TIMING OF FRASNIAN (EARLY LATE DEVONIAN) CARBON ISOTOPIC AND ALAMO IMPACT EVENTS, SOUTHERN NEVADA

Recent integrated work on Frasnian (early Late Devonian) geochemistry and event stratigraphy has identified a series of significant carbon isotopic shifts within shelf and epicontinental basin settings on several continents. In this study, we examined the carbonate carbon isotope geochemistry of lower to middle Frasnian (falsiovalis, transitans, punctata, and hassi conodont zones) platform and upper slope strata of the Guilmette Formation and enclosed Alamo Breccia Member, which was emplaced at the western edge of Laurussia by the oceanic, mid-punctata Zone Alamo Impact Event.

In western Lincoln County, southern Nevada, the type Alamo Breccia Member comprises four units, designated A to D, in descending order. Unit A consists of multiple tsunamites capping underlying megabreccia; unit B is chaotic, poorly sorted polymict megabreccia; unit C consists of huge, semi-autochthonous carbonate-platform megablocks barely separated from underlying intact beds; and unit D is primarily thin, monomict breccia formed as a fluidized bedrock-detachment surface under unit C blocks. Laterally, units C and D disintegrate into unit B, which rests directly upon underlying intact strata.

Analysis of 80 micritic carbonate geochemical samples taken through ~175 m of the lower Guilmette Formation at the type Alamo locality shows two potentially significant carbon isotopic shifts. The first, which occurs within the falsiovalis Zone near the poorly constrained Middle-Upper Devonian boundary, is a positive, ~2.5 per mil (d¹³C_carb V-PDB) increase that separates generally lighter values (average d¹³C_carb=-2.1 per mil) below from heavier values (average d¹³C_carb=0.21 per mil) above. The second, more marked shift is a negative-positive, ~5 per mil couplet within the upper part of a unit C megablock. This shift separates lighter values (average d¹³C_carb=-0.97 per mil) in units D-C below from heavier values (average d¹³C_carb=1.3 per mil) in units B-A above. The presence of this isotopic event in unit C, which is well constrained within the punctata Zone, indicates that it must have closely preceded the Alamo Impact by <0.1 m.y. These isotopic events are being compared against those recorded in other regions, including the Holy Cross Mountains, Poland, to evaluate the synchroneity and potential cause(s) of the excursions.