2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 9
Presentation Time: 4:00 PM


TAPANILA, Leif, Department of Geosciences, Idaho State University, 921 S. 8th Ave, Pocatello, ID 83209-8072, THOMASON, Carrie, Geosciences, Idaho State University, 921 S 8th Ave, Stop 8072, Pocatello, ID 83209-8072 and ANDERSON, Julia, Minnesota Geological Survey, 2642 University Ave. W, St. Paul, MN 55114-1057, tapaleif@isu.edu

The early Frasnian Alamo impact preserved in southeast Nevada provides empirical data to test theoretical models on the geometry and facies relationships of bolide impacts on marine targets. Ongoing stratigraphic and surficial mapping of the disturbed and post-event sedimentary record serve to characterize and provide limits for the conceptual Ring and Runup realms situated shoreward of the crater rim. Incision of pre-impact carbonate rocks to a depth of ~100 m among Ring realm outcrops thins considerably to <10m in Runup realm outcrops of the northern Hiko Range, and suggest a stepwise rather than gradual detachment surface. The thickness of overlying breccia deposits positively correlates with the depth of incision and results in a post-impact seafloor topography with a slope of ~1°. Facies variations immediately above the breccia delineate an offshore transition from largely barren peritidal environments in the Runup realm through an intermediate zone of sandy barforms to a highly fossiliferous subtidal Ring realm. Further offshore and closer to the crater rim, the Pahranagat Range records sediment starvation following the impact in a fully subtidal portion of the Ring realm. Increased thickness of terminal breccia deposits appears to correspond to topographically low areas of the seafloor, providing accommodation space and limiting erosive processes during the waning phases of the catastrophe. Channelized depressions having 20m of seafloor relief are tempered by an overall along-strike slope <2°. Further detailed stratigraphic studies in adjacent ranges will use GIS applications to compile a quantitative dataset for spatiotemporal analysis.