Flexure and Sedimentation in Sevier Foreland: Part 3, a Model for Distal Foreland Sedimentation, Big Horn Basin, Wyoming

N. S. Winslow, P. L. Heller

The initiation of thrusting in the Sevier orogenic belt is recorded in the distal part of the adjacent foreland basin by deposition of fine-grained sediments, not by conglomerates. Lower Cretaceous nonmarine deposits in the Big Horn basin consist of mudstones, limestones, and sandstones of the upper Cloverly Formation and conglomerates of the basal Cloverly Formation. These conglomerates are part of an extensive network of fluvial gravels spread throughout the Western Interior. Flexural and paleohydraulic modeling has shown that it is difficult to derive the basal Cloverly conglomerates in the Big Horn basin from the Idaho-Wyoming thrust belt to the west. Instead, the fine clastics of the upper Cloverly Formation document thrust-induced flexural subsidence during the Apti n to Albian.

Evidence for a west-dipping slope along the distal margin of the foreland basin is found within the Greybull Sandstone Member of the upper Cloverly Formation. The Greybull sandstone was deposited by streams flowing from the craton, across central Wyoming and into the foreland trough to the west.

A model of foreland-basin sedimentation can be distilled, in which coarse-grained clastics are localized near the thrust front whereas fine-grained detritus fills the bulk of the basin. Flexural bending of the distal foreland region creates a thrustward-dipping slope along which fluvial sediments may be transported toward the thrust front. Hence, the initiation of Sevier foreland-basin sedimentation is not recorded by the first occurrence of western-derived conglomerate. Instead, nonmarine mudstones, limestones, and eastern-derived fluvial sandstones represent the earliest distal sedimentary response to the Sevier orogeny.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.