NEW CONSTRAINTS ON THE INTERPRETATION OF LOW-ANGLE NORMAL FAULTS IN SOUTHEASTERN IDAHO

Gently dipping normal faults in the southern Bannock Range of SE Idaho have previously been interpreted as evidence for a regional detachment system originating and slipping at a low inclination on the basis of geometrical relations between faults and bedding in lacustrine sediments of the upper Miocene to lower Pliocene Salt Lake Formation (Carney and Janecke, 2005). New field data from three key locations lead to a different interpretation. 1) Measurements in the Clifton basin (Clifton 7.5’ quadrangle) indicate that beds dip consistently to the SE at ~32°, with a cut-off angle against the west-dipping Clifton fault of ~44°. These data are inconsistent with the existence of the Clifton basin anticline of Carney and Janecke (2005). 2) In the Pocket basin ~2 km to the NW, bedding in the Salt Lake Formation is more variable but inclined generally to the east into the west-dipping Pocket basin fault. We find no evidence for the Pocket basin syncline of Carney and Janecke (2005) or for inferred near-concordance of bedding with the fault surface in the vicinity of the fault. Our new bedding data are based upon a series of shallow pits excavated to avoid surficial deformation – which is widespread in these fine-grained deposits, particularly on slopes. Fault orientations were determined from a series of points of known elevation at which fault location is well established in published mapping. 3) A contact mapped as the Clifton fault on the west side of Rattlesnake Ridge (Steely et al., 2005; Weston Canyon 7.5’ quadrangle) is tentatively reinterpreted as a west-dipping Miocene unconformity, offset in many places by moderately to steeply dipping normal faults that were active after and not during deposition of the Salt Lake Formation. While our data do not preclude the existence of a regional detachment fault at depth, they are well explained by Miocene and younger domino-style normal faulting. Cretaceous backthrusting is hypothesized to account for locally large stratigraphic separations, including younger-over-older relations in Neoproterozoic and Cambrian rocks at Rattlesnake Ridge. The complex detachment model envisaged in earlier work is not required. Low-angle normal faults in SE Idaho differ appreciably from the Sevier Desert detachment of west-central Utah, the interpretation of which remains unresolved.