Volcanically impacted continental rift basins that experience coeval subsidence and volcanism challenge the conventional sedimentation models for continental rift basins. A field study was undertaken at the eastern margin of Bearhead basin within the southeastern Jemez volcanic field of north central New Mexico and synthesized with previous workers' research. 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 about 550 m of lava and primary pyroclastic deposits and minor fluvial sediments. As a result, little or no coarse-grained epiclastic 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 generally 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 conventional rift basin. Pyroclastic-flow, surge, fall, and block-and-ash-flow deposits dominate the stratigraphic record with fewer hiatuses as basin filling progressed. In the 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, and thus 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. Bearhead basin is an overfilled, volcanically impacted rift basin. Therefore, the knowledge gained from the Bearhead basin case study, using both sedimentological and volcanological perspectives, may provide an analogue for better understanding older, less well exposed basins.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
Smith, Gary A.
First Committee Member (Chair)
Fischer, Tobias P.
Second Committee Member
Meyer, Grant A.
rift basin, Bearhead Basin, Jemez Mountains, rhyolite, pyroclastic deposits, volcaniclastic deposits
Edelman, Ashley Elizabeth. "Characterization of a volcanically impacted, overfilled continental rift basin within the southeastern Jemez Mountains, New Mexico." (2010). https://digitalrepository.unm.edu/eps_etds/22