Jurassic palynostratigraphy of Bornholm, Baltic Sea, Denmark

Samples of the Rønne Formation have been collected from exposures at Munkerup, Sose Bugt and Galgeløkke and n.vo boreholes, Galgeløkke-1 and -2, at Galgeløkke south of Rønne (Fig. 1). The Hasle Formation was sampled at the type section south of Hasle harbour, at Korsodde and from the Levka-1 and Hasle-1 boreholes. The Baga Formation, that consists of the previously defined Levka, Sorthat and Baga beds (Gry, 1969; Gravesen et al. 1982), was sampled from Mo sections within the Hasle Klinkerfabrik clay pit, from the coastal section at Korsodde, and from four cored boreholes the Levka-1, 106, 107 and 109 at the Hasle Klinkerfabrik clay pit (Nielsen & Koppelhus 1989, 1991; Koppelhus & Nielsen 1994). The samples were processed for their palynological content using the standard techniques developed at the Geological Survey of Denmark (Poulsen et al, 1990). They were examined by transmitted light microscopy. All of the slides are stored in the collections of the Department of Stratigraphy. at the Geological Survey of Denmark.

Foraminiferal biostratigraphy of the post mid-Miocene in two boreholes in the Danish North Sea

Cutting samples from two exploration wells, Cleo-1 and Kim-1, in the Central Trough area in the northwestern part of the Danish North Sea, have been investigated for foraminiferal content in the section above the prominent mid-Miocene event. Benthonic foraminifera have been used to produce a stratigraphic subdivision by reference to the standard NSB zonation of King. The NSB 12 to NSB 17 zones (Middle Miocene to Middle Pleistocene) have been identified above the event. These zones can be related to the paleo water depth zonation. Paleoenvironrnental reconstruction shows that sediments from the subject interval from Cleo-1 were deposited in a shallower situation than equivalent deposits in Kim-1. A conspicuous hiatus is identified in Cleo-1 at the Pliocene-Pleistocene boundary.

Generation of accomodation space around the D-1 fault during the Oligocene, Danish North Sea

Implications of a sequence stratigraphic study of the Oligocene sediments in the Norwegian-Danish Basin around the D-1 well are presented. The study is based on seismic mapping, log interpretations and detailed dating. This paper focuses on the evolution of the accommodation space along and across the D-1 fault since the fault generated deviations in the available accommodation space and has a significant influence on the depositional geometry of the sequences. The results show that displacement across the D-1 is fairly constant along strike during deposition of the older Oligocene sequences. This is in contrast to the youngest Oligocene sequence which has a higher displacement rate and thus significant variations along strike. This emphasises, that the understanding of the structural evolution of local faults in a basin dominated by elastic sediment infill as the Tertiary North Sea has to be as detailed as the sequence stratigraphy, because the fault may have had a significant influence on the depositional geometry, and hence the complete sequence stratigraphic interpretation.

Correlation of Miocene North Sea sequences with the Danish land area based on foraminifera

In the present study comparisons are made between the Miocene North Sea sequences and the Danish onshore formations based on foraminiferal biostratigraphy. Foraminiferal faunas of four onshore borings previously analyzed by Kristoffersen have been reviewed. Biozones NSB 9 - NSB 13 of the offshore sequences were all found in the Danish Miocene formations, and the obtained results promise well for a future more detailed correlation between the North Sea and the Danish land area.

Origin of a deep buried valley system in Pleistocene deposits of the eastern central North Sea

In the North Sea, the sedimentary development of the late Tertiary and early Quaternary was dominated by deltaic sedimentation in a fast subsiding basin. During the Pleistocene, pronounced climatic changes affected the sedimentation of the area and progradation of the delta systems ceased. The Middle and Upper Pleistocene sedimentary successions consist of alternations of marine and fluvial deposits, partly reworked during glacial periods. Seismic records from the Danish sector of the North Sea reveal numerous deep incisions cut down from various levels of the Middle and Upper Pleistocene successions. These incisions are concluded to form a pattern of buried valleys. Detailed seismic stratigraphic analysis shows the occurrence of various internal unconformities within these buried valleys. It is concluded that the valleys originate from a river system developed in periods of repeated sea-level changes. Pluvial erosion during glacial sea-level lowstand and glacial meltwater action is proposed to have been responsible for the origin of the valley system. Thus, in Middle and Upper Pleistocene glacial periods drainage and associated sediment transport occurred from Northwest and Central European land areas via a presently buried river system in the southeastern North Sea towards a depositional basin north and northwest of the Danish North Sea sector.

Tertiary fluvial deposits of Jylland, Addit area

Sandy fluvial deposits of Tertiary age are described from three sand pits in the Addit area with up to 28 metres high outcrops. The deposits form up to 20 metres thick fining-upwards successions of mainly large-scale tabular cross-sets of medium and coarse sand interpreted as fluvial bars. Individual successions are capped by thinly interbedded sands and silts and occasionally completed with a coal bed. Concave-up bounding surfaces separate the fining-upwards successions. Channel switching, breaks in active channel aggradation, rapid scour- and filling events and chute activity created bounding surfaces described as a 4-tier hierarchy. The palaeocurrents derived from trough foresets suggest that the channel course was straight rather than sinuous. The current directions were generally to the south and southwest.

