The seafloor after a bolide impact: sedimentary and biotic signatures across the Late Devonian carbonate platform following the Alamo Impact Event, Nevada, USA
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- Tapanila, L., Steenberg, J.R., Johnson, C.J. et al. Facies (2014) 60: 615. doi:10.1007/s10347-014-0398-0
Eighty measured stratigraphic sections across the Late Devonian carbonate platform of Nevada (USA) document the uppermost terminal Alamo Breccia and overlying sediments, which record the waning energy at the end of an impact and the recolonization of the post-impact seafloor. Four sedimentary styles of terminal breccia, recognized by the continuity of normal grading versus reworking, and dolomitization, define patterns of sedimentary accommodation across the platform. Examined in combination with the first post-impact facies, the field area can be divided into deep subtidal, shallow subtidal, and peritidal zones with increased distance from the inferred crater center. Farthest away, peritidal outcrops have very low accommodation, and the terminal breccia is physically reworked and dolomitized. Biotic signals are rare in this zone. However, we find rare but exceptional deposits of impact fallout lapilli at or above the top of the breccia. The shallow subtidal region records reworked and pristine grading of the terminal breccia, which at several locations document burrowing directly into the top of the Alamo Breccia, confirming infaunal recolonization prior to post-impact sediment accumulation. Rare occurrences of erosional terminal breccia produced rockgrounds at the seafloor, some showing evidence of bioerosion. Deeper subtidal localities, closest to the crater center, preserve the thickest, continuously graded terminal breccias owing to high accommodation at the end of the event. Body and trace fossils are common in the first overlying lithofacies, although the abundance of oxidized firmground surfaces in deep water settings suggest these deposits were sediment-starved and fossil assemblages are strongly time-averaged.