The Late Devonian (382 Ma) Alamo Impact Event was caused by a km-scale bolide impact that devastated a 200 km portion of the expansive carbonate platform in the Great Basin. Shortly following the event, faunas recolonized the surface of the Alamo impact breccia without any evidence for biotic extinctions or any long-lived geochemical change to the environment. As such, the Alamo Impact Event is comparable to moderately frequent (103-106 yr) catastrophic physical events, such as large storms, tsunamis, and other modest bolide impacts, that shape carbonate platform evolution. This project will conduct detailed field mapping of the Guilmette Formation in southern Nevada to provide the first high resolution 3D model of platform strata before, during and after the Alamo Event and allow site-specific correlation of impact features with overlying elements of recovery. Objectives of the project include (1) testing and refining the current model for marine-target impacts; (2) assessing the hypothesized Devonian activity of a major E-W structural lineament in the Great Basin; and (3) evaluating faunal recolonization and facies evolution as a function of the physical attributes of the seafloor left by the Alamo impact event. These objectives will be accomplished in a novel way by using Vulcan GIS software, originally designed for the mining industry, to compile stratigraphic thickness, paleocurrent, sedimentary and paleontological attributes from more than 100 stratigraphic sections of the Guilmette Formation exposed in outcrops of southeast Nevada. Maps produced from these data will be used to identify depositional pathways and depocenters of Alamo Breccia subunits; to calculate net rock volume displacement from impact excavation and subsequent debris-flow and tsunami deposition; and to recreate the post-impact seafloor topography and relative bathymetry in the study area. Sedimentary, faunal and ichnological data collected above the impact deposits will be compared to underlying physical parameters of the post-impact seafloor, e.g., topography, relative bathymetry, substrate type, and geographic position, to identify variations in short-term (<100 yrs) and long-term (~500 kyrs) recovery of the carbonate platform ecosystem. The Alamo research will involve the training of two graduate students in field geology and GIS applications. More significantly, research activities will merge with a comprehensive educational outreach effort through the Impact Initiative on Great Basin Field Geology. Partnering with the U.S. Department of Education's GEAR UP programs in Nevada and Idaho, our Impact Initiative includes a four-pronged approach to provide STEM education and training opportunities to K-12 teachers and students, with an emphasis on involving female high school students in research.

Project Report

Nearly 380 million years ago, back when Nevada was covered by warm ocean water on the west coast of North America, a large meteor blasted into the coastline. Seafloor rocks the size of houses were crumpled and tossed about, and hail-stones made of pure limestone rained from the sky. Giant waves of ocean water surged into Utah and then retreated westward, leaving gouges filled with sediment in its wake. When the roiling waves calmed back to normal, the shallow seafloor was completely changed. How this catastrophic event shaped the environment and its marine life is the subject of the Alamo Impact Project. After mapping dozens of rock outcroppings in the mountains of Nevada, our results show the outer edge of the crater was at least 111 km (60 miles) wide, and the central part of the crater blasted down nearly 3 km (2 miles). This makes the Alamo crater one of the largest of the past 500 million years, and one of the best exposed marine craters in the world. A study on the fossils below and above the impact deposits shows little to no change in the types of animals or their community structure. Evidently the impact was not severe enough to have long-lasting effects on the biosphere, even though the depression made by the crater altered the geography of the coast for tens of thousands of years. Our research on the Alamo impact was leveraged to teach people from the region about their natural surroundings. Four separate field courses taught geology to teenage women and K-12 educators in southern Nevada. Groups hiked to important Alamo impact sites, learned how geologists make observations, and developed projects that used the scientific method to test hypotheses. Teachers developed lesson plans that could be directly incorporated in their future classes. All participants left the experience with a deeper appreciation for the history of their region and the landscape in which they live.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Yusheng Liu
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Idaho State University
United States
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