FROM A METEOR IMPACT TO A REEF ECOSYSTEM: LATE DEVONIAN BENTHIC RECOVERY FOLLOWING THE ALAMO IMPACT IN NEVADA

The Late Devonian (early Frasnian) Alamo Impact struck the shallow carbonate platform of SE Nevada, producing the regional Alamo Breccia Member of the Guilmette Formation. In many outcrops the catastrophic Alamo Breccia terminates with a series of graded beds, the last of which provides a time-significant stratigraphic datum horizon marking a return to pre-impact hydrodynamic conditions. Ongoing research measures cm-scale stratigraphic sections of outcrops in SE Nevada to document the return of benthic faunas following the Alamo Impact.

Post-impact deposits and fossils often show marked lateral variability, however some pervasive benthic trends are summarized. Trace fossils penetrate the upper part of the Alamo Breccia, including the datum horizon, suggesting a rapid exploitation of the seafloor by opportunistic burrowers. These often are followed by sediment-stabilizing laminar stromatoporoids, roughly 1.5–2.0 m above the datum. Three meters above the datum, shelly open marine faunas including brachiopods, gastropods and crinoids become more abundant, and in two localities (Mount Irish and Hiko Ranges) these faunas form the base of reefs containing thick tabular stromatoporoids and rarer colonial rugosans. Reef development ranges from smaller patch reefs (8–20 m wide) with amalgamated thicknesses of 6 m, to larger reefs up to 500 m wide and at least 42 m thick. These represent the south-westernmost occurrence of reef ecosystems on the Laurentian continent during Frasnian time.

Preceding the Frasnian–Famennian mass extinction events by ~5 Ma, the Alamo Impact and the faunal recovery sequence preserved in the Guilmette Formation provides a significant opportunity to examine the localized effect of a meteor impact on a marine ecosystem. Initial research of the post-impact faunas in Nevada suggests a high capacity for the Frasnian ecosystem to recover from the transient localized effects caused by the Alamo Impact.