Facies Analysis at Triassic/Jurassic Boundary: Support for Eustatic Control of Global "Sequence Stratigraphy"

David L. Loughman

Recognition of global seismostratigraphic sequences and their eustatic interpretation has been a key geological concept of the past decade. Published criticism recently has led Exxon staff to constrain their sequence boundaries through the onshore geological record, thus placing the data base partially in the public domain. This act is a major step, since field testing of the eustatic model has been surprisingly limited, despite the latter's widespread use in interpreting basin histories. The type of field calibration required is exemplified by a comprehensive global analysis of the marine stratigraphic record from the basal Sevatian substage (Norian) to the Semicostatum zone (Sinemurian). This 20 Ma interval was originally selected because of discrepancies between Exxon' and previous eustatic curves, and for its sound biostratigraphic control. Four key areas were studied at outcrop, in varied paleogeographic and paleotectonic settings (Peru, Nevada, northwestern Europe and Pacific Canada). Fieldwork was supported by an extensive literature survey, including key areas of the eastern U.S.S.R. Interpretation involved depth assessment for each facies and the construction of bathymetric curves, which contrasts with the Exxon objective of recognizing depositional surfaces consistent with the seismostratigraphic model. A significant similarity between the 23 bathymetric curves created indicates eustatic control. The bathymetric data also constrain rates of global sea level change. Important global shallowing phases during Crickmayi zone and Angulata zone times were recognized in the study, consistent with the Exxon model. The Exxon perception of the Hettangian as a sea level lowstand is not supported and requires further discussion.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.