Pliocene Development of Modern Basin and Range Topography Revealed by a Dismembered Miocene-Pliocene Rift Basin, Southeast Idaho

S. M. Carney
Utah State University, Dept. of Geology, Logan, UT

Geologic mapping of the Clifton 7.5 ft quadrangle in SE Idaho indicates that the modern Basin and Range topography and strain field are only a few million years old and were superimposed on unrelated rift basins associated with the 12 to 4 Ma Bannock detachment system. The Miocene–Pliocene Salt Lake Formation was deposited during WSW-ward directed extension on the Bannock detachment system. Piedmont gravels were deposited after the initial uplift of the modern topography and after deposition and deformation of the Salt Lake Formation.

Tephra correlations from 22 locations in the study area show that deposition of the Salt Lake Formation occurred >11.8 Ma and <4.4 Ma. Several observations indicate that uplift of the N-trending Clifton horst postdates deposition of the Salt Lake Formation. (1) The horst exposes Neoproterozoic Pocatello Formation from the footwall of the Bannock detachment system, yet no detritus of the Pocatello Formation has been identified in the Salt Lake Formation. (2) Similar depositional facies characterize the tuffaceous Salt Lake Formation on either side of the horst. (3) An angular unconformity at the base of the Pliocene-Pleistocene piedmont gravels reveals erosion of the Tertiary Salt Lake Formation, lower Paleozoic to Neoproterozic rocks, and, locally, Neoproterozoic Pocatello Formation prior to deposition of the gravels. (4) Uplifted piedmont gravels indicate >600 m of throw on the east side of the horst after their deposition.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid