Vtngen 15, 3140 Alsgarde, Denmark. 15th May, 1996. The Berriasian(Lower Cretaceous) Robbedale Forma­ 1) was taken as an opportunity to make an ichnological tion of Bomholm, Denmark consists of uncemented study of a part of the Robbedale Formation. fine-grained sand (0sterborg Member) and medium to coarse sand and gravels (Langbjerg Member) (Gravesen 1982). Depositional environments fluctu­ ated between decidedly nonmarine and marinecondi­ Stratigraphy and palaeogeography tions (Gravesen, Rolle & Surlyk 1982). Trace fossils are abundant and especially well preserved in the Bomholm is situated in the Baltic Sea south of Swe­ 0sterborg Member and were first described by Gry den within the Tomquist-SorgenfreiLineament and is (1968) and Jux & Strauch (1968). They have been strongly block-faulted (Gry 1968). Down faulting of mentioned and illustrated (i.e. Gravesen et. al. 1982; the blocks as a result of Late Kimrnerianextensional Noe-Nygaard & Surlyk 1988), but an up-to-date ap­ tectonics brought about the deposition of the Nyker praisal is overdue. A new Conichnus isp. is proposed Group (Ziegler 1987; Gravesen et al. 1982). in the present paper. Inland exposures of the forma­ Sedimentation was controlled by tectonics, changes tion are deteriorating rapidly owing to abandonment in sea-level (eustasy) and fluctuationsof climate (Gry of the quarries.However, the appearance of a new ex­ 1968; Gravesen et al. 1982). Fault-control on sedimen­ posure adjacent to the RlllnneGolf Course (pit II) (Fig. tation has been proposed for the Nyker Group by Nielsen et al.: Robbedale Formation 115 Fig. 1. Map showing geographical location of the island Bornholm, Denmark and the studied locality. After Gravesen et al. (1982). Germany . 300 Km — Limit of mesozoic deposits Robbedaie Formation Quarry at Rønne Golf Course Aakirkeby Rønne Gravesen et al. (1982) and Noe-Nygaard & Surlyk described as composed of a lower clay-rich limnic and (1988). The Nyker Group comprises a transgressive fluviatile Rabekke Formation followed by the Robbe- to regressive phase. At the time of deposition (Berria- daie Formation. The Robbedaie Formation consists of sian), Bornholm was situated at the edge of a narrow a lower Østerborg Member deposited during transgres- seaway connecting the Danish Embayment and the sion and an upper Langbjerg Member which, together Polish Trough, i.e. a boreal North Sea and the Tethyan with the superjacent Jydegård Formation, was depos- Sea (Doré, 1991). Later tectonic inversion phases in ited during regression (Gravesen 1982). The Jydegård Late Cretaceous and Neogene seem to be responsible Formation is a clay- and sand-dominated, lagoonal and for the present structural configuration. The uplift dur- fluviatile sediment (Gravesen et al. 1982; Noe- ing the inversion phases is thought to be more than 1 Nygaard, Surlyk & Piasecki 1987; Noe-Nygaard & km in the Tornquist-Sorgenfrei lineament (Japsen Surlyk 1988). 1993). Direct evidence of the age of the Robbedaie Forma- The Nyker Group, which unconformably overlies tion is not available, but the Jydegård Formation has the Middle Jurassic Bagå Formation and is itself been dated as Late Berriasian to Valanginian using unconformably overlain by Upper Cretaceous marine ostracodes and dinoflagellates (Christensen 1974; sediments of the Amager Greensand Formation (Gry Surlyk 1980; Piasecki 1984). The upper part of the 1968; Gravesen et al. 1982), has traditionally been Rabekke Formation also has yielded ostracodes, which 116 Bulletin of the Geological Society of Denmark NE SE Fig. 2. Conceptual stratigraphie scheme including Rabekke Formation, Robbedale Formation (Østerborg Member Amager Greensand Formation and Langbjerg Member) and SB Jydegård Formation. Sequence stratigraphie notations: HST, highstand system tract; SB, Jydciïârd l-'orinalion sequence boundary; TS, transgressive surface. « a, HST