Analysis of the fault geometry of a Cenozoic salt-related fault close to the D-1 well, Danish North Sea

A normal detaching fault in the Norwegian-Danish Basin around the D-1 well (the D-1 fault) has been mapped using seismic sections. The fault has been analysed in detail by constructing backstripped-decompacted sections across the fault, contoured displacement diagrams along the fault, and vertical displacement maps. The result shows that the listric D-1 fault follows the displacement patterns for blind normal faults. Deviations from the ideal displacement pattern is suggested to be caused by salt-movements, which is the main driving mechanism for the faulting. Zechstein salt moves primarily from the hangingwall to the footwall and is superposed by later minor lateral flow beneath the footwall. Backstripping of depth-converted and decompacted sections results in an estimation of the saltsurface and the shape of the fault through time. This procedure then enables a simple modelling of the hangingwall deformation using a Chevron model with hangingwall collapse along dipping surfaces. The modelling indicates that the fault follows the salt surface until the Middle Miocene after which the offset on the fault also may be accommodated along the Top Chalk surface.

Mid-Miocene progradational barrier island and back-barrier deposits, central Jylland, Denmark

This paper reports on a c. 15 m thick section of marine, coastal sand, that combine a hitherto less described fades succession transitional to the Miocene fluvio-deltaic and open marine deposits. The section is exposed in a pit located 10 km northeast of the sandy fluvio-deltaic brown coal bearing strata of the Søby-Fasterholt area, known as the Odderup Formation (Middle Miocene). The section comprises more than 6 m of trough cross-stratified, low-angle cross-stratified and parallel-laminated, sand and gravel, deposited on the foreshore and backshore of a barrier island. Locally these deposits are eroded by a steepsided washover channel with a sandy fill. The overlying back-barrier deposits are up to 9 m thick, and are initiated by a thin bituminous mud bed with a mixture of terrigeneous and marine organic matter. Above, a series of 0.2-2 m thick beds of tangential to sigmoidal cross-stratified sand, often with well preserved topsets, are deposited. These beds are interbedded with ripple-laminated and parallel-laminated sand. Burrows of Ophiomorpha, possibly Skolithos and Arenicolithes occur in the back-barrier deposits and in the top of the washover channel-fill. The succession of well-preserved coastal fades-belts indicates that the depositional area was characterized by a large sediment supply in combination with a rising sea-level. The section was probably formed fairly rapidly during a coastal progradation more or less contemporaneously with the general transgression that led to the deposition of the brackish- marine Hodde Formation and later the fully marine Gram Formation.

Foraminiferal biozonation in the Early Pleistocene in the Central North Sea

Pliocene and Pleistocene deposits from 66 boreholes from the central and northern North Sea have been examined for their content of benthic foraminifera. In this area the Pliocene/Pleistocene boundary is generally placed at the bottom or middle of the Elphidium oregonense Subzone or at the last local occurrence of Cibicides grossus. A detailed study of the temporal relationship between these two species in the western part of the Danish sector shows, that the last local occurrence of Cibicides grossus in this area is older than the Elphidium oregonense Subzone. On the basis of the results concerning the Lower Pleistocene an emendation of the foraminiferal biostratigraphy for this sub-series is proposed and related to previous work on the biostratigraphy and palaeogeography of the North Sea area. The investigation indicates the existence of two distinct new subzones within the Elphidiella hannai/Cassidulina teretis range: The oldest of the two new subzones is an Acme-zone with Buliminidae as the characteristic taxon, and it is named the Stainforthia/Bulimina Subzone. The depositional environment was a boreal shelf with a water depth presumed to exceed 100 m. Based on its stratigraphic position and enviromental indications the Subzone is referred to the Tiglian stage. The youngest of the two new subzones is a local Range-zone, defined by the presence of the arctic species Elphidiella gorbunovi, and it is named the Elphidiella gorbunovi Subzone. The depositional environment was an arctic shelf with a water depth of less than 50 m. The occurrence of Elphidiella gorbunovi in the central North Sea thus indicates a cold interval in either the Eburonian or the Menapian stage. Succeeding the Elphidiella gorbunovi Subzone, the foraminiferal fauna and the sediment indicate increasingly near-coastal environment and a warming of the climate.

A stratigraphic interpretation of the Oxfordian-Valaginian deposits in the Danish Subbasin

Six sequences are identified in the Upper Jurassic and lowermost Cretaceous Flyvbjerg-, Børglum- and Frederikshavn Formations of the Danish Subbasin. Sequence boundaries and maximum flooding surfaces are interpreted on the basis of log analysis, biostratigraphy and lithology of core samples. Parasequences and parasequence sets are mainly recognized in shallow marine sediments. The gamma ray and SP logs of the sequences have an overall cyclic character, indicating the presence of upward fining deposits overlain by upward coarsening deposits. A blocky log pattern or an upward decreasing trend lowermost in the sequence represents sanddominated sediments, and it is interpreted as lowstand deposits caused by forced regression. The overlying interval with upward fining deposits comprise transgressive deposits. The interval between maximum flooding surface and the upper sequence boundary has an upward coarsening trend, and it probably includes the highstand deposits.