STRATIGRAPHIC RELATIONSHIPS ACROSS THE PENNSYLVANIAN-PERMIAN BOUNDARY, OSCURA AND SAN ANDRES MOUNTAINS, NEW MEXICO

Detailed lithostratigraphy and biostratigraphy in the Oscura-San Andres Mountains of Socorro-Doña Ana Counties, central and southern NM, establish lithofacies relationships and correlation across the Pennsylvanian-Permian boundary. Traced southward from its type section in the northern Oscura Mountains (T6S, R4E), the lower part of the Bursum Formation (mainly nonmarine redbeds) is laterally equivalent to the Panther Seep Formation (mainly marine shale, sandstone and limestone), the middle Bursum grades into the lowermost (marine) interval of the Hueco Group, and the upper part of the Bursum is equivalent to an interbedded purple/red shale and limestone interval of the lower Hueco. Key to these correlations is the equivalence of the medial Bursum Schwagerina limestones in the Oscura Mountains and the Schwagerina limestones at the base of the Hueco in the northern San Andres Mountains. These limestones can be correlated lithostratigraphically, and contain a fusulinacean assemblage dominated by large species of Schwagerina, including S. andresensis Thompson and S. grandensis Thompson, of early Wolfcampian age. The southernmost section of Bursum Formation lithotypes (although with fewer redbeds than the type Bursum) is at sec. 36, T11S, R3E (Big Gyp Mountain), whereas the Bursum equivalent section is Panther Seep overlain by Hueco at sec. 12, T12S, R2E (Rhodes Spring). Total Wolfcampian stratal thickness varies little between the central San Andres and the northern Oscura Mountains. In contrast, between Mockingbird Gap (T8S, R5E) and the northern Oscura Mountains (T6S, R5E), the majority of the Panther Seep Formation grades northward into a much thinner Madera Group (Atrasado Formation) section. This indicates significant facies changes during the Missourian-Virgilian that reflect a tectonic hingeline between the rapidly subsiding Orogrande basin and the Oscura shelf. In contrast, the transition between Panther Seep/lower Hueco and Bursum lithotypes during the Virgilian-Wolfcampian transition respresents lateral facies changes within a more quiescent tectonic setting.