A FUNDAMENTAL CHANGE IN SUBSIDENCE STYLE EXISTS ACROSS THE ANCESTRAL ROCKY MOUNTAINS AT THE PENNSYLVANIAN-PERMIAN BOUNDARY (Invited Presentation)

The classical view of late Paleozoic intraplate deformation of SW Laurentia, the Ancestral Rocky Mountains (ARM), is protracted basin subsidence facilitated by basin-bounding reverse faults throughout the duration of the event. Data challenges that view and indicates a fundamental change in the driver of subsidence at or near the Pennsylvanian-Permian boundary.

Pennsylvanian strata proximal to basin-bounding faults typically record a local Precambrian basement provenance and alluvial fans or fan deltas adjacent to those faults suggest that sediment was shed across those faults from the adjacent highland. Locally, Pennsylvanian strata demonstrate growth structures and are folded with the hinge line subparallel to basin-bounding faults. Tectonic subsidence curves of the Anadarko, Eagle, northern Paradox, and Taos Trough basins produce the highest rate of subsidence in the latest Atokan through the Desmoinesian, whereas the Late Pennsylvanian represents a period of relatively little subsidence.

During the early Permian, another uptick in the rate of tectonic subsidence is demonstrated on available subsidence curves, but early Permian strata record a demonstrably different history than Pennsylvanian strata. Notably, early Permian strata are predominantly undeformed and cut across older basin-bounding faults or folds in Pennsylvanian strata. Early Permian strata commonly form angular unconformities with Pennsylvanian strata and onlap adjacent highlands forming a buttress unconformity with Precambrian rocks. Provenance data from early Permian strata exhibit mixed local and far-field provenance signatures.

These observations indicate that classically defined ARM deformation, consisting of subsidence facilitated by offset on a basin-bounding fault, peaked in the Middle Pennsylvanian and was over by the latest Pennsylvanian, which also indicates that intracratonal deformation ceased by the end of the Pennsylvanian. Early Permian strata across the ARM region postdate Pennsylvanian compressional structures and commonly overlie adjacent Precambrian-cored highlands at a buttress unconformity indicating that widespread epeirogenic subsidence drove sediment accumulation in the early Permian, potentially through dynamic subsidence, preexisting crustal density heterogeneity, or both.