CONSTRAINTS ON THE AGE OF EXTENSIVE FLUVIAL FACIES OF THE UPPER SANTA FE GROUP, ALBUQUERQUE AND SOCORRO BASINS, CENTRAL NEW MEXICO

Many 40Ar/39Ar ages and geochemical correlations determined from primary and fluvially recycled tephra and volcanic flows in the upper Santa Fe Group (SFG) of the Albuquerque and Socorro basins constrain the timing of development of external drainage in these basins and provide a basis for correlation in the Rio Grande rift. The hallmark of the upper SFG, the onset of major regional fluvial deposition (ancestral Rio Grande, ARG) and major tributaries, began in the northern basin before deposition of a 6.9-Ma Peralta Tuff tephra. Major fluvial fans, not clearly tied to the northern ARG, developed between 7.0 and 9 Ma. In the Socorro area to the south, playa-