Abstract

Structural and stratigraphic analysis of the Marsh Valley, Lava Hot Springs, and Wakley Peak areas south of the Snake River Plain in southeast Idaho is based on geologic mapping, stratigraphic sections, gravity maps and models, reflection seismic data, and well control. Interpretation of the data suggests that Neogene extension occurred in two phases. Phase 1 extension started about 10 Ma, creating east-dipping tilt blocks and half grabens filled with the late Miocene to Pliocene upper member of the Salt Lake Formation. These are bounded by high- to low-angle west-dipping normal faults interpreted to be part of the regional top-to-the-west Bannock detachment fault system. Diverse mafic to felsic volcanic activity accompanied Phase 1 extension, including the 7.3 Ma Yago Creek basaltic eruptive center and 6.6–6.0 Ma rhyolite domes. Phase 2 extension started after 4 Ma and resulted in widely spaced moderate- to high-angle Basin and Range normal faults, range-block uplifts, and large basins such as Marsh Valley. Phase 2 extension was accompanied by 3.3 Ma mafic volcanism in the Wakley Peak area.

The Dempsey Creek subbasin (DCSB) in the Lava Hot Springs area formed before 9.3 Ma above the Bannock detachment and continued filling with sediment after 7.0 Ma. It was filled with <1.3 kilometers of mostly footwall-derived fluvial and lacustrine sediments. Interpretation of the gravity and reflection seismic data suggest that during and after its formation, the DCSB was cut by moderately west-dipping normal faults.

Phase 1 faulting in the north-south trending Marsh Valley and surrounding ranges began after 10 Ma as breakup of the upper plate of the Bannock detachment fault progressed westward. In Marsh Valley, this extension opened a basin that contains as much as 1.5 km of the upper member of the Salt Lake Formation, and formed the east-tilted Bannock Range block. North of an east-west trending Lava Hot Springs transfer zone, Marsh Valley is narrower, and experienced less extension and subsidence.

Gravity data indicate that several east-dipping half-grabens occur beneath Marsh Valley. One geophysical interpretation suggests that west-dipping normal faults bounding the half grabens sole into the Bannock detachment fault at a depth of 2.5 kilometers or less. Together, these normal faults and the detachment accommodate as much as 12 kilometers of late Miocene extension in central and southern Marsh Valley. The greatest extension occurred along a single secondary breakaway zone west of Arimo Ridge and the southern Portneuf Range. A second geophysical interpretation either does not require a detachment fault or indicates it is deeper, and suggests that late Miocene extension in Marsh Valley totaled less than 2 kilometers. In this scenario, the basin is slightly shallower than in the first model, and the Salt Lake Formation does not lie in fault contact with the Bannock detachment. Based on geologic mapping in the ranges surrounding Marsh Valley though, a detachment scenario is favored.

During Phase 2 extension, moderate- to high-angle Basin and Range faults cut the Bannock detachment fault and uplifted the western and possibly eastern flanks of Marsh Valley to form the modern Bannock and southern Portneuf Ranges respectively. Pliocene to Pleistocene subsidence along these faults created a west tilted half-graben that filled with as much as 1.0 to 1.5 kilometers of fine-grained fluvial-lacustrine sediment and fanglomerate comprising the formation of Marsh Valley. Phase 2 extension along several east-striking normal faults resulted in progressively greater subsidence southward in Marsh Valley.