BASIN STRUCTURE OF PROPOSED LATE MIOCENE TO PLIOCENE SUPRADETACHMENT BASINS IN SE IDAHO BASED ON DETAILED GRAVITY AND GEOLOGIC DATA

Gravity methods are used to constrain the structural architecture and geologic history of two Late Miocene to Pliocene basins, the Deep Creek half-graben and Cottonwood Valley, southeast Idaho. These basins have been interpreted by previous authors to have developed above the regional Bannock detachment fault, then modified by younger high angle Basin and Range block faulting. This study addresses the internal structure of basins filled with the Salt Lake formation and formed above detachment faults or multistage rotated normal faults. Cottonwood Valley is interpreted to have formed at the breakaway of the proposed detachment fault while the Deep Creek half-graben formed 25 km to the west of the breakaway zone but still over the proposed detachment fault.

Gravity profiles were acquired across the two basins with a 0.32 km station spacing to create gravity maps and interpretive cross sections of the gravity profiles based on 2 ½-D models constrained by previous geologic mapping. Previously mapped geology and structural interpretations of the Deep Creek half-graben correlate well with the gravity, with the exception that basin depth varies from the original cross-sections. The original cross-section has a basin depth of 0.8 km while a maximum basin depth of 1.3 km was modeled with half-graben normal fault dips above the proposed detachment ranging from 40 to 65 degrees across the basin. The previously interpreted structure of Cottonwood Valley suggests the presence of a 4 km deep basin formed above a single listric normal fault that changes from a dip of 80 degrees at the surface to nearly flat at 4 km. This fault is interpreted as the breakaway zone for the Bannock detachment fault. The gravity data does not support this interpretation, but suggests that Cottonwood Valley formed above several normal faults with dips ranging from 45 to 75 degrees and a maximum basin thickness of 1.1 km. There does not appear to be one single fault that acts as the breakaway. A large gravity low beneath mapped lower Paleozoic and Neoproterozoic exposures along the southern Portnuef Range in eastern Cottonwood Valley indicates a low-density body in the subsurface beneath the high-density units exposed at the surface. This low-density body is interpreted as upper Paleozoic through Mesozoic units below a ramp in the Paris thrust.