Paleogeographic, volcanologic and tectonic significance of the upper Abiquiu Formation at Arroyo del Cobre, New Mexico
— Gary A. Smith

Detailed study of the sedimentology and petrology of the upper member of the Abiquiu Formation west of Abiquiu, New Mexico, indicates that this member is composed of three stratigraphic intervals reflecting distinctive provenance and depositional processes. Interval I 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 from the north across the distal outflow sheet of the Amalia Tuff that was dispersed at least 60 km westward from its source at the Questa caldera. 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) and the paleogeographic reconstruction of Ingersoll et al (1990) require revision. Interval I of the upper Abiquiu Formation is a variant of the Esquibel petrosome that contains significant (> 25%) basement-derived detritus. Interval II is a mixture of the Esquibel and Cordito petrosomes. The presence of the Cordito petrosome in interval III along the far western margin of the Rio Grande rift suggests that development of accommodation space due to eastward tilting of the San Luis basin in the late Oligocene (~26 Ma) was small compared to the sediment supplied from the Latir volcanic field; therefore a westward paleoslope was maintained across most of the basin and into the Abiquiu embayment. Southward dispersal of Latir-derived sediment near Abiquiu suggests, however, that faulting along the western margin of the rift influenced drainage patterns.