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Stratigraphic and structural relations in trench exposures and geomorphology at the Big Burn, Lily Lake, and Lester Ranch sites, Bear River Fault Zone, Utah and Wyoming

April 12, 2019

This report provides trench photomosaics, logs and related site information, age data, and earthquake event evidence from three paleoseismic trench sites on the Bear River Fault Zone. Our motivation for studying the Bear River Fault Zone—a nascent normal fault in the Rocky Mountains east of the Basin and Range physiographic province—is twofold: (1) the intriguing conclusion from previous work that the neotectonic history of the fault may have begun in the middle to late Holocene and consists of only two surface-rupturing earthquakes and (2) the question of whether large scarps (>10 meters in height) observed along the fault represent net tectonic displacement, which, given a two-event history, would put the displacements among the largest in the Basin and Range region. In presenting our trench and initial geomorphic interpretations, this report lays the groundwork for further exploration of these issues.