New Mexico Geological Society Annual Spring Meeting — Abstracts

We summarize Miocene clastic deposition in the Española Basin (EB) and examine how drainage systems may respond to tectonic and paleoclimatic controls. The EB is a half-graben tilted west towards the Embudo–Santa Clara–Pajarito fault system (ESPFS). An arm of the EB, called the Cañada Ancha graben (CAG), extends SE away from the ESPFS. A faulted structural platform lies NW of the ESPFS. Syn-rift deposits of the Santa Fe Group are particularly well exposed in the EB, and we could readily map western alluvial slope, central basin floor, and eastern alluvial slope lithofacies assemblages. Abundant tephra and fossils provide exceptional age control. Previous studies documented an increase in ESPFS throw rates at 15-11 Ma and decreased subsidence rates in the CAG after 13.5-13.0 Ma. Paleodrainage changes included: 1) coarsening ca. 13.2 Ma; 2) progressive westward progradation of the eastern alluvial slope during 18-10 Ma and a narrowing of the basin floor, with a particularly rapid advance at 13.5-11 Ma; 3) development of a gravel-bearing ancestral Rio Chama by 12 Ma; and 4) shifting of the axial river onto the northwest structural platform after 11 Ma, at least episodically. The progradation of the eastern alluvial slope is interpreted to be controlled by increased activity along the basin master fault (ESPFS) and slower subsidence in the CAG. Poorly constrained middle Miocene progradation also occurred for eastern alluvial slope deposits in the San Luis Basin to the north, and streams of the western alluvial slope carried coarser bedload at ~14 Ma. These observations imply a paleoclimatic influence for the rapid 13.5-11 Ma progradation, driven by increased discharge and stream competence. Increased precipitation, higher subsidence rates along the ESPFS, and emergence of the nearby Jemez volcanic field perhaps facilitated integration and headward elaboration of streams draining the Colorado Plateau, forming a single river roughly coincident with the location of the modern Rio Chama. High throw rates on a fault west of the ESPFS at 11-8 Ma facilitated the shift of the San Luis Basin-draining, axial river onto the northwest structural platform. But another driver for this westward shift may be larger sediment delivery from streams draining the eastern alluvial slope compared to the western alluvial slope and axial river.