PETROGRAPHY, MINERALOGY, AND WHOLE-ROCK GEOCHEMISTRY OF THE BIOTURBATED ZONE AND THE MUDSTONE1/HALITE1 UNIT IN THE LOWER PERMIAN RUSTLER FORMATION (LOS MEDAÑOS MEMBER) IN THE NORTHERN DELAWARE BASIN, SOUTHEAST NEW MEXICO, USA

The Permian Rustler Formation (Fm) in the Delaware Basin (southeast New Mexico and west Texas) comprises five members that were deposited during transgressive and regressive events at the end of the Permian. The Los Medaños, the lowermost member of the Rustler, marks the beginning of a shift from saltpan environment that led to the deposition of the underlying evaporitic Salado Fm to a marine/saline mudflat environment in the Rustler. In this study, we highlight differences between a basal bioturbated zone and the overlying Mudstone1/Halite1 (M1/H1) unit in the Los Medaños and aim to understand the sediment distribution in the first deposits of the Rustler based on petrography, mineralogy, and whole-rock geochemistry. Results from collected samples indicate a graywacke composition for the dark gray bioturbated zone and a litharenite for the reddish brown M1/H1—chemical classification likely influenced by the high salt content in both samples—whereas field descriptions classify them both as fine to very fine halitic to muddy quartz sandstones to siltstones. Framboidal pyrite found in the bioturbated zone attests of microbial actions that contributed to reducing conditions in the unit, while the reddish-brown color and the abundance of iron oxides in M1/H1 show oxidizing conditions. Trace element analysis revealed high strontium (Sr), hafnium (Hf) and zirconium (Zr) concentrations in M1/H1 along with a high chromium (Cr) concentration in both units. While Sr could derive from gypsum present in M1/H1, and Hf and Zr from zircon, there are uncertainties regarding the high Cr concentration. Finally, zircon grains were not observed in the bioturbated zone heavy mineral fraction when they were found in M1/H1, suggesting different dispersal mechanisms that kept heavy minerals and probably coarser sediments at the basin margins during transgressions and carried them into deeper parts of the basin when the water receded.