CHARACTERIZATION OF A VOLCANICALLY IMPACTED, OVERFILLED CONTINENTAL RIFT BASIN WITHIN THE SOUTHEASTERN JEMEZ MOUNTAINS, NEW MEXICO

Volcanically impacted continental rift basins that experience coeval subsidence and volcanism challenge the conventional sedimentation models for continental rift basins. A field and petrographic study was undertaken at the eastern margin of Bearhead basin within the southeastern Jemez volcanic field of north central New Mexico. The 7.5-km wide basin is part of the Rio Grande rift. A brief episode of rhyolitic volcanism at 6.5-7.1 Ma overfilled the basin with ~650 m of lava and primary pyroclastic deposits and very minor fluvial sediments. As a result, little or no coarse-grained detrital sediments were shed from adjacent footwall highlands and deposited into the basin along its margins, which is a trait of conventional rift basin models. Additionally, the reworked volcaniclastic sediments, which are mostly observed in the lower stratigraphy, were longitudinally transported through the basin, even at the basin margins, as opposed to transverse deposition on the footwall-derived alluvial fans. The prominent sedimentary clast types shift from older andesite to mostly younger rhyolitic volcaniclastic facies and lavas depicting the change in the type and composition of volcanism; this pattern contrasts with unroofing of progressively older rocks to provide sediment into a traditional rift basin. Pyroclastic-flow, surge, fall, and block-and-ash-flow deposits dominate the stratigraphic record with fewer hiatuses as basin filling progressed. In Bearhead basin, the accumulation rate of mostly volcanic fill sourced from coeval dome complexes, including vents within the basin, exceeded the subsidence of the basin, overfilling the basin. This differs from the conventional models, where subsidence typically exceeds the accumulation rate of sedimentary fill sourced from uplifted highlands along and near the basin margin. Volcanism can, therefore, obscure typical features that are used to recognize coeval tectonism. Careful examination of thickness variability and evidence of syndepositional faulting are key to recognizing contemporaneous volcanism and rifting because topographic relief may be muted or absent along basin boundaries with volcanic constructional processes, rather than uplift and erosion, dominating evolution of landforms and depositional patterns.