Surficial Geologic Map of the West Cache Fault Zone and Nearby Faults, Box Elder and Cache Counties, Utah

Map 172, 1999, 20 pages, 2 plates, $6.95

ABSTRACT

The West Cache fault zone (WCFZ) extends about 56 kilometers (35 mi) along the west side of Cache Valley in northern Utah, and another 37 kilometers (23 mi) into southern Idaho. I mapped about 9 1 0 square kilometers (350 mi2) in northern Utah, including the WCFZ and three other nearby Quaternary faults. Mapping was compiled at a scale of 1:24,000 and is published at a scale of 1:50,000.

The Quaternary geology of the region is dominated by late Pleistocene deposits of Lake Bonneville. The lake lasted from about 30,000 to 12,000 14C yr B.P., but the lake occupied Cache Valley in northern Utah from only about 25,000 to 13,000 14C yr B.P. Static levels of Lake Bonneville are recorded by shoreline complexes which are useful datums for evaluating late Pleistocene structure and stratigraphy. I mapped 9 Lake Bonneville surficial units and 2 major Lake Bonneville shorelines, as well as 24 other Quaternary geologic units, 6 pre-Quaternary bedrock units, and several minor Lake Bonneville shorelines.

Three normal faults are recognized in the WCFZ of Utah, and they are, from north to south, the Clarkston, Junction Hills, and Wellsville faults. These three faults are geographic divisions of the WCFZ, although evidence suggests that the Clarkston fault is a seismically independent structural segment.

Quaternary normal faults in three other adjacent areas may also be associated with the WCFZ, but they are not included because of their lack of demonstrable continuity with the WCFZ and lack of paleoseismic data. These nearby faults are, from north to south, the Dayton, Hyrum, and Mantua faults. Together, the WCFZ and nearby faults form a distinctive right-stepping, en echelon pattern in the western half of Cache Valley in northern Utah and southern Idaho.

Detailed surficial geologic mapping of the WCFZ and nearby faults in this study identified stratigraphic and structural relationships not described in previous mapping. Although data suggest a late Pleistocene age for most recent displacement along the faults, surficial evidence for the age of surface faulting in some locations is ambiguous. No conclusive evidence of Holocene faulting was found.

I have therefore proposed several trench locations to help determine the size and timing of earthquakes along the WCFZ and nearby faults. Correlations of events on various sections of the WCFZ and nearby faults will be used to help assess fault continuity, segmentation, and rupture patterns.