Abstract

Geologic mapping, basin analysis, and tephra correlations in the Clifton and Malad City East 7.5-minute quadrangles in southeastern Idaho indicate that the modern Basin-and-Range topography is only a few million years old and was superimposed on unrelated rift basins associated with the ~12 to < 4 Ma Bannock detachment system. The Miocene-Pliocene Salt Lake Formation in the greater Cache Valley area was deposited during three sub-episodes of west-southwest extension on the Bannock detachment system.

Depositional systems within the Salt Lake Formation (>10.27 ± 0.07 Ma to < 5.1-4.4 Ma) evolved in response to the changing structural configuration of rift basins in the hanging wall of the Bannock detachment system. Early alluvial fans derived from the underlying Paleozoic rocks were replaced by broad, saline/alkaline lakes during the translation phase on the detachment fault. Later, as the hanging wall broke up internally freshwater lakes and deltas occupied newly created NE-tilted half graben, and eventually filled them with braided streams. The Bannock detachment system collapsed the Cache-Pocatello Culmination of the dormant Sevier fold-and-thrust belt, much like the Sevier Desert Detachment collapsed the Sevier Culmination and the Wasatch Fault collapsed the Wasatch Culmination.

Deposition and tilting of the Salt Lake Formation ceased in the middle to late Pliocene (<2 Ma?) before uplift of the Clifton Horst along a new system of Basin-and-Range normal faults. This later episode of normal faulting uplifted and exposed metamorphic rocks in the footwall of the Bannock detachment system for the first time, and resulted in the deposition of < 200 m of Pliocene-Pleistocene(?) piedmont gravel and conglomerate deposits in angular unconformity on the exhumed bedrock of the horst and the adjacent, down-faulted Salt Lake Formation. External drainage may explain the sparse sedimentary record of Basin-and-Range faulting in this area.

The young age of both the large-magnitude extension (starting before >10.27 to and ending after 5.1 or 4.4 Ma) and of the modern Basin-and-Range topography (developing after 4.4 to or 5.1 Ma, probably 2 to 3 Ma and younger) in the Cache Valley region supports recent analyses that show both westward and eastward younging of extension from the central Basin-and-Range province. The synrift deposits in southeast Idaho illustrate a common evolutionary sequence in the Basin-and-Range province: 1) Deposition in broad basins primarily during early large-magnitude extension along listric low-angle normal faults, 2) formation of smaller half graben, reworking of older strata and continued deposition during break-up of the hanging wall of the low-angle normal fault along closely spaced normal faults, 3) and subsequent small-magnitude extension along steeper, widely spaced, N-striking range-front faults that dismember the earlier detachment system.