New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed study of the sedimentology and petrography of the upper member of the Abiquiu Formation at Arroyo del Cobre, west of Abiquiu, revealed three stratigraphic intervals reflecting distinctive provenance and depositional processes. Interval consists of fluvial sandstone and siltstone derived from erosion of volcanic rocks in the San Juan Mountains and Precambrian rocks of the Tusas Mountains north of Abiquiu. Interval II consists primarily of pumiceous debris-flow deposits derived from erosion of the 26.5 Ma Amalia Tuff erupted in the Latir volcanic field northeast of Taos. This interval also contains detritus indicative of San Juan and Tusas Mountains provenance, suggesting drainage, at times catastrophic, from the north across the distal outflow sheet of the Amalia Tuff, which was dispersed at least 60 km westward from its source at the Questa caldera. Volcanism-induced sedimentation following eruption of the Amalia Tuff affected a larger area than previously recognized. Interval III is characterized by fluvial facies, notably finer grained than those of interval I, with petrographic characteristics consistent with derivation entirely from the Latir volcanic field. This study suggests that the petrosomes defined by Ingersoll and Cavazza (1991; SEPM Spec. Pub. 45) and the paleogeographic reconstruction of Ingersoll et al. (1990, GSA BulL) require revision. Interval I of the upper Abiquiu Formation is avariant of the Esquibel petrosome (derived from the San Juan Mountains) that contains significant (~25%) basement-derived detritus revealing the significance of relief in the Tusas Mountains. It is not clear whether this provenance represents remnant relief of the Laramide Brazos uplift or is related to early-rift tilting. Interval II is a mixture of the Esquibel and Cordito (derived from the Latir volcanic field) petrosomes. The presence of the Cordito petrosome in the Abiquiu Formation was not incorporated into the Ingersoll et al. (1990) paleogeographic synthesis. The dominance of Latir-derived sediment in interval III along the far western margin of the Rio Grande rift suggests that eastward tilting of the San Luis Basin in the late Oligocene (~26 Ma) was small compared to the volume of sediment supplied from the Latir volcanic field, so that awestward pa!eoslope was maintained across most of the basin and into the Abiquiu embayment. Southward dispersal of this sediment at Arroyo del Cobre suggests, however, that rift-bounding faults were active and influenced drainage patterns. The Arroyo del Cobre outcrops will be examined during the 1995 NMGS Fall Field Conference.