ABSTRACT: Isolating the Effects of Thrust Belt Tectonism from Relative Sea Level History: An Example from the Cretaceous Western Interior

Virginia S. Hill, Paul L. Heller, William E. Frerichs, Charles Angevine

The major controls on foreland basin stratigraphic sequence development are changes in rate of subsidence, sea level fluctuation, and sediment supply. The relative influence of tectonics and eustatics in foreland basins can be discriminated by analyzing relative subsidence across the basin using detailed biostratigraphic control. Theoretical considerations predict that thrust belt tectonism will result in rapid, asymmetric basin subsidence. Subsidence can be synchronous within the flexural wavelength of the basin and time transgressive in the more distal parts of the foreland as the wave of sediment filling moves across the basin. In contrast, eustatic events should occur penecontemporaneously throughout the basin and result in progradation of tabular stratigraphic sequen es.

Age and paleobathymetric data determined from foraminifera of late Cenomanian to early Campanian marine deposits from Utah, Wyoming, Colorado, and Kansas provide the framework for reconstructing and correlating subsidence histories from the Sevier belt across the adjacent foreland basin and on to the stable craton. Comparison of cumulative differential subsidence curves shows that despite a shared pattern of seven water-deepening events, only the westernmost sections show the characteristic signal of foreland basin subsidence. Those events that occurred during a time of regional asymmetric subsidence may be related

to thrust belt tectonism. Only one event shows a change large enough to require eustatic controls. Other events are probably related to eustatic changes or, less likely, secular changes in sediment supply rates. Regardless, the longer term (< 10 m.y.) tectonic signal can be clearly separated from the shorter term (^sim 2 Ma) water-deepening events.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990