QUANTITATIVE STRATIGRAPHIC MODELING OF THE LATE DEVONIAN ALAMO IMPACT, EASTERN NEVADA

Ancient impact craters concealed by sedimentary cover and dismembered by orogenic processes require detailed stratigraphic mapping of surface and fill patterns in order to define their size and geometry. We outline our approach and results from mapping the Alamo impact crater based on >70 measured sections covering >13.5 stratigraphic km of the Guilmette Formation in southeast Nevada. The Late Devonian marine-target Alamo impact crater is filled with Alamo Breccia deposits and overlying upper Guilmette platform carbonates. We developed a relational database and ArcGIS tools to quantitatively test previous models of the Alamo deposit by creating structurally restored isopach maps for part of the Guilmette Formation. Automated GIS scripts facilitated the generation and validation of isopach models, which were found to be internally consistent, repeatable, and robust, except where geographic sampling density was low. Results confirm previous studies suggesting that the detachment surface underlying the breccia shallows away from crater center, but increased resolution of our model shows that the detachment surface and overlying breccia deposits are more heterogeneous than originally predicted. Isopach models indicate a minimum volume of 954 km³ of total Alamo Breccia, nearly half formed by megablock platform slabs, and half deposited as surge and resurge breccias. These volumetric estimates provide new insights on mechanisms for wave generation and mass balance transfer in marine-target impacts.