ABSTRACT

Oligocene, mostly volcaniclastic, fluvial sedimentation in the Rio Grande rift of northern New Mexico has been ascribed to either aggradation in response to explosive volcanism or initial subsidence of extensional basins. These competing interpretations are tested by numerical simulations and by stratigraphic and petrographic study of the Oligocene-lower Miocene Abiquiu Formation. The upper 450 m of the > 600-m-thick Abiquiu Formation is tuffaceous volcaniclastic sediment that accumulated during explosive volcanic activity in the Latir volcanic field, about 150 km to the northeast. The formation extends westward beyond the most conspicuous rift-bounding faults. Simple numerical simulations suggest that two volcanism-related processes may have contributed to sedimentation. First, stream longitudinal profiles adjust upward as a result of (a) increasing headwater elevation in volcanic constructional topography and (b) increased sediment load. These combined effects may produce as much as 100 m of deposition at distances in excess of 150 km from the volcanic source area. The second volcanic mechanism is flexural subsidence resulting from the load of volcanic fields and flanking volcaniclastic aprons, and may account for 300 m or more preservation space at a distance of 150 km. Correlation of stratigraphic intervals of the Abiquiu Formation defined by lithofacies geometry and sandstone composition reveals thickening of strata into the rift across rift-bounding and intra-rift faults, indicating subsidence-driven sedimentation. Petrographic and ⁴⁰Ar/³⁹Ar geochronologic data reveal that sediment was derived mostly or wholly from ignimbrites erupted in the Latir volcanic field, including widespread deposition of pumiceous debris-flow deposits following emplacement of the 25.1 Ma Amalia Tuff. Deposition of the Abiquiu Formation was therefore most likely caused by a combination of riftbasin subsidence and volcanism-related isostatic subsidence and adjustments of stream profiles.