GEOCHEMICAL EVALUATION OF DISTAL, ONSHORE CHANNEL DEPOSITS FROM LATE DEVONIAN, MARINE ALAMO IMPACT EVENT, NEVADA AND UTAH

Anomalous thin, sandy, middle- to inner-platform, Upper Devonian channel deposits in north-central Nevada, west-central Utah, and southeastern California give distal, onshore evidence of the ~382 Ma marine Alamo impact event. Major, minor, trace, and rare earth element analyses of one locality in Nevada (DVG) and one in Utah (CON), using neutron activation analysis and X-ray fluorescence spectrometry, further demonstrate the anomalous nature of the channel deposits relative to background carbonate-platform rocks and strengthen the hypothesized link with the Alamo Event. Major element data clearly show an enrichment of siliciclastic components in the channels, also evidenced petrographically by greater quartz sand and clay fractions, relative to background carbonate adjacent to the channels. A wide suite of non-siderophile minor and trace elements, including Sc, Cr, As, Rb, Zr, Sb, Cs, Ba, Hf, Ta, and Th, show significantly greater enrichment in the channel deposits than in adjacent carbonates. C1-normalized rare earth element abundance patterns are similar in both the channels and background carbonates, although elemental concentrations are an order of magnitude higher in the channels. However, rare earth concentrations in the channels and background rocks are both significantly depleted relative to average upper continental crust and North American shale composite (NASC) standards. Comparison of selected, C1-normalized siderophile element values from DVG, CON, and NASC show that for Fe, Co, Ni, Ir, and Au, the channel deposits are variably enriched relative to background carbonates, although, with the exception of Ir, both channels and carbonates are depleted up to an order of magnitude compared to NASC. Iridium concentrations from the channels at DVG and CON reach maxima of 183 ppt and 191 ppt, respectively, which are higher than the previously reported maximum of 133–139 ppt from Alamo Breccia located more proximal to the impact site. In background carbonates, Ir values are much lower, generally ≤25 ppt and, in some samples, below detection limits. Unlike other siderophile elements, however, the Ir at DVG and CON is anomalously enriched relative to NASC, indicating an extraterrestrial component, which further supports a causal link between the distal channel deposits and the Alamo Event.