PROXIMAL TO DISTAL GRAIN-SIZE TRENDS IN HEMATITE INCLUSION-RICH QUARTZ, LATE DEVONIAN ALAMO IMPACT EVENT, NEVADA, UTAH, AND CALIFORNIA
Petrographic measurement of 1110 quartz grains and 12,760 studs from eight proximal to distal Alamo Event localities indicates that both the diameter of studded grains and the diameter of individual studs decrease away from the inferred impact site. Proximal to the impact site, the mean diameter of studded grains is nearly 190 µm, while at more distal sites the mean diameter is 150 µm or less. Similarly, at proximal localities the mean diameter of hematite studs is over 15 µm, while at more distal sites the mean diameter is less than 10 µm. Qualitatively, the abundance of studded quartz grains also decreases from proximal to distal sites. The present euhedral to subhedral, hematite-rich inclusions in the Alamo quartz grains are clearly overprinted diagenetically. However, the observed coarser-to-finer grain-size trend away from the impact site, seen in both studded grains and individual studs, together with other petrographic data suggest that the initial formation of the studded grains was linked directly to impact and/or immediate post-impact processes. In-progress geochemical and electron microscopic analyses of the hematite inclusion cores are seeking to further identify if a relict primary impact signal is retained within the inclusions.