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 (Berg's "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 source 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.

Like many thrust-derived conglomerates, the Beartooth Conglomerate contains intraformational, syndepositional folds, faults, and angular unconformities that can be used with provenance modeling to define the kinematics of uplift during conglomerate deposition. By utilizing the results of our provenance modeling and the orientations of the basal and intraformational angular unconformities, we have been able to retrodeform stepwise an area-balanced cross section through the eastern Beartooth uplift. Retrodeformation of the cross section indicates that 3.8 km of uplift occurred before deposition of the oldest part of the proximal Beartooth Conglomerate. Further uplift of ∼1.7 km and total horizontal shortening of ∼3.7 km accompanied ∼1.3 km of displacement on the Beartooth fault. Cross sections indicate that additional uplift of ∼1 km and horizontal shortening of ∼2 km accompanied displacement on the Line Creek fault. Our data indicate that during the early stages of uplift by fault-propagation folding, sediment derived from the uplift bypassed the proximal realm and was deposited in distal settings. Bypassing occurred because the eventual footwall of the Beartooth fault was uplifted during fault-propagation folding. Viewed from this perspective, the early fine-grained fills of many Laramide basins may owe their existence to a combination of fine-grained source material and sediment bypassing during the early stages of Laramide uplifts.

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