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The depositional histories of three piggyback basins are related to the surface and subsurface thrust geometries that controlled early Eocene basin development during the final phase of shortening in the Idaho-Wyoming-Utah thrust belt. Early Eocene strata of Bear Lake Plateau basin, Fossil Basin, and the LaBarge basin occupy similar structural positions on the Absaroka, Crawford, and Hogsback thrust sheets respectively. The basin depocenters formed above flat décollement surfaces of underlying thrust sheets. Trailing basin margins formed above the trailing footwall ramps of the thrust sheets that immediately underlie the basins. Leading basin margins formed along the leading edges of thrust sheets that are underlain by the ramps of structurally lower thrusts. Sedimentary facies distributions and sediment dispersal patterns were controlled by uplift of the basin margins rather than by subsidence of depocenters. Periods of basin margin uplift are identified by the presence of alluvial fan sediments shed from the uplifted margins, patterns of onlap and overstep along and across the margins, and cross-cutting structural relations of folded or faulted early Eocene strata along the basin margins. Uplift was accomplished through reactivation of slip along the trailing footwall ramps of the Crawford and Absaroka thrusts, late slip on the Hogsback trailing ramp, and slip on the LaBarge and Calpet thrusts in the Hogsback footwall. The late thrust slip in the interior of the thrust belt was a type of break back, or out-of-sequence thrust slip, that deviated from the previous foreland-directed sequence of thrusting.

Reactivated thrust uplift and footwall uplift are the two uplift processes common to the three basins studied in this report. Reactivated thrust uplift was generally confined to the trailing ramp areas of thrust sheets, where slip along the trailing ramp produced ramp-rooted imbricate thrusts and fault-propagation folds. Footwall uplift was confined to the leading basin margins, where the basin margins and the leading edges of the thrust sheets beneath them were translated up the ramps of structurally lower thrusts. The reactivation of thrusts beneath piggyback basins across the width of the Wyoming-Idaho-Utah thrust belt is attributed to impeded slip along the frontal thrusts of the belt. Slip impedance along the thrust front is attributed to the interaction of four processes: buttressing of the frontal thrusts by coeval foreland basement uplifts, fault deflection above preexisting foreland basement warps, decrease in regional wedge taper from the combined effects of foreland subsidence and thrust belt erosion, and rheologic changes related to stratigraphic changes toward the foreland.

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