GEOMETRY AND EVOLUTION OF SELECTED NEOGENE BASINS IN THE BASIN AND RANGE PROVINCE OF SOUTHEAST IDAHO BASED ON GRAVITY, SEISMIC, AND GEOLOGIC DATA

Geologic mapping, well control, and seismic data are combined with over 1400 new and 2253 existing gravity stations to develop a detailed picture of the structure, stratigraphy and evolution of Marsh Valley, northern Cache Valley, and surrounding basins. Gravity maps and 2 ½ D forward gravity models constrained by the other data indicate two distinct phases of extension in the study area: Phase 1 (about 10 - 5 MA) and Phase 2 (about 4 - 2? MA).

Phase 1 normal faulting and deposition of the late Miocene to Pliocene Salt Lake Formation was probably controlled by the underlying, west-dipping Bannock detachment fault system. This fault forms a breakaway zone above a ramp in the Paris thrust within Cottonwood Valley, continues southward into Utah, and northward along the eastern margin of the Dempsey Creek subbasin near Lava Hot Springs. The upper plate of the Bannock detachment remained relatively intact during the early stages of extension, allowing coarse grained Salt Lake Formation deposits to be overlain by extensive lacustrine deposits. Later extension of the upper plate was not concentrated at the breakaway zone, but was distributed throughout the plate, breaking it into numerous, mostly east to northeast-tilted half grabens that resulted in deposition of the Salt Lake Formation within more isolated subbasins. Maximum Phase 1 basin depths modeled by gravity are 2.5 km or more in southern Marsh Valley, and 1.1 to 1.3 km in surrounding basins.

Phase 2 moderate to high angle normal faulting, which produced the topographic basins and ranges seen today, was accompanied by deposition of Pliocene to possibly Pleistocene sediments. Phase 2 extension reactivated Phase 1 faults or created new faults that cut them, causing widening and large scale subsidence of Marsh Valley towards the south, and separation of Marsh Valley from northern Cache Valley due to uplift of Oxford Ridge. Phase 2 faults, when combined with Phase 1 faults, created several grabens within Marsh Valley and accommodation zones between Marsh Valley and northern Cache Valley. Near the western and southern margins of Marsh Valley, but not directly adjacent to the range front, gravity data indicate that Phase 2 basins are as much as 1.5 km deep. This suggests that the range fronts have retreated since the deep basins were formed, or younger faults have widened Marsh Valley.