Milton et al. (1990) divided ..... The Simon Petroleum Technology North Sea dino- cyst zonation was ... Eocene section encompassed by the Simon Petroleum.
Marine and Petroieum Geology, Vol. 13, No. 3, pp. 295-312, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain, All rights reserved 0264-8172/96 $15.00 + 0.00 ELSEVIER
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Palaeocene biostratigraphy and sequence stratigraphy of the UK central North Sea David C. Mudge* 6 Ki/mardinny Crescent, Bearsden, Glasgow G61 3NR, UK
Jonathan P. Bujak The Lexis Group, Albion House, 9Albion Avenue, Blackpool, Lancashire FY3 8NA, UK Received 14March 1995; revised 15August 1995; accepted 19August 1995 The Palaeocene of the central North Sea can be divided into four stratigraphic sequences named the Ekofisk, Maureen, Lista and Forties. These sequences display consistent and predictable patterns on electric log data and are bounded by high-gamma log peaks which represent thin condensed mudstones. Five additional subsequences are recognized within the Lista sequence. Biostratigraphic analysis has enabled the Palaeocene to be divided into six dinocyst zones and 11 subzones based on last occurrence or last abundance events that are considered to have chronostratigraphic significance across the North Sea basin. The dinocyst zonation has been integrated with bioevents based on other microfossil groups, and correlated with onshore NW European sections, permitting calibration with the standard worldwide nannoplankton and planktonic foraminiferal zones. These bioevents display a consistent relationship with the stratigraphic sequences, allowing the construction of a confident stratigraphic framework for the Palaeocene. Keywords: Palaeocene; stratigraphy; North Sea
The integration of well log data with a high-resolution dinocyst zonation has allowed the development of a detailed stratigraphic framework for the Palaeocene rocks of the central North Sea. This area includes the Moray Firth Basin, South Viking Graben and northern part of the Central Graben, which were sites of thick sand-prone deposition during the Palaeocene (Figure 1). Four regionally mappable stratigraphic sequences are defined, separated by high-gamma mudstones that are interpreted as maximum condensation surfaces. These bounding surfaces can be correlated over the whole of the study area, as well as further afield over a large part of the North Sea basin. Calibration of the surfaces with dinoflagellate cyst (dinocyst) and other microfossil bioevents in examined wells across the basin gives confidence that the surfaces represent time lines. In the Moray Firth Basin, a number of subsequences can also be identified and mapped over more limited areas. This study uses a database of more than 400 released wells for log interpretation and 40 wells for biostratigraphic analysis. The purpose of the present paper is (1) to define six dinocyst zones and 11 subzones in the central North Sea; (2) discuss the correlation of these zones and subzones with onshore sections around the S and SE margins of the Palaeogene North Sea basin and hence, * C o r r e s p o n d e n c e to D. C. M u d g e
with worldwide chronostratigraphy; (3) calibrate the zones with other microfossil events to establish an integrated succession of bioevents including planktonic and benthic foraminifera, radiolaria, diatoms, dinocysts and pollen; (4) define four stratigraphic sequences and associated subsequences in the Palaeocene; and (5) map these sequences over the central North Sea. A variety of wells have been selected to illustrate the log expression of the stratigraphic sequences in differing facies and the relationship of sequences to microfossil bioevents. Details of sequence boundary depths in 28 wells are given in Table 1. A tabulation of bioevent depths is included in the Palaeocene-Eocene North Sea synthesis of Mudge and Bujak (in press).
Stratigraphic analysis The development of an integrated stratigraphic framework for the Palaeocene of the North Sea basin follows the principles and methodology established by Mudge and Bujak (1994) in their stratigraphic analysis of the Eocene in this area. In their paper they identify a succession of log cycles displaying characteristic bowshaped gamma-sonic patterns, which are separated by high-gamma log peaks. They define these cycles as stratigraphic sequences and their bounding high-gamma log peaks as surfaces of maximum condensation. These definitions are based simply upon
M a r i n e and Petroleum Geology 1996 V o l u m e 13 N u m b e r 3
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Palaeocene biostratigraphy and sequence stratigraphy of UK central North Sea: D. C. Mudge and J. P. Bujak empirical interpretation of well data and carry no genetic connotation. Thus the stratigraphic sequences reflect waxing and waning of sedimentation in the basin, corresponding to cycles of sediment progradation, aggradation and retrogradation on the basin margins. The bounding surfaces record the effect of greatly reduced sediment input into the basin and may have a number of origins, including major transgression associated with an abrupt increase in water depth and basin margin flooding, climatic changes, variations in sediment supply to the basin or changes in rates of tectonic uplift or subsidence. Mudge and Bujak (1994) erected a high-resolution Eocene dinocyst zonation for the North Sea, defining a succession of dinocyst, foraminiferal, diatom and radiolarian bioevents that occur consistently across the basin. The dinocyst succession is almost identical to that previously described from onshore localities in Denmark, NW Germany, The Netherlands, Belgium and southern England, and includes cosmopolitan taxa, permitting correlation of North Sea Eocene stratigraphy with worldwide chronostratigraphic schemes. Mudge and Bujak (1994) demonstrated that individual Eocene sequence boundaries consistently fall between the same bioevents, giving a high degree of confidence that these boundaries represent time lines. Occasional bioevents that appear to be anomalous in
some wells probably result from poor sample quality. Mudge and Bujak (1994) also showed that the Eocene sequences can be mapped over a large part of the North Sea basin, retaining their internal character from the East Shetland Basin in the north to the Central Graben in the south, a distance of 400 km. Similar sequence stratigraphic and biostratigraphic relationships are also present in the Palaeocene succession of the North Sea basin, with four stratigraphic sequences being identified. These are the Ekofisk, Maureen, Lista and Forties sequences, each named after the major rock unit that they contain. Typical log patterns for the four Palaeocene sequences are shown in Figure 2; the overlying Dornoch and Balder sequences of Lower Eocene age are described in Mudge and Bujak (1994). In sand-poor distal wells, e.g. in the Central Graben, individual sequences display characteristic bow-shaped gamma log patterns, often mirrored by the sonic and resistivity responses, which are bounded by high-gamma log peaks. Sandstones present in the Forties sequence interrupt, but do not destroy, the log patterns. In sand-rich, more proximal wells in the Moray Firth basin, thick sandstones are developed in the Lista sequence, which can also be subdivided into five subsequences based upon the recognition of a series of subordinate high-gamma log peaks. These subsequences can be separated over only limited parts of the Moray Firth basin and South
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Figure 1 Structural e l e m e n t s of the central N orth Sea area, showing location of reference wells
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Marine and Petroleum Geology 1996 Volume 13 Number 3
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Palaeocene biostratigraphy and sequence stratigraphy of UK centra/ North Sea: D. C. Mudge and J. P. Bujak
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