FOUR LATE MIOCENE FAULT SETS, DOWNEY EAST QUADRANGLE, PORTNEUF RANGE, IDAHO

The Downey East 7.5 min. quadrangle, covers the southwest side of the Portneuf Range and adjacent southern Marsh Valley, immediately north of Red Rock Pass, SE Idaho, approximately 50 miles south of the Snake River Plain. The area is within the hanging wall of the Paris-Putnam thrust fault. Bedrock strata include Neoproterozoic to Middle Ordovician siliciclastic and carbonate rocks of the Cordilleran miogeocline. Neogene basin fill of the Late Miocene to Pliocene Salt Lake Formation is a composite of lacustrine, volcaniclastic, fluvial, and alluvial sediments.

At least four Tertiary fault sets are recognized. The oldest set trends NW-SE, and is older than or coeval with deposition of the lower Salt Lake Fm (about 10 Ma). Younger NE-SW trending faults cut the Salt Lake Fm. and the older set, and may be related to Yellowstone Hot Spot tumescence or Snake River Plain subsidence. The third fault set includes range-front faults and trends NNW-SSE, orthogonal to the modern strain field. In the headwaters of Marsh Creek the main range-bounding, west-dipping fault steps leftward to the north, producing an anticline-syncline pair within the accommodation zone. The youngest fault set in the map area strikes east-west, cuts late Tertiary to early Quaternary sediments, and is likely a result of Quaternary stress transfer from the Portneuf Range-front fault to the more seismically active Oxford-Dayton fault to the south on the west side of Cache Valley.

Neogene basin fill (i.e. Salt Lake Fm.) mapped north, south, and east of the Downey area includes three distinct depositional environments: 1) Basal fanglomerate and debris flow, 2) low- energy ash-choked lake, 3) and moderate-energy alluvial fan to fluvial system. Miocene strata located in the Downey region and to the north and east may have formed as a single basin in the hanging-wall of the Bannock Detachment system, after 10 Ma.

Late Tertiary and Quaternary incision of Paleozoic bedrock and Tertiary basin-fill demonstrate drainage reversal. Quaternary-Pliocene boulder conglomerates fill south-flowing incised valleys of the Salt Lake Fm. and mantle strath terraces. Major element geochemical analysis of basalt boulders and orientation of paleo-valleys indicate a southward Plio-Pleistocene drainage direction toward the Bonneville basin in contrast to the modern northward drainage direction.