Late Devonian Alamo Impact, Southern Nevada: Analysis of Hematite Inclusions within Quartz Grains

ABSTRACT
The early Late Devonian (~382 Ma) Alamo event was produced by a marine bolide impact near the offshore margin of a large Paleozoic carbonate platform fringing western North America, with an impact site located in the current area of south-central Nevada. Effects of the impact, including the Alamo Breccia and related deposits, are now found in numerous mountain ranges in south-central Nevada, western Utah, and southeastern California. A key feature characterizing the Alamo deposits is the common occurrence shocked-quartz grains from impact ejecta that contain abundant inclusions of hematite after pyrite and magnetite. The sizes of the hematite inclusions and containing quartz grains found across the Alamo impact site show evidence for size sorting and a direct correlation between proximity to the impact site and inclusion size. Nine slides from eight locations proximal-to-distal from the impact were analyzed. The average size of the proximal hematite inclusions is 15.98 μm and the average size of the distal inclusions is 8.07 μm. The exceptions are the Confusion Range (CON, western UT) and Bat Mountain (BAT, SE CA) data. The CON location appears to be distal, but has an average inclusion size of 15.4 μm. This anomaly is possibly due to irregular impact plume movement. The BAT data appear to be proximal, but have an average inclusion size of 8.3 μm. In the Late Devonian this area was distal, but post-impact strike-slip faulting has moved this impact breccia block to the northwest. The Alamo is one of the only known impact sites with such hematite inclusions in the shocked quartz grains. These inclusions are useful in supporting previous estimates of the impact size and may provide a valuable tool for tracking proximal-to-distal fallout from the event.