The Surficial Geology and Neotectonics of Hansel Valley, Box Elder County, Utah

Hansel Valley, located at the north end of the Great Salt Lake in Box Elder County, Utah, has exposures of the lacustrine sediments of the Little Valley, Bonneville, and Gilbert lake cycles. A 1:50,000 scale map was constructed of the surface geology. Although no trenches were dug for this study, about 240m of logs were compiled in an arroyo in lake bottom sediments. Sediments from at least three lake cycles were found in this gully: 1) compact bottom deposits from the Little Valley cycle; 2) bottom sediments from an intermediate cycle (the Hansel Valley cycle); and 3) beach gravel and bottom sediments from the Bonneville cycle. Evidence gathered indicates that a previously undescribed lake, the Hansel Val ley cycle, which reached a maximum elevation of about 1342 m (4400 feet). Thermoluminescence (TL) dating, supplemented by ostracode identification and stratigraphic position, dates the Hansel Valley cycle at about 80 ka (late Oxygen Isotope Stage 5). Hansel Valley is seismically very active and the site of the largest and only historic earthquake to rupture the ground surface in Utah. Scarp heights up to 50 em were measured from the 1934 M6.6 event, which was contiguous with an older 6 km long scarp that crosses Lake Bonneville recessional shoreline s. Scarp heights range from 1.6 m to 9.0 m and control recessional shorelines (instead of simply displacing them). This morphologic evidence suggests that a portion of the scarp was formed underwater, and that slumping occurred along the trace of the fault. The fault scarp intersects the gully mentioned above and reveals highly fractured sediments with 11 main faults within a 240 m zone. Movement on individual faults ranges from 0.1 to 2.5 m, with a net displacement of 1.3 m down to the east, which agrees with the offset measured on the scarp on both sides of the gully. Most faults offset Little Val ley, and transgressive Bonneville shoreline sediments, but are not continuous through intensely convoluted Bonneville lake bottom sand, silt and clay. Two units of Bonneville bottom sediments show convolutions, features and slump blocks. roll structures, liquefaction Fault scarps, liquefaction features, and subsurface faults indicate one pre-BonneviIIe, possibly two Bonneville, and one post-Bonneville-age large earthquakes