Jared R. Morrow, 
Department of Earth Sciences, University of Northern Colorado, ZDA
 
Distal Effects of Late Devonian Marine Alamo Impact, Western U.S.A.: Tsunamites, Seismites & Ejecta 


Evidence of the early Late Devonian (~383 Ma) Alamo Event, which formed by a bolide impact into an marine setting west of the ancient North American craton margin, is found in the megabreccia, tsunami deposits, seismically disrupted beds, and impact ejecta preserved in over 22 mountain ranges of the Great Basin region, western U.S.A.  The oceanic Alamo crater is obscured by post-Late Devonian tectonics, erosion, and deposition; however, abundant shocked quartz, carbonate accretionary lapilli, ejected lithic fragments and conodont microfossils, and a minor iridium anomaly prove an impact origin for the Event.  Estimates of crater size and site based on conodonts ejected from target rocks indicate that the Alamo crater was >44 km across and that the original crater site was located in an offshore, submarine slope setting west of an extensive, shallow-marine carbonate platform bordering western North America.  On the carbonate platform, distal, onshore evidence of the impact includes high-energy tsunami channel deposits up to 6 m thick, which may have extended ~350 km onshore from the impact site; seismically disrupted platform strata; and abundant, fine-grained, aerially sorted ejecta.  Ongoing research is examining the micro-stratigraphy, petrography, geochemistry, and paleogeogeographic distribution of these distal, onshore tsunami beds.