IMPACT OF AN IMPACT: BENTHIC RECOVERY IMMEDIATELY FOLLOWING THE LATE DEVONIAN ALAMO EVENT

The Guilmette Formation of SE Nevada provides a rare opportunity to observe the recovery sequence of a shallow marine ecosystem immediately following an extraterrestrial catastrophe. The Alamo Event, a subcritical impact, was not synchronous with any known major faunal extinctions, and it preceded the Frasnian-Famennian mass extinction events by more than 3 Ma.

A 175 km semicircular region of SE Nevada preserves the Alamo Breccia Member of the Guilmette Formation, which records tsunami-generated debris flows and turbidites from a major bolide that struck the shallow continental platform during the Frasnian (early Late Devonian). The energy of the Alamo Event was sufficient to cause major disruptions on the shallow carbonate platform and have a destructive effect on its ecosystems.

Examination of the terminal graded beds at the top of the Alamo Breccia sequence at multiple localities reveals solitary burrows (Teichichnus) and simple ichnofabrics. Burrowers obviously were the first pioneers on the scene following the deposition of the impact breccia sequence, and these sediment layers are devoid of body fossils. Bioturbation intensity increases upward with the addition of Thalassinoides, Planolites and other ichnotaxa. The pioneering burrows directly underlie tabular stromatoporoids and other benthic invertebrates, which are overlain by stromatoporoid and coral biostromes and localized bioherms. The ecological succession begins with soft-bodied deposit feeders and proceeds through sediment stabilizers to reef-associated climax communities.

The rapid colonization of the seafloor by benthic invertebrates demonstrates (1) that the early Frasnian ecosystem, consisting largely of cosmopolitan taxa, was resilient to major environmental catastrophes, and (2) that the Alamo Event did not produce long-lasting alterations to preexisting physicochemical marine conditions.