Paper No. 9
Presentation Time: 8:00 AM-6:00 PM
PROXIMAL TO DISTAL GRAIN-SIZE TRENDS IN HEMATITE INCLUSION-RICH QUARTZ, LATE DEVONIAN ALAMO IMPACT EVENT, NEVADA, UTAH, AND CALIFORNIA
HUTCHINS, Kieri L., Geological Sciences, San Diego State University, 5500 Campanile Drive, GMCS 237, San Diego State University, San Diego, CA 92182-1020,
MORROW, Jared, Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, 237 GMCS, San Diego State University, San Diego, CA 92182-1020, SANDBERG, Charles A., U.S. Geol. Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225 and HARRIS, R. Scott, Department of Geosciences, Georgia State University, Atlanta, GA 30302, jmorrow@geology.sdsu.edu
A characteristic feature of proximal to distal deposits resulting from the ~382 Ma marine Alamo impact event is the presence of common to abundant quartz grains containing large, distinct hematite inclusions (or “studs”) formed after euhedral to subhedral pyrite and magnetite. The inclusion-rich quartz grains occur solely in breccia matrix, clasts, and sandy channel units from all recognized Alamo Event Realms; they are also a provenance indicator where they are reworked into post-Event clastic units. The studded quartz grains commonly contain additional alteration features, including fluid micro-inclusions, curviplanar microfractures, polycrystalline domains, anomalously reduced birefringence, and oriented, shock-diagnostic planar microstructures (planar fractures and planar deformation features). Hematite inclusions occur both as rinds embedded in quartz grain rims and as isolated crystals enveloped within grain interiors, in some cases showing an apparently penetrative microfabric that rarely cross-cuts or deflects planar microstructures.
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.
