PLIOCENE DEVELOPMENT OF MODERN BASIN AND RANGE TOPOGRAPHY REVEALED BY A DISMEMBERED MIOCENE-PLIOCENE RIFT BASIN, SOUTHEAST IDAHO

Geologic mapping in the Clifton 7.5' quadrangle in southeastern Idaho indicates that the modern Basin and Range topography and modern stress field are only a few million years old and were superimposed on unrelated rift basins associated with the Bannock detachment system. The Miocene-Pliocene Salt Lake Formation in the greater Cache Valley area was deposited in a large rift basin(s) in the hanging wall of the Bannock system of low-angle normal faults. The Salt Lake Formation is exposed on the east and west sides of the younger Clifton horst block in depositional contact with lower Paleozoic carbonate rocks, and in the Clifton horst block in low-angle fault contact with Neoproterozoic bedrock.

Tephra correlations from approximately 20 locations in the adjacent Malad City East quadrangle show that deposition of the Salt Lake Formation began more than 10.27 Ma and continued after 4.5 to 5.1 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 yet no detritus of the Pocatello Formation has been identified in the Salt Lake Formation. (2) Similar facies and depositional systems characterize the tuffaceous Salt Lake Formation on either side of the horst. Early alluvial fans derived from the underlying Paleozoic rocks were replaced by saline/alkaline lakes, freshwater lakes, and delta environments with final deposition in braided streams. (3) Uplifted Pliocene-Quaternary pediment gravels indicate >600 m of throw on the east side of the horst after their deposition.

We document a major phase of sedimentation in the hanging wall of the Miocene-Pliocene, WSW-dipping low-angle normal faults, followed by structural disruptions of the older rift basin(s) during a separate younger phase of faulting along N-S striking high-angle normal faults. The anomalous young age (4.4 to 5.1 Ma) of the modern Basin and Range topography in the Cache Valley region may indicate outward migration of the Basin and Range province.