BENTHIC ECOSYSTEM RECOVERY FOLLOWING THE ALAMO IMPACT EVENT: A PRELIMINARY ASSESSMENT

The Late Devonian Alamo impact, recorded by the Guilmette Formation in Nevada, targeted the carbonate platform on the western passive margin of North America. Our goal is to evaluate the potential effects of this impact on benthic community structure by comparing fossil deposits below and above the Alamo Breccia, both at sites proximal and distal to the impact crater. We present here findings from one of seven study sites at Mail Summit near Hiko, Nevada. Measured sections at bed-scale include 90 m of pre-impact lower Guilmette unit (Ledge-forming interval) and 98 m of post-impact upper Guilmette Formation.

The pre-impact lower Guilmette at Mail Summit is composed of peritidal cycles of laminated dolomite tidal flat deposits with rare lagoonal skeletal wackestone or subtidal stromatoporoid boundstone. Bulbous and domical stromatoporoids are abundant and dominated by Actinostroma, and commonly associated with Arcostroma and Trupetostroma. Digitate Stachyodes are almost always found as constituents of stromatoporoid boundstones, with Amphipora largely confined to very shallow subtidal deposits. Discrete lenses of tabulate corals and brachiopods are rare. By contrast the lower 50 m of the upper Guilmette (post-impact) contains mostly subtidal cycles of burrowed mudstones and wackestones. Stromatoporoid boundstones are common in the upper 50 m and contain a similar fauna to the pre-impact lower Guilmette formation. Genera observed in upper Guilmette boundstones include Actinostroma, Trupetostroma, Stromatopora, Atelodictyon and Stachyodes. Amphipora packstones are commonly found as tops of subtidal cycles and bases of rare peritidal cycles. Corals are more diverse and abundant than in the lower Guilmette. After the impact, facies are overall deeper and record greater diversity and abundance of stromatoporoids and corals. Analyses of adjacent localities will provide a test for the correspondence between facies and benthic structure to help isolate eustatic from impact-related signals.