New Mexico Geological Society Annual Spring Meeting — Abstracts

Previous work on the western margin of the Rio Grande rift, in the area of the southern San Luis basin and northern Espafiola basin, has purported that rift subsidence did not begin until middle to late Miocene (Baldridge et aI., 1994, GSA Bull 105, p.1538; Large and Ingersoll, 1997, JSR 67:3, p.462). We have found evidence for earlier onset of rift subsidence in the Qligocene-early Miocene Abiquiu Formation, which straddles the margin of the Rio Grande rift and Colorado Plateau near Abiquiu, New Mexico. Structural and stratigraphic relationships between Mesozoic strata ofthe Plateau and the upper member ofthe Abiquiu Formation provide evidence for syndepositional subsidence ofthe Abiquiu embayment, a shallow structural bench ofthe rift adjacent to the deeper San Luis and Espafiola basins. The data presented here indicate riftward stratigraphic thickening in the upper Abiquiu Formation across rift-bounding faults.

Stratigraphic thickness between intervals, defined by vertical changes in lithofacies and clast composition, increases from the Plateau to the rift. Compositional changes, evident petrographically and in mesoscale, reflect the introduction ofnewly erupted material from the Latir volcanic field (~60 km to the northeast in the Taos area) to the depositional system. Such Latir-derived volcanic clasts as tuff, sanidine and quartz-bearing pumice each make their first appearance progressively higher in the stratigraphic sections toward the rift, as do lithofacies transitions within the upper Abiquiu. These signs point to deposition ofthe upper Abiquiu Formation having taken place while the basin was subsiding along rift-bounding faults.