Abstract: Epeirogeny, Lithospheric Flexure, and Differential Vertical Basement Movements in the Cretaceous Western Interior Basin, USA

Ming Pang, Dag Nummedal

Two-dimensional backstripping analysis and forward flexural modeling have been applied to a set of six regional stratigraphic transects across the U.S. Cretaceous Western Interior basin to determine its subsidence history. The transects were constructed from well logs and outcrop sections, and encompass strata of late Cenomanian to early Campanian age. This stratigraphic interval corresponds largely to the Greenhorn and Niobrara cycles and precedes the Laramide Orogeny.

Subsidence profiles derived by backstripping indicate that the tectonic subsidence consists of three distinct patterns or components. Over much of the Western Interior, from the Sevier thrust front to the eastern cratonic interior, there exists a regionally consistent "background" component of 150 to 200 m. Because of its large amplitude, and because all published sea level records show a net eustatic sea level fall from late Cenomanian to early Campanian, the "background" component is interpreted as epiorogenic subsidence rather than eustatic signals. Recent mantle convection models (Gurnis, 1988, 1990) suggest that such epiorogenic subsidence could result from changes in the dynamic topography of North American Plate during the Cretaceous. Superimposed on the epiorogenic subsidence re local differential subsidence which appears most pronounced in the Piceance Creek and Williston basins, and lithospheric flexure confined to the western part of the Western Interior. The observed differential subsidence occurred chiefly during the Niobrara cycle by localized vertical basement movements, but their causes remain uncertain to us. Lithospheric flexure, which has long been recognized as the dominant control on the Cretaceous Western Interior basin, displays considerable spatial and temporal variations. The flexural depression, subcircular in map view, was centered in central Utah during the Greenhorn cycle and shifted northward to western Wyoming during the succeeding Niobrara cycle. This suggests that thrusting activities were not uniformly distributed along the length of the Sevier belt. Forward flexural modeling suggests that the overall flexural subsidence pattern can result from flexure of a lithosphere with a elastic thickness of 80 (+/-10) km under spatially concentrated crustal loads.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994