MARTIN, MICHAEL A., Department of Geology, Univ. of Manchester, Manchester, UK

ABSTRACT: Sequence Stratigraphic and Trace Fossil Analysis of the Upper Jurassic Fulmar Formation, NW Central Graben

The sequence stratigraphic interpretation of bioturbated sediments has been greatly improved by recent advances in trace fossil analysis. This will be exemplified by the presentation of a high resolution sequence stratigraphic framework for the highly bioturbated Upper Jurassic Fulmar Formation of the Western Platform (Central Graben, UK North Sea). Within the study area lies the Kittiwake and Gannet oil fields.

Up to five sequences have been identified, their bounding surfaces often amalgamating with transgressive surfaces. The sediments can be

seen to onlap to the north and north-west. The identification of depth, salinity and substrate changes reflected in the trace fossil assemblages (ichnofabrics) allows for the picking of bounding surfaces at any scale.

On the sequence scale; downward shifts in facies can be identified from shallowing events picked out by, for example; the abrupt transition from an Anconichnus ichnofabric (offshore) to a more proximal Ophiomorphaichno fabric. Such shifts are typically reflected in lithological changes (i.e. increase in grain size) and can often be picked on well logs. Where biostratigraphic data is sufficient the sequence boundaries can be corroborated by the absence of biozones. However, in less obvious cases i.e. where no major facies shifts and only poor biostratigraphic data exists, the pinpointing of major depositional hiatuses/candidate sequence boundaries is possible from the identification of specific ichnofabrics, for example the Glossifungites ichnofacies, typically represented by the Thala sinoides trace.

On the parasequence scale; subtle changes in ichnofabrics, can reflect marine flooding surfaces, which are occasionally associated with grain size fluctuations, but rarely show on wire line logs.

No lowstand systems tracts have been clearly identified on the Western Platform, but are thought to exist in the basin to the east.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.