EVIDENCE OF SUBAERIAL ENVIRONMENTS IN THE MESOPROTEROZOIC DRIPPING SPRING QUARTZITE, APACHE GROUP, ARIZONA

The 1.26 Ga Dripping Spring Quartzite (DSQ) comprises the prominent siliciclastic formation of the Apache Group (AG) of Arizona. From the base to top, the DSQ is composed of the Barnes Conglomerate (BC; including both matrix- and clast-supported conglomeratic facies), the Middle Member (MM; consisting of arkosic sandstones, interbedded with lenticular conglomerates and siltstones), and the Upper Member (UM; comprised of primarily finely-laminated siltstones and mudstones). The DSQ was deposited over a time span of ~160 Ma and during this time, deposition appears to have included both marginal marine and fluvial aquatic depositional environments, punctuated by periodic subaerial-exposure and the development of fully terrestrial (dryland) environments. Both the MM and the UM include intervals dominated by thin-bedded, hematite-cemented siltstones and shales that show abundant evidence for desiccation as extensive, mud-cracked bedding surfaces. We interpret these mud-cracked, siltstone-shale intervals to record the periodic desiccation of interchannel environments in coastal fluvial systems. Other sedimentary structures commonly seen on bedding surfaces of siltstones and shales include roll-ups, gas domes and vesicles, reticulated surfaces, sand folds and ridges, and wrinkle marks, all interpreted to be biologically-mediated. Some roll-up structures also contain organic-walled microfossils, which we attribute to cyanobacteria. Suspect biogenic features in the DSQ compare closely to the surface structures formed by modern biological soil crusts (BSCs)- common features of dry interchannel environments of Sonoran Desert alluvial fans. Our results suggest that more detailed studies of fine-grained lithofacies within the DSQ may provide additional insights for interpreting the detailed depositional history of the DSQ, as well as clues for early colonization of land surfaces by microbial ecosystems.