Late Jurassic Seismic Sequences, Inner Moray Firth, U.K. North Sea: Do Local Influences Invalidate a Key Segment of Exxon's Sea-Level Chart?

UNDERHILL, JOHN R., University of Edinburgh, Edinburgh, U.K.

Exxon workers used seismic reflector terminations in the Late Jurassic of the Inner Moray Firth (IMF) to define seven sequence boundaries within the late Oxfordian-Berriasian marine interval. The seven are incorporated in their global chart showing relative changes of coastal onlap and eustatic sea level because it was considered that data from the IMF showed no evidence that tectonics caused the unconformities. However, the new interpretation casts doubt on Exxon's conclusions.

Interpretation of a comprehensive seismic and well data base has only led to the recognition of five sequences within a late Oxfordian-Berriasian (J2) megasequence. Geometric, thickness, and sedimentary facies variations in this megasequence demonstrate the likelihood that synsedimentary tectonism created them in a fully marine domain, rather than coastal onlap in a basin that was tectonically quiescent. Additional evaluation of the onlap-defined sequence boundaries suggests that they were largely the result of extensional activity caused by fault-block rotations in association with half-graben development. Furthermore, the work shows that onlaps within a fully marine sequence should not be used to define coastal onlap because they may represent purely sedimentary processes, such as s bmarine fan avulsion untied to any rise in sea level. Indeed, independent work generally shows that such sedimentary responses are more likely to be associated with sea-level lowstands when sediment supply rates are much greater.

The reappraisal of an integrated data base from the IMF type area casts doubt on the local controls on and global applicability of the Late Jurassic section of Exxon's sea-level chart.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)