Geological Survey of Denmark and Greenland Bulletin 15, 2008, 29-32 In Denmark most of the water used in private households, in the industry and for irrigation in agriculture comes from sub- surface aquifers. Some of the most important aquifers in Jyl - land, western Denmark, are sand layers deposited from 23 to 15 Ma ago, in the Early Neogene (Early to Middle Miocene). About 23 Ma ago, in the Early Miocene, the coastline ran NW–SE across present-day Jylland (Rasmussen 2004). Global climatic variations led to major sea-level changes (Zachos et al. 2001), which in combination with increased sediment trans- port from the north (the present Norway) resulted in deposi- tion of several huge, fluvio-deltaic sand systems intercalated with marine clay (e.g. Rasmussen 1961; Rasmussen 2004; Rasmussen & Dybkjær 2005). The Geological Survey of Denmark and Greenland (GEUS) and the regional Environment Centres (the former counties (amter)) in Jylland are working in close cooperation to study the Early Neogene succession; the main purposes are: (1) to find new aquifers, (2) to map the extent of known aquifers and clarify their mutual relationships, in order to evaluate the size of the water resources and optimise production, and (3) to protect the aquifers from pollution due to leaching from the surface. In order to map the complex sedimentary succession, it has been necessary to combine several geological disciplines, includ- ing seismic interpretation, sedimentology, correlation of geo- physical logs, and biostratigraphy (e.g. Dybkjær 2004; Ras- mussen 2004; Rasmussen et al. 2004; Piasecki 2005; Ras- mussen & Dybkjær 2005; Dybkjær & Rasmussen 2007). This article shows some results of a detailed dinoflagellate cyst stratigraphy, which is based on an extensive database (Fig. 1). We present here for the first time a dinoflagellate cyst zonation for the complete Neogene succession in the Danish area. Dinoflagellates and their cysts Dinoflagellates are eukaryotic, single-celled organisms that occur as motile cells with two flagella, one which encircles the cell, and one longitudinal flagellum. These organisms, which also occur abundantly at present, include autotrophs, phagotrophs, sym- bionts and parasites. Photosynthetic species (autotrophs) ac - count for about half of the living genera and play an important role in the marine ecosystem as primary producers. Some marine species produce toxins or cause red tides and several types of shellfish poisoning. Dinoflagellates have left a rich fossil record, mainly of organic-walled cysts, in Mesozoic and Ceno zoic rocks; examples of fossil dinoflagellate cysts are shown in Fig. 2. Some dinoflagellate species form cysts as part of their life-cycle; others form cysts as a survival strategy, dur- ing unfavourable environmental conditions (e.g. low sea sur- face temperatures or lack of nutrients). The morphology of a cyst reflects the morphology of the motile dinoflagellate. The rapid evolution of the relatively complex, fossilisable dinofla- gellate cyst wall makes these fossils ideal for biostratigraphic purposes (e.g. Fensome et al. 1996). Material and methods The dinoflagellate cyst zonation scheme presented here is based on data from more than 50 onshore and offshore bore- holes and a series of onshore exposures (Fig. 1). The sediment samples from the boreholes and exposures were processed at the palynological laboratory at GEUS using standard paly- nological preparation methods. A new Neogene biostratigraphy for Denmark Karen Dybkjær and Stefan Piasecki © GEUS, 2008. Geological Survey of Denmark and Greenland Bulletin 15, 29–32. Available at: www.geus.dk/publications/bull 29 9°E Wells with biostratigraphy (49) Wells without biostratigraphy (5) Exposures (25) 50 km 53°N 55°N 57°N 8°E 10°E 12°E North Sea Jylland Sweden Germany Denmark Frida-1 Lone-1 Tove-1 S-1 100 km Fig. 1. Map of Denmark showing the location of studied wells and expo- sures. The insert map shows the location of the offshore exploration wells included in this study. The Danish Neogene stratigraphy has traditionally been based on molluscs and foraminifers whereas early studies of dinoflagellate floras were limited (Piasecki 1980). However, new comprehensive studies of dinoflagellate cysts throughout the Danish Neogene have resulted in a high-resolution strati - graphy of the succession, even in marginal marine deposits where foraminifers and molluscs rarely occur. Dinoflagellate cysts in strata of the central North Sea Basin were studied in offshore, hydrocarbon exploration wells, for comparison with the onshore, marginal, marine deposits and to obtain data from the youngest Neogene that is not represented by onshore strata. The resulting stratigraphy is a so-called interval zona- tion, i.e. all zones are defined as successions of rocks between a lower and an upper event of fossil appearance or disappear- ance. For example the Unipontidinium aquaeductum Zone is defined from the first appearance of U. aquaeductum to the first appearance of Achomosphaera andalousiense (Figs 2, 3). However, the zone is also characterised by other associated events (stratigraphic first and last occurrences of other charac- teristic species) and by the dinoflagellate cyst assemblage in general. Species with relatively common and consistent occur- rence and with reported stratigraphic potential from outside the North Sea Basin, e.g. in the North Atlantic realm, were preferentially selected for the definition of the zones. The zona- tion can therefore easily be compared with other interna- tional/North Atlantic dinoflagellate zonations, despite the fact that much of the studied material was deposited in the rela- tively enclosed and possibly periodically brackish environ- ments of the North Sea Basin. Dinoflagellate cyst zonation The Danish Neogene is formally divided into 19 dinoflagel- late cyst zones from the uppermost Oligocene to the top of the Pliocene, with one zone in the Oligocene, 15 zones in the Miocene and three zones in the Pliocene (Fig. 3). The Pliocene is considered tripartite; Zanclean, Piacenzian and Gelasian (see Gradstein et al. 2004). The zonation has a high resolution in the uppermost Oligocene, the Lower Miocene and the Middle Miocene (1, 8 and 4 zones respectively) and a lower resolution in the Upper Miocene and Pliocene (3 and 3 zones respectively). The zones will be formally defined in a forthcoming paper. The proposed dinoflagellate cyst strati - graphy for Denmark correlates well with other formal strati- graphies in the North Sea Basin, e.g. in Germany (Köthe 2003), the Netherlands (Munsterman & Brinkhuis 2004), Belgium (e.g. Louwye et al. 1999) and the United Kingdom (e.g. Head 1998), as well as with dinoflagellate cyst stratigra- phies outside this basin in the North Atlantic realm (de Verteuil & Norris 1996), in spite of differences in selection of diagnostic species. The new Danish biozonation is also cor- related with Neogene chronostratigraphy and with nanno- plankton biostratigraphy (see Gradstein et al. 2004). Geological results The new dinoflagellate cyst stratigraphy has been extensively tested in boreholes and exposures throughout Jylland (Fig. 1). The application provides a detailed stratigraphic frame- work for improved geological interpretations. Three major, prograding deltaic sand systems with excellent potential as reservoirs for drinking water are recognised in the Lower Miocene to lowermost Middle Miocene in the central parts of Jylland: the Billund sand, the Bastrup sand and the Od - derup Formation (e.g. Dybkjær 2004; Rasmussen 2004). In addition, several problematic issues of Neogene geology in the Danish region have been solved: 1. The stratigraphic position of the Vejle Fjord Formation (Larsen & Dinesen 1959) is now definitively established as lat- est Oligocene (Brejning Clay) to earliest Miocene (Vejle Fjord Clay and Sand) (Dybkjær 2004; Dybkjær & Rasmussen 2007). 2. The upper, silty and sandy part of the Sofienlund For - mation is time equivalent to the Vejle Fjord Clay and Sand. The lower Ulstrup Clay and the Sofienlund Clay are time equivalent with the Brejning Clay (Unpublished data 2006, K. Dybkjær). 3. The Oligocene–Miocene boundary can be recognised in the eastern North Sea Basin as the last occurrence of common Deflandrea phosphoritica (Figs 2, 3). This event corresponds to the transition from the Brejning Clay to the Vejle Fjord 30 25 µm 25 µm A B C D 25 µm 25 µm Fig. 2. Stratigraphically significant dinoflagellate cysts from the Miocene of Denmark. A: Unipontidinium aquaeductum. B: Achomosphaera anda - lousiense. C: Chiropteridium galea. D: Deflandrea phosphoritica. Clay and to the Sequence Boundary B (Rasmussen 2004; Dybkjær & Rasmussen 2007). 