Tectonic Significance of Paleocene Alluvial Sequence, Clark's Fork Basin, Wyoming-Montana

Abstract The Clark's Fork Basin developed in response to Laramide deformation along the east-verging Line Creek-Beartooth Thrust complex during Paleocene time. This basin is a westwardly asymmetric syncline in which subsidence and progressive westward migration of its structural axis was driven by tectonic loading. The Willwood Formation which is exposed along its orogenic margin was deposited in response to structural development of this basin. Deposition of a Torrejonian fluvial interval at the base of the Willwood Formation was controlled by the northwest-trending Bennett Creek Fault. Torrejonian sedimentation is contemporaneous with major Beartooth mountain-flank deformation, although the development of mature paleosols indicates that thrust-induced subsidence did not exceed sedimentation. Basal Willwood sediments interfinger with distal equivilents of Fort Union Formation quartzite conglomerates to the south. The appearance of coalesced alluvial fan deposits in the Tiffanian marks the development of sharp fault-generated relief maintained by continued deformation along the Beartooth Thrust. These hanging-wall-sourced fans built transversely off the eastern flank of the nascent Beartooth Uplift during Tiffanian to Clarkforkian time. This synorogenic package displays intraformational unconformities, unroofing sequences and megacyclicity and exhibits a rotative offlap relationship indicating accelerated basin-margin uplift. Several line-sourced fluvial packages onlapped tilted alluvial fan sequences during Clarkforkian time. Middle Clarkforkian fluvial units depositionally thin against the blind thrust culmination structure of the Line Creek Thrust and thicken in a paleovalley developed in subjacent fan sediments. In the late Clarkforkian, the north-flowing longitudinal basin drainage system onlapped the southernmost fan sequence.