Sequence and systems tract interpretation of the epicontinental Oligocene deposits in the Danish North Sea

Abstract

A sequence stratigraphical scheme has been established for the siliciclastic Cenozoic deposits in the southeastern North Sea Basin. The present paper addresses the problem of recognizing systems tracts by means of logs, using the Oligocene sequences as an example. The Oligocene sequences are characterized by an overall prograding seismic reflection pattern, and systems tracts cannot be interpreted from seismic sections alone.The sequences are dominated by marine clay deposits, and abrupt changes in lithological facies are rarely seen. The gamma-ray log trends indicate retrograding, aggrading and prograding stacking patterns.The lowstand deposits are identified as prograding sedimentary bodies. Coarse-grained sharp-based lowstand deposits are found basinwards of the depositional shoreline break of the preceding sequence, and further basinwards they are more fine-grained. Fan deposits are not recognized. A thin interval of transgressive deposits including upward fining clayey sediments is found at the top of the lowstand deposits and in the landward direction at the top of the older highstand deposits. The maximum flooding surface is identified by a high gamma-ray peak. The overlying highstand deposits thicken in the landward direction, and are here characterized by an upward coarsening trend. They thin in the basinward direction and become a condensed clay interval.Deposition took place during a tectonically quiet period in an epicontinental basin with a gently southwestward dipping sea floor. The lowstand deposits comprise forced regressive deposits and prograding deposits, and a deep ramp model is, therefore, suggested for these deposits.

Oligocene climate changes controlling forced regression in the eastern North Sea

“…2), dominated by SW-ward prograding siliciclastic units, which are unconformably overlying the Eocene hemipelagic deposits (Michelsen et al, 1995(Michelsen et al, , 1998Schiøler et al, 2007) and reach over 1000 m of thickness. Michelsen and Danielsen (1996) and Danielsen et al (1997) showed that the Oligocene sediments originated from the north east, and that the depocenters of the individual sequences migrated westwards, parallel to the pinch-out line of the Zechstein salt and onto the northern flank of the Paleozoic Ringkøbing-Fyn High.…”

3D Seismic Analysis Reveals the Origin of Ambiguous Erosional Features at a Major Sequence Boundary in the Eastern North Sea: near Top Oligocene

“…Sequence stratigraphic studies have shown that the Oligocene was characterized by south and southwestward progradation (Michelsen & Danielsen 1996;Danielsen et al 1997, Michelsen et al 1998). In the eastern North Sea area the progradational units generated an E-W striking palaeosiope with an overall dip towards the south during the Oligocene as outlined by the time-structure map in Figure 1.…”

A mid-Oligocene cooling (Oi-2b) reflected in the dinoflagellate record and in depositional sequence architecture. An integrated study from the eastern North Sea Basin

“…The previously used sequence stratigraphic subdivision (Michelsen et al, 1998;Michelsen and Danielsen, 1996;Danielsen et al, 1997) was primarily based on well log analysis supported by 2D seismic sections. However, the introduction of 3D seismics in sequence stratigraphic analysis calls for a revision of the subdivision.…”

“…At this time the sedimentation of mainly hemipelagic clays of the Eocene was succeeded by prograding muddy and locally sandy sediments from Fennoscandia (Michelsen et al, 1998;Schiøler et al, 2007). The Oligocene clinoforms exceed 1 km in thickness in the Norwegian-Danish Basin (Schiøler et al, 2007) and are subdivided into a number of sequences showing progradation from the east e northeast (Michelsen and Danielsen, 1996;Danielsen et al, 1997). The structure and development of these sequences have been the subject of many sequence stratigraphic studies (Michelsen and Danielsen, 1996;Danielsen et al, 1997;Clausen et al, 1999Clausen et al, , 2000 but attempts to correlate the sequence development to glacioeustatic changes (Miller et al, 1991) have previously been hampered by the poor datings of the sequence boundaries (Clausen et al, 1999).…”