Tihomir Marjanac

Dunlin Group Sequence Stratigraphy in the Northern North Sea: A Model for Cook Sandstone Deposition

The Dunlin Group in the northern North Sea, consisting of the Johansen, Amundsen, Burton, Cook, and Drake formations of late Sinemurian- Toarcian age, hosts important hydrocarbon reservoirs in the Cook Formation sandstones. The Johansen Formation is associated with a relative fall of sea level and is interpreted to be a large sandstone delta confined within a broad incised valley at the base of the group. During a later stage of relative sea level rise, the finer grained Amundsen and Burton formations were deposited. The overlying Cook Formation consists of four sandstone tongues, each of which is characterized by a lower zone of sharp-based, upward-coarsening, thinly bedded shoreface sandstones and siltstones (reflecting forced regression during falling relative sea level) and an erosively based upper zone of thin tidal flat and thick deltaic/estuarine sandstones (reflecting lowstand incision, as well as initial progradation and subsequent transgressive backfill of estuaries during relative sea level rise). The Drake Formation shales were deposited during continued relative sea level rise. Several types of erosional surfaces are recognized within the studied succession: (1) sequence boundaries occur at the base of the Johansen Formation and within the Cook Formation, and represent the bottoms of incised valleys that truncate the underlying ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received May 4, 1995; revised manuscript received March 27, 1996; final acceptance September 12, 1996. 2Department of Geology/Paleontology, University of Zagreb, 10000 Zagreb, Croatia. 3Department of Geology/Geophysics, University of Wyoming, Laramie, Wyoming 82071. This paper was part of the sequence stratigraphy research project at the University of Bergen, Norway, and is a product of studies on the architecture of Lower Jurassic shelf sandstones of the northern North Sea. The work was sponsored by STATOIL through a VISTA fellowship awarded to T. Marjanac, and this support is gratefully acknowledged. We are grateful to D. Mellere and L. M. Falt for discussion and encouragement during the work. We also thank Else Lien for drafting the sections and Jane Ellingsen for reprographs. Constructive criticism from D. Nummedal, J. Webb, and K. T. Biddle on the final version of the manuscript is much appreciated.