GOOD TIMES AFTER AN IMPACT: BENTHIC COMMUNITIES FOLLOWING THE LATE DEVONIAN ALAMO IMPACT, SE NEVADA

The Late Devonian Alamo impact formed a complex crater that excavated >10,000 km² of epicontinental carbonate platform, preserved today in eastern Nevada. The effect of the Alamo impact on benthic communities is assessed in the context of sequence stratigraphy and facies. Rank abundances of >3,000 fossils from 37 taxa (class to genus level) were separated into groups of similar sampling localities using agglomerative cluster analysis, and the pre- and postimpact faunas were compared using ANOSIM tests. Results of cluster analysis show that communities generally correspond to an offshore facies gradient, from restricted nearshore lagoons, to subtidal restricted and open marine communities. Comparison of these communities before and after the impact show little dissimilarity (R = 0.1-0.2), and overall the fauna shows an increase in taxonomic diversity after the impact. At five localities, bioherms and mudmounds develop a few meters above the impact breccia; these facies are either rare or absent on the preimpact platform in the field area. Other observations that corroborate the notion of resilient faunal recovery include abundant burrows in the top of the Alamo impact breccia, and the continuation of carbonate productivity and cycle deposition for the remainder of the highstand systems tract following the impact. Taken together, the Alamo impact appears to have had no lasting negative biological effects on the regional carbonate platform. Benthic recovery by the cosmopolitan Frasnian fauna appears to have reinstated the preimpact community, and exploited a broader range of seafloor habitats introduced by the impact.