Paper No. 8
Presentation Time: 1:30 PM-5:30 PM
RECONSTRUCTING THE AFTERMATH OF THE LATE DEVONIAN ALAMO METEOR IMPACT IN THE TYPE SECTION REGION OF THE PAHRANAGAT RANGE, SE NEVADA
The long term sedimentological and biological consequences of the Frasnian Alamo Impact Event (AIE) are evaluated with a series of stratigraphic sections stretching 11 km along strike, focusing on the Upper Guilmette Formation deposited immediately following the impact. Carbonate lithofacies include bioclastic wackestones, alternating light-dark nodular mudstone, minor skeletal grainstone, and channelized intraformational conglomerates and bioclastic breccias. All facies are consistent with deposition between fair-weather and storm wave base on a shelf gradient. Carbonate-dominated facies occur in three shallowing upward cycles that receive increasing siliciclastic input that ultimately terminates the third carbonate cycle with extensive quartz arenites derived from eastern terrestrial sources based on detrital zircon and paleoflow data. Carbonate cycles 1 to 3 appear to record a T-R sequence, with sediment starved firmgrounds at the base followed by aggraded nodular mudstones and bioclastic wackestones terminated by quartz arenites. Deposition of the three carbonate cycles correlate to the construction of biostromes and bioherms in the northern Hiko and Irish Ranges, and the influx of siliciclatics at the end of cycle 3 correlates with a major regression recorded by the karsted reef-top at Irish. Restricted circulation from these organic buildups is observed in the northern part of the Pahranagat with the alternating nodular limestone and the presence of bioclastic debris carried by channels with SW paleoflow; whereas the southern exposures of the Pahranagat consist of more open marine conditions that supported a diverse invertebrate fauna. Continued refinement of the paleogeographic reconstruction of Pahranagat outcrops serve as a baseline to evaluate the faunal recovery and succession after the AIE and help correlate other localities in the vicinity to create a basin-wide reconstruction of the impact's aftermath.