4. A significant hiatus is recognised between the Vejle Fjord and Arnum Formations in the northern and central parts of Jylland. In the southern parts of Jylland a time-equivalent fluvial-deltaic sand system, the Ribe Formation, was de - posited (Dybkjær & Rasmussen 2000; Dybkjær 2004; Rasmussen 2004; Rasmussen & Dybkjær 2005). 5. The sand-rich succession in the Salten profile and in nearby gravel pits (Addit, Voervadsbro) was formerly referred to the late Early Miocene to early Middle Miocene Odderup Formation. Dinoflagellate cyst analysis has shown that this 31 5 10 15 20 b a NN21 NN20 NN19 NN18 NN17 NN16 NN15/ NN13 NN12 N N 11 NN10 NN9 NN8 NN6 NN5 NN4 NN3 NN2 NN1 NP25 NN7 L L M M E E Holocene Pleistocene Pl io ce ne M io ce ne Oligocene A ge ( M a) Epoch Stage N an no pl an kt on zo na tio n Dinoflagellate cysts zonation Dinoflagellate events Onshore zonation Offshore zonation Gelasian Piacenzian Zanclean Messinian Tortonian Serravallian Langhian Burdigalian Aquitanian Chattian 23.03 20.43 15.97 13.65 11.61 7.25 5.33 3.60 2.59 1.81 Amiculosphaera umbracula Amiculosphaera umbracula Impagidinium multiplexum Bitectatodinium tepikiense Melitasphaeridium choanophorum Barssidinium pliocenium Barssidinium evangelinae Erymnodinium delectabile Reticulosphaera actinocoronatum Barssidinium evangelinae Selenopemphix armageddonensis Hystrichosphaeropsis obscura Palaeocystodinium spp. Gramocysta verricula Achomosphaera andalousiense Unipontidinium aquaeductum Unipontidinium aquaeductum Distatodinium biffii Deflandrea phosphoritica, common Chiropteridium galea Caligodinium amiculum Thalassiphora pelagica Thalassiphora rota Cordosphaeridium cantharellus Exochosphaeridium insigne Homotryblium spp. abundant Labyrinthodinium truncatum Cousteaudinium aubryae Ectosphaeropsis burdigalensis Exochosphaeridium insigne Sumatradinium hamulatum Palaeocystodinium miocaenicum H. obscura H. obscura S. armageddonensis M. choanophorum B. pliocenicum I. multiplexum G. verricula G. verricula A. andalousiense U. aquaeductum L. truncatum A. andalousiense A. umbraculaA. umbracula U. aquaeductum L. truncatum C. cantharellus C. cantharellus E. insigne E. insigne C. aubryae C. aubryae C. galea C. galea D. phosphoritica D. phosphoritica S. hamulatum S. hamulatum T. pelagica T. pelagica C. amiculum Homotryblium spp. Homotryblium spp. Fig. 3. Stratigraphic scheme presenting the new dinoflagellate cyst zonation correlated with the Neogene nannoplankton zonation and chronostrati - graphy. The time scale is according to Gradstein et al. (2004). The diagnostic dinoflagellate cyst species are shown in red. 32 succession is older than previously assumed and is part of the Vejle Fjord Formation/Billund sand system (Rasmussen et al. 2006). 6. The Hodde transgression occurred in the early Langhian, earliest Middle Miocene and limits the age of the underlying Odderup Formation to latest Early Miocene – earliest Mid - dle Miocene (Piasecki 2005). 7. The youngest onshore Neogene deposits are of Tortonian age (Late Miocene) as no Messinian or Pliocene dinoflagel- late cysts have been recorded. The earlier Sæd Formation, upper Gram Formation sand and the Neogene sand in cliffs at Ho Bugt are considered of Tortonian age and referred to the Gram Formation (Piasecki 2005). 8. A series of maps of the Cenozoic succession in the Danish North Sea area was produced by Rasmussen et al. (2005). The age determinations of the Neogene succession are based on the dinoflagellate cyst stratigraphy presented here. Future perspectives The new dinoflagellate cyst zonation is not only a valuable tool for unravelling the Danish Neogene succession. The Neogene succession in Denmark is presently being correlated with other Neogene successions, e.g. in Germany, the Neth - er lands and Poland, in order to elucidate the Neogene geol- ogy of the North Sea Basin. The zonation also leads to an improved understanding of the subsidence and tilting of the North Sea Basin during the Neogene, and a better understanding of the petroleum sys- tems in the hydrocarbon-producing provinces in the North Sea area. This may lead to new discoveries and increased pro- duction from known oil/gas-fields (Rasmussen et al. 2005). 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