GEOLOGY OF DOWNEY WEST QUADRANGLE: BANNOCK COUNTY, IDAHO

The Downey West 7.5'quadrangle is located within the eastern Basin and Range province in southeastern Idaho. It was mapped under the USGS EdMap program. The quadrangle is bordered on the south and west, respectively, by uplifted Neoproterozoic strata of Oxford Peak and Wakley Peak. Most of the map area is composed of Quaternary basin fill. Alluvial fans fill the southern and western borders of the map, with a small, yet complicated, section of Paleozoic rocks (part of the Bannock Range) exposed in the southwest corner of the map.

The basin fill is composed of a mixture of Early to Late Pleistocene paludal, alluvial and eolian deposits covered locally by 14.5 ka Bonneville flood gravels. Marsh, distal alluvial and loess sediments interfinger along the southern edge of the Marsh Creek floodplain. These sediments also include a thick undated ash layer. Miocene-Pliocene Salt Lake formation rocks are found only near the frontal fault of the Bannock range.

The Cambrian carbonate rocks exposed in the southwest corner dip west and are cut by low-angle, west-dipping, older normal faults and east-dipping, younger normal faults that may cut Quaternary alluvial fans. Southern Marsh Valley is the deepest portion of the valley. An accommodation zone may exist that transfers down-to-the west movement from the east side of Marsh Valley to the east side of Malad Valley to the southwest, and the west side of Cache Valley to the south.