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Mary Beth Gray

Associate Professor of Geology

 

 

Kinematic history of a foreland uplift from Paleocene synorogenic conglomerate, Beartooth Range, Wyoming and Montana

 

P.G. DeCelles, M.B. Gray, K.D. Ridgway, R.B. Cole, P. Srivastava, N. Pequera, D.A. Pivnik, Department of Earth and Environmental Sciences, University of Rochester, Rochester NY 14627

 

 

Abstract

An integrated structural, sedimentological, and provenance study of the upper Paleocene Beartooth Conglomerate along the eastern flank of the Beartooth Range in Montana and Wyoming yields new information about Laramide tectonics in the northern Rocky Mountain foreland and has implications for models of fault-propagation folding and the development of synorogenic basins in thrust-faulted terranes. During the Laramide orogeny, the Beartooth Conglomerate was deposited by alluvial fans and a coarse braided fluvial system along the eastern flank of the range in response to displacement on the Beartooth fault, a 30¡-35¡ west-dipping thrust fault that places Precambrian rocks on Paleozoic and Mesozoic cover rocks. Displacement along the Beartooth fault produced a large fault-propagation fold (BergÕs "fold-thrust") that eventually was transected by the fault along the entire eastern flank of the range. Lithologic provenance modeling indicates that the Beartooth Conglomerate was produced by unroofing of the Upper Cretaceous through Precambrian section of the eastern Beartooth Range. The model results indicate that source-area relief during deposition of the lower part of the Beartooth Conglomerate was relatively low (~600 m) while the soft, mudstone-dominated Upper Cretaceous part of the course section was being eroded from the crest of the uplift. Coarse, alluvial-fan accumulation commenced when resistant Paleozoic carbonate and quartz-arenitic rocks were exposed and source-area relief increased to ~1,400 m. Relief steadily diminished thenceforth to ~600-800 m, as the upper part of the source section was eroded from the Beartooth Range, leaving only a resistant carapace of Paleozoic carbonate rocks and Precambrian crystalline rocks. Source-area relief was controlled both by bedrock composition of the source terrane and by individual episodes of uplift.