POST-LARAMIDE HISTORY OF NORTH-CENTRAL WYOMING
Abstract
North-central Wyoming and surrounding areas have been the subject of geological investigations for more than a century. However, the presently accepted history of the region has not been revised in relation to changes in the science of geology. This study evaluated the assumptions on which this history was based and developed a new set of assumptions in keeping with recent advances. Assumptions previously used include: (1) physiographic and orogenic mountains originate at the same time and from the same processes; (2) all significant deformation occurred during the Laramide Orogeny; (3) all streams transverse to structure were superposed from the highest level of basin fill; and (4) the Subsummit Peneplain is a pediment and represents the maximum level of basin fill. I assumed that (1) physiographic mountains are not necessarily as old as the orogenic structures they contain; (2) significant deformation has occurred in post-Laramide time as an effect of the Yellowstone Hotspot; (3) any explanation of transverse streams must be documented by analysis of regional geological data; and (4) much of the Subsummit Peneplain is a composite of non-cyclic surfaces and therefore has no relation to the level of basin fill. The Subsummit Peneplain on the east flank of the Bighorn Mountains is a planation surface created in a subtropical climate in late Eocene time. It marks the level of Eocene basin fill. Slow regional uplift and net erosion occurred during Oligocene through early Pliocene time. Relatively minor deposition of air-fall ash punctuated this erosion but was rapidly removed by streams. Late Pliocene-early Pleistocene uplift associated with migration of the Yellowstone Hotspot and climatic change created the details of the present landscape. Thus, much of the landscape is a relic of Eocene time preserved by the ineffectiveness of more recent tectonic and climatic processes of landform evolution.
Degree
Ph.D.
Subject Area
Geology
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