Influence of Recurrent Laramide Structural Growth on Sedimentation and Petroleum Accumulation, Lost Soldier Area, Wyoming

Abstract

Recurrent anticlinal growth during Late Cretaceous and early Tertiary (Laramide) time in the Lost Soldier area of south-central Wyoming profoundly affected local patterns of sedimentation, erosion, and petroleum accumulation. Uplift was accomplished by folding at Lost Soldier and by arching in the adjacent Granite Mountains area. During earliest Campanian time, shale beds thinned depositionally across the rising areas. Abnormally thick sequences of sandstone beds accumulated across shoals of structural origin at Lost Soldier, during both early Campanian and earliest Maestrichtian time. But during intervening late Campanian time, two episodes of abrupt local anticlinal folding resulted in erosional truncation of successively older beds beneath unconformities. Strong pulses of uplift in Maestrichtian and middle Paleocene time produced angular unconformities adjacent to the Granite Mountains uplift, gentle arching at Lost Soldier, and increasing amounts of arkosic sediment in the adjacent basin, culminating during the early Eocene in uplift and a flood of arkosic debris. Subsequent folding in late Eocene time segmented the ancestral Lost Soldier anticline, and a culminating pulse of mountain-flank thrusting followed, probably in early Oligocene time. Episodes of deformation lasted approximately 1-2 m.y., whereas intervals of quiescence and subsidence lasted from 2.5 to 5 m.y. Subsidence progressed fairly uniformly, except for sharp increases during Maestrichtian and early Eocene time; amounts of uplift on structures were nearly uniform during each pulse until latest Paleocene time when uplift of the Granite Mountains accelerated. Throughout Laramide time the ancestral Lost Soldier anticline continuously occupied an intermediate structural position between the Great Divide basin and the Granite Mountains uplift. By middle Paleocene time, successive pulses of deformation had developed nearly sufficient closure on the ancestral anticline to trap as much petroleum as is now in the area. Later folding during early late Eocene time increased the closure and segmented the ancestral fold. Folds formed later, possibly during late Tertiary and Quaternary time, are barren of petroleum. New prospects for exploration in the Great Divide basin can be inferred from the facies changes, variations in patterns of sedimentation, and relation of beds at unconformities, all resulting from recurrent structural growth in the Lost Soldier area.