CALIFORNIA OR BUST: RECOGNITION AND SIGNIFICANCE OF LATE DEVONIAN ALAMO IMPACT DEPOSIT NEAR DEATH VALLEY

The previously mapped Lost Burro Formation at Bat Mountain, just southeast of Death Valley National Park, California, is actually a displaced part of the Middle to Late Devonian carbonate platform of eastern Nevada and western Utah. The upper part of Lost Burro member lb3 and all of member lb4 are here correlated with the widespread Guilmette Formation of eastern Nevada and western Utah. At Bat Mountain, an anomalous, 1.5-m-thick, high-energy channel deposit discovered in the middle of the Guilmette is linked to the early Late Devonian (punctata Zone), marine Alamo Impact Event. A punctata Zone age for the deposit is corroborated by an overlying, ~8.5-m-thick limestone interval containing the index gastropod Orecopia sp., which occurs in punctata Zone rocks above Alamo channel deposits in Nevada and Utah, and the middle Frasnian index brachiopod Tenticospirifer utahensis. This channel deposit has a sharp, erosive base with local injection into underlying platform strata; polymict lithic carbonate-clast breccia with inverse grading and chaotic bedding in the lower 50 cm; and a quartz sand-rich matrix. Altered, fine- to very fine-grained quartz sand grains in the matrix contains common penetrative inclusions or “studs” of hematite, pseudomorphous after pyrite pyritohedrons. Such studded quartz grains uniquely characterize Alamo Event deposits in both impact-proximal and -distal settings across the upper slope, platform margin, and platform.

The Bat Mountain deposit resembles other distal Alamo Event Runoff Realm channels, oriented radial to the impact site and previously mapped in western Utah and northern Nevada. These drained the post-impact, megatsunami-flooded platform. However, the present location of Bat Mountain, due south-southwest of the inferred Alamo Impact site, reinforces existing paleotectonic models showing that significant (i.e., 100+ km) westward, post-Late Devonian displacement has occurred at this site. This displacement was across stepped northwest-trending, right-lateral strike-slip faults including the Las Vegas lineament, the Stewart Valley fault, and the Death Valley-Furnace Creek fault zone, which collectively offset and cut the southern end of the carbonate platform at the latitude of Las Vegas, Nevada.