LIU, XIJIN, and WILLIAM E. GALLOWAY, Department of Geological Sciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Paleobathymetry Determination and Restoration of Depositional Profile, An Example From the Cenozoic North Sea Basin

Paleobathymetry determination is a critical element in tectonic subsidence calculations and quantitative sequence stratigraphic analysis. However, resolution of the paleobathymetry of deep basin sediments based onbenthic foraminifera data is commonly poor. This is especially true for the Cenozoic North Sea basin fill, which contains agglutinated foraminiferal species which are difficult to relate to the global depth-dependent biozones.

A new approach is proposed for rift basins where accommodation space for sedimentation is created predominantly by post-rift thermal subsidence. First, tectonic subsidence at a given location is determined bybackstripping without considering paleobathemetry. Second, theoretical thermal subsidence is calculated based on McKenzie's stretching model, using a stretching factor obtained by fitting the present magnitude of tectonic subsidence since rifting determined from backstripping to the theoretical one. An estimate of paleobathymetry along a transact at a given time is then obtained by subtracting the backstriped value from the calculated theoretical value. Calibration can be made wherever the paleobathymetry in shallow marine region is known from lithological and micropaleontological analysis. The technique was applied to the Central North Sea. The paleobathymetry was about 200-400 m during the Paleocene, increased in the early Eocene (300-400 m), sharply decreased (100-200 m) in the early Oligocene, and increased again (200-400 m) in the late Oligocene through Miocene.

With known paleobathymetry and decompacted thickness for individual stratigraphic sequences, one can restore the original depositional profile, which is useful not only in reconstruction of depositional history but also in predicting the occurrence of reservoir facies.

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