DISTAL, ONSHORE EFFECTS IN WESTERN UTAH OF MARINE, LATE DEVONIAN ALAMO IMPACT EVENT

The early Late Devonian (early Frasnian, ~382 m.y.) Alamo Impact targeted a marine, off-platform site in southern Nevada. Distal, onshore evidence of the impact, which is contained within the shallow-water, inner carbonate-platform Guilmette Formation exposed in western Utah, comprises megatsunami uprush/backwash channels, seismites, and ejecta. Diagnostic criteria for identifying and distinguishing the onshore Alamo channels include their conodont biostratigraphic age, stratigraphic position, anomalous sedimentology and sedimentary structures, and altered, probably impact-ejected quartz grains.

Conodont biostratigraphy has constrained the age of two distal, onshore Alamo channel deposits in the Confusion Range and Burbank Hills, Utah, as within the middle part of the punctata Zone, thus establishing a correlation with well-dated proximal outer-platform and deep-water Alamo deposits to the west. The onshore Alamo channels are identified stratigraphically within the Guilmette relative to such widespread, regional marker units as the basal yellow slope-forming member and post-Alamo stromatoporoid reef facies. The anomalous channels are characterized by an erosive base and complex internal fabric comprising coarsely graded breccia and microbreccia, heterolithic clasts, common semi-autochthonous rip-ups, abundant altered quartz grains, convoluted laminations, and irregular cross bedding. Quartz grains within the very fine-grained sand-to-silt fraction show abundant fractures, inclusions, hematite-crystal studding, and rare planar fractures and PDFs, which are all diagnostic of the shocked-quartz populations within proximal Alamo deposits. The narrow grain-size range of the altered quartz within the onshore channels suggests that sorting has occurred, possibly through aerial transport and deposition of the grains as impact ejecta. Shallow-water carbonate beds directly beneath the Alamo channels locally display a massive, pseudo-brecciated fabric resulting possibly from impact seismic energy or emplacement force of the overlying channels. In the Dugway Range, Utah, the occurrence of additional Alamo-candidate beds currently under study suggests that megatsunami from the impact may have swept up to 350 km onshore from the probable crater site.