MIXED CARBONATE-SILICICLASTIC SEDIMENTATION AND ACCOMMODATION TRENDS IN THE BUG SCUFFLE LIMESTONE, SACRAMENTO MOUNTAINS, NM

The Sacramento Mtns in southern NM contain Pennsylvanian mixed carbonate-siliciclastic strata deposited during regional shortening and a transition into global icehouse, offering an opportunity to study accommodation succession in a dynamic tectonic and climatic setting. Mississippian carbonates were deposited on a ramp that dipped southward into the Tobosa basin, but by the Late Pennsylvanian the platform had evolved into a narrow, west-facing shelf with the Pedernal uplift to the east and the Orogrande basin to the west. The Pedernal landmass was a source of siliciclastics by the Morrowan, and mixed carbonate-siliciclastic deposition persisted throughout Pennsylvanian.

This study concerns the accommodation history of the Desmoinesian Bug Scuffle Mbr. of the Gobbler Fm. (Morrowan?-Desmoinesian). The Bug Scuffle platform, located along the eastern Orogrande basin, is dissected by an E-W trending siliciclastic thoroughfare at Alamo Canyon. Published data indicate regional transitions from marine carbonates along the west face of the Sacramento Mtns to marginal marine siliciclastics to the east, and basinal facies to the south and west.

New measured sections from the Dog Canyon area were compared with published data to interpret sequence architecture of the Bug Scuffle carbonate platform. Fissile mudstone, crinoidal turbidites and cohesive slide blocks abruptly overlie shallow marine siliciclastics at the base of sections, indicating deepening and suppression of siliciclastic input. Skeletal wackestone and packstone cycles stack into transgressive systems tracts, while highstand cycles grade into crinoid grainstones. Blackened crusts and mottled cycle tops indicate subaerial exposure of subtidal facies and are common tops in highstand cycles. Two composite sequences are interpreted from stacking trends and facies proportions. Above the upper sequence boundary, shingled mounds and thin-bedded, nautiloid wackestone suggest increased accommodation during terminal Bug Scuffle deposition, in contrast with previous studies that suggest progradation throughout. This work represents preliminary findings in a larger effort to compare tectonic and eustatic signals in Pennsylvanian carbonates from the Sacramento Mountains with coeval deposits in other Ancestral Rocky Mountains basins.