New Mexico Geological Society Annual Spring Meeting — Abstracts

The southern Rio Grande rift constitutes a broad (150 km) region of mid-late Cenozoic crustal extension that is characterized by 20 fault blocks. The history of individual rangeboundary faults can be deduced by using the sedimentology and provenance of strata deposited in complementary basins. This approach is especially applicable to a 100 km stretch of the southern rift from Truth or Consequences to Las Cruces, where basin-fill strata ranging in age from Late Oligocene to Pleistocene are widely exposed.

Deposition of the Hayner Ranch Formation (upper Oligocene-mid Miocene) took place in a north to northeast-tilted half graben whose footwall was the Caballo Mountains. By the end of deposition of the Hayner Ranch Formation, stratigraphic separation on the Caballo fault system had reached ~1615 m. During deposition of the upper Miocene Rincon Valley Formation, an additional 850 m of stratigraphic separation occurred on the Caballo fault block and the Sierra de las Uvas began to rise, the latter creating a west-tilted half graben with ~550 m of stratigraphic separation on its boundary fault. The two late Miocene basins were linked by a gypsumprecipitating playa lake.

Deposition of the Camp Rice and Palomas formations (Pliocene-lower Pleistocene) was accommodated by continued movement along the faults bordering the Caballo Mountains (760 m, strat. sep.) and Sierra de las Uvas (100 m strat. sep.), as well as by new faults that uplilfted the Red Hills, Animas Mountains, Rincon Hills, northwestern Sierra de las Uvas, Cedar Hills, San Diego Mountain, and Robledo Mountains. During this time the ancestral Rio Grande flowed through six basins, with the location and width of the floodplain largely controlled by basin symmetry. Periodic kilometer-scale shifting of the axial river toward the Caballo footwall was a response to active faulting and basin tilting. Following deposition of the Camp Rice and Palomas formations (post-0.8 Ma), many pre-existing and new faults experienced activity, including 40 m of offset on the Caballo fault.