A POTENTIAL SEISMIC ORIGIN FOR LATE HOLOCENE FLUVIAL AGGRADATION AND INCISION, EASTGATE, CHURCHILL COUNTY, WEST-CENTRAL NEVADA

An intriguing section of fine-grained fluvial sediments filling an axial valley upstream of Eastgate—a narrow canyon cut through Tertiary volcanic rocks in Churchill County, west-central Nevada—holds clues about processes affecting Holocene landscape development at this locale. The area is in the Basin and Range province, with fault bounded mountain ranges and intervening valleys filled with gravelly alluvium on piedmonts, sand and fine gravel in ephemeral axial streams, and playa sediments in valley centers. The region has been tectonically active during the Quaternary and historical earthquakes within 25 km have produced surface ruptures with vertical offsets exceeding 3 m. The local mountain front about 2 km downstream from the study site is bounded by the Eastgate fault, for which other researchers have documented at least one ground-rupturing event during the Holocene, possibly around 4.4 ka. At the study site stream aggradation followed by incision has formed near-vertical cliffs roughly 8–10 m high of fluvial silt, clay, and mostly fine sand deposits. These deposits extend around 1.3 km downstream to the narrowest point in the canyon where bedrock (megabreccia blocks in tuff of Elevenmile Canyon) is cut by relatively straight side drainages, and where there are landslide deposits, at least some of which are modern. AMS ¹⁴C dates on organic sediment collected near the base, middle, and top of the study section have calibrated radiocarbon ages of 4.16 ± 0.08 cal ka B.P. (3.78 ± 0.02 ¹⁴C ka B.P., USGS-1157), 3.00 ± 0.08 cal ka B.P. (2.88 ± 0.02 ¹⁴C ka B.P., USGS-1423) and 1.94 ± 0.05 cal ka B.P. (1.98 ± 0.02 ¹⁴C ka B.P., USGS-1422) respectively, suggesting aggradation of the section began prior to about 4.2 ka and ended after about 1.9 ka. The young age of the uppermost sediments is supported by a weakly developed A/Bw/C surface soil profile. A working hypothesis for the section is that a landslide within the narrow canyon temporarily blocked or substantially slowed flow in the channel, causing the stream to first aggrade and then incise once normal flow was restored. We interpret the potential similarity in age of the base of the section and last rupture of the Eastgate fault as evidence that this landslide was triggered by seismicity.