2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 11:30 AM

Tracing the Meter- to Kilometer-Scale Carbonate Seafloor Topography Following the Alamo Impact Event, Late Devonian, Nevada

ANDERSON, Julia, Minnesota Geological Survey, 2642 University Ave. W, St. Paul, MN 55114-1057 and TAPANILA, Leif, Department of Geosciences, Idaho State University, 921 S. 8th Ave. Stop 8072, Pocatello, ID 83209-8072, tapaleif@isu.edu

The Pahranagat Range in SE Nevada exposes carbonate sequences preserving the sedimentological and biological aftermath of the 382 Ma Alamo Impact Event. A series of high resolution stratigraphic sections spanning 15 km along strike measured the uppermost Alamo Breccia and the Upper members of the Guilmette Formation to characterize the surface topography of the event breccia and subsequent evolution of the carbonate platform. Three styles of terminal Alamo Breccia are recognized based on unit thickness, grading, level of reworking and overlying carbonate facies. High accommodation zones contain thick, well-graded event breccia and are overlain by burrowed brachiopod wackestones, whereas two styles of low accommodation have coarse and often reworked breccias that are capped by bioturbated firm- to hardground surfaces. In places, tilted megaclasts (>50 m) of the breccia form angular rockground surfaces at the top of the Alamo Breccia and show evidence of Trypanites borings. Subsequent carbonate facies suggest that high accommodation zones of the terminal Alamo Breccia represent slightly deeper water than those of lower accommodation. Lateral variability in low-to-high accommodation zones suggests local surface topography of < 1° after deposition of the Alamo Breccia, with amplitudes of ~10 m over a horizontal distance of 500 m. Recognition of the styles of breccia termination suggests stronger time averaging of benthic fauna in areas of low accommodation. At the kilometer scale, outcrops at the northern end of the study area record a restricted marine signature of aggradational mudstones that is gradationally replaced to the south with more open marine fauna and storm-influenced facies. Post-impact variations in carbonate facies support recent interpretation that the Pahranagat deposits were located within the fault-terraced Ring Realm of the impact structure. Detailed stratigraphic mapping in adjacent ranges will test and constrain this model and will further compare faunal recovery in the various post-impact environments.