MIOCENE EXTENSIONAL BASINS IN THE ELK CITY REGION: BASIN AND RANGE FAULTING IN NORTH-CENTRAL IDAHO

Documenting the relationship between faulting and contemporaneous sedimentation offers crucial insight into the tectonic and geomorphic evolution of a region. In north-central Idaho, Tertiary sedimentary deposits have been well-characterized for their placer gold mining value, but little work has been done to characterize their tectonic significance. We present results from new detailed geologic mapping, structural analysis, LiDAR-derived imagery, and U-Pb zircon geochronology of these Tertiary sediments and related faults from the Elk City, Idaho area to argue that these sediments are likely Miocene and associated with Basin and Range extensional tectonism.

The tectonic setting for the Elk City area during Miocene fluvio-lacustrine sediment deposition is characterized by north-striking, high-angle, down-to-the-east normal faults which juxtapose unconsolidated sediments against Mesoproterozoic metasedimentary rocks of the Lemhi subbasin of the Belt-Purcell Supergroup and Late Cretaceous granitic rocks of the Atlanta lobe of the Idaho batholith. These sedimentary deposits thicken towards the basin-bounding normal faults, and consist of gravel, sand, silt, and mud displaying fluvial channels and cross-bedded bar deposits. We have identified two major fault-bounding basins in the Elk City area, which include the Elk City basin east of the Buffalo Gulch fault and the Newsome Creek Valley basin adjacent to the Newsome Creek fault. Additional smaller deposits are located adjacent to other north-striking unnamed structures. Due to the style, geometry, and magnitude of these faults, we propose that they are Miocene and possibly younger, and that the associated basin deposits represent the northern continuation of Basin and Range extensional faulting. Normal faulting would have occurred during the Cenozoic, producing small syntectonic half-graben basins in their hanging walls that intercepted Miocene paleo-valleys. We speculate that other historic high-level placer gold deposits in north-central Idaho such as those at Florence and Warren may also owe their existence to Miocene Basin and Range extensional tectonism.