2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 7
Presentation Time: 1:30 PM-5:30 PM


MORROW, Jared R., Dept. of Earth Sciences, Univ of Northern Colorado, Greeley, CO 80639 and SANDBERG, Charles A., Geologist Emeritus, U.S. Geol Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225, jrmorro@bentley.unco.edu

Ejecta from the mid-Frasnian (early Late Devonian), marine Alamo Impact Event are currently recognized at least 140 km radially from the inferred impact site in southern Nye County, south-central Nevada. Qualitative data suggest that the greatest concentration and farthest transport of ejecta occurred in a north to northeast direction from the target site, although ejecta material occurs in virtually all studied sections displaying evidence of the impact. Within the Alamo Breccia, several types of ejecta are recognized, including: 1) abundant mono- and polycrystalline, hematite inclusion-rich shocked quartz grains, which were derived from the Lower to Middle Devonian Oxyoke Canyon Sandstone and Middle Ordovician Eureka Quartzite, present at depths as much as 1.5 km below the targeted seafloor; 2) common carbonate accretionary spherules or "lapilli" randomly scattered in the megabreccia or concentrated within normally graded, tsunami-reworked lapilli blocks; 3) common to rare Middle Ordovician, Silurian, and Devonian conodonts with color-alteration indices ranging from CAI 1 to 4.5, redeposited mainly within these blocks; and 4) rare, alteration-rimmed lithic "bombs" displaying mega- and microscopic deformation features suggestive of emplacement in a plastic or semi-molten state. These bombs contain onshore- and offshore-derived debris including carbonate and quartzite clasts, recrystallized calcite clumps, and shocked-quartz and possibly kaolinitized-feldspar grains.

Ejecta in onshore, intertidal to shallow-subtidal, carbonate-platform settings include these four types and occur mainly within the upper half of the megabreccia lens and throughout the overlying, capping tsunamite beds. In offshore, deep-water settings, the ejecta, which are concentrated in the upper 1-2 m of megabreccia channel deposits, include isolated shocked quartz and lapilli together with deformed quartzite clasts up to 25 cm across. Overall, the heterolithic composition of quartz-grain and lithic ejecta indicates deep impact excavation of multiple sedimentary and metasedimentary target layers. Furthermore, the complex ejecta stratigraphy within the megabreccia and capping units demonstrates extensive hydraulic modification of ballistically transported impact deposits by the subsequent tsunami.