Langbjerg Member Ü - Studied section Robbedale Formation Østerborg Member- TS Riihckkc Formatidii ,—•„'V~_-"--'' •-•-, SB Bagå Formation indicate an age of Lower to Upper ?Berriasian (Chri- stensen 1974). Sedimentology A new sequence stratigraphie approach is attempted The sedimentology of the Robbedale Formation has here, in which the Rabekke Formation is regressive been discussed by Gravesen (1982,1986) and Gravesen and constitutes the lowstand system tract. These de- et al. (1982), but the section described here has not posits are followed by a transgression and deposition been previously available* The two members are of prograding to aggrading and normal regressive sedi- mainly distinguished on the basis of grain size. The ments of the Robbedale and Jydegård Formation dur- Østerborg Member is silty fine-grained sand and ing the highstand system tract (Fig. 2). From seismic Langbjerg Member is medium to coarse sand and grav- lines and samples from the sea floor just south of els. In the definition of the members, Gravesen et al. Bornholm it has been shown that there apparently is (1982) also included plant material and ichnology. At no evidence of relatively deep marine sedimentation the Rønne Golf-course the transition from Østerborg above the Rabekke Formation. Instead there is a direct Member to Langbjerg Member is exposed (Figs 3 & transition from the Rabekke Formation to the Robbe- 4). No shelly fossils are preserved in the sediment. dale Formation (Fredningsstyrelsen 1986). This could be due either to erosion or that the Nyker Group was developed on a basement high where deeper marine Units A and E conditions were never reached. The transgression Description: Units A & E both consist of white, fine- seems to have regional extension. In Scania (Sweden) grained sand including some lenses or layers of fine to there is a similar paralic succession in Lower Creta- medium-grained sand. The sand is well-sorted, con- ceous, but no hiati or erosional surfaces are mentioned taining subangular to well-rounded grains. The small- (Norling 1981). The model presented here suggests that est grains are the most rounded. The only primary struc- the change in grainsize from Østerborg Member to tures are plane layering and the above-mentioned lenses Langbjerg Member is merely a change in environment of coarser material. The plane layering is defined by during a period of prograding to aggrading conditions thin drapes of mica. Bioturbation increases from sparse and not a change from transgressive to regressive con- to common through unit A. Bioturbation varies be- ditions (Fig. 2). During continuing progradation un- tween common and sparse through unit E. der normal regressive conditions, lagoonal and fluvial sedimentation of the Jydegård Formation overlies the Interpretation: These two units are grouped as faciès coastal environments of the Robbedale Formation. 1. In faciès 1 physical reworking probably was a the Nielsen et al.: Robbedale Formation 117 Lithology F2 2b : : Mud 7m - Sand 4 Coal Intraclasts : Fl 3b : o 6m - n Structure n Collapse structures 2 3a : ¥ Medium/coarse sand lenses Medium trough cross-bedding 5m - Ichnology "V Conichnus : E 1 conosinus isp. n. Cylindrichnus isp. 4m - 2c ' 4, Ophiomorpha nodosa 1 Skolitkos linearis 3 Ü Teichichnus rectus - Thalassinoides suevicus 3m 1 form 1 _A n Thalassinoides suevicus 2 form 2 : D2 4a : D1 3 6 4b ! 2m i : Q 5a : : B 2 2 la ; lm - : A 1 1-2 2a t ^1 111 o C a» öß C 3 w U E o O =3 a .fes •ë cj B o Fig. 3. Log illustrating the examined part of the Robbedale Formation. The relative abundance of each ichnotaxon is indicated by the number of symbols (sparse, common, abundant). Bioturbation index (BI) after Taylor & Goldring (1993). 118 Bulletin of the Geological Society of Denmark Fig. 4. Overview of the Rønne Golf Course pit at the
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