Stratigraphy and Tectonic Significance of the Tunp Conglomerate in the Fossil Basin, Southwest, Wyoming

Abstract

The Tunp Member of the Wasatch Formation in the Fossil basin of southwestern Wyoming was deposited by debris flows containing abundant, very poorly sorted to unsorted coarse debris in a mudstone matrix. Deposition occured on alluvial fans where small braided streams reworked the toes of the debris flows generating minor fluvial deposits. Tunp sediments are preserved in three separate north-south trending belts adjacent to the northern Fossil basin and deposits in each belt had separate sources in discrete highlands. The Tunp on Commissary Ridge was generated by passive uplift of the Absaroka sheet over a ramp in the Darby thrust. The deposits on Rock Creek and Dempsey ridges are related to motion on the Tunp thrust. The Tunp on Boulder Ridge was derived from the hanging wall of the Crawford thrust but it is not clear whether this was a passive or active source. Stratigraphic relations of these Tunp deposits with dated beds in the Wasatch and Green River formations indicate that the belts of Tunp are successively older to the west and that passive rotation of the Absaroka sheet preceded movement on the Tunp thrust which, in turn, preceded the generation of a source on the Crawford. Deposition of the Tunp therefore records a sequence, progressively younger to the west, of minor adjustments to compression during the last phase of thrusting.