New Mexico Geological Society Annual Spring Meeting — Abstracts

A summary review of previous and ongoing research in the Rio Grande rift constrains the timing, character, and patterns of aggradation and incision of basin-fill in this tectonically active region. Regional incision of the synorogenic Santa Fe Group marks a transition from widespread aggradation, which dominated much of the SE Basin and Range, to incision associated with the Rio Grande Valley. The transition from aggradation to incision is generally well understood south of the Española basin. To the north, it is complicated by extensive volcanic activity, the character of strain accommodation zones developed between basins, and higher relief and greater precipitation in upland drainages. Distinct geomorphic surfaces associated with local depositional tops of the Santa Fe Group provide a record of tectonic and climatic controls on landscape development. Older geomorphic surfaces and associated deposits are preserved along basin margins, typically in updip positions of tilted basin floors. Younger surfaces conservatively constrain the beginning of regional entrenchment to between 0.7-1.2 Ma. Widespread and nearly synchronous incision of basin fill by the ancestral Rio Grande occurred during a time of increased magnitude of climatic oscillations.

Extensive Mio-Pliocene erosion surfaces provide paleogeomorphic clues as to the interactions between sediment supply and tectonics. Widespread erosional surfaces, recognized along the margins of the Española, Albuquerque, and Socorro basins, formed between about 7-5 Ma, and mark a transition from earlier internal drainage and fairly rapid sediment accumulation, to a later time of drainage integration and slower accumulation of coarser-grained sediment. The angularity of Mio-Pliocene unconformities indicates that erosion occurred slightly after (or possibly during) deformation. The weakly deformed character of overlying sediments suggests that Plio-Pleistocene aggradation continued during times of lower tectonic activity. The reduction in sedimentation rates and transition from internal drainage to fluvially integrated drainage supports an overall reduction in tectonic activity, which would allow upstream basins to fill and integrate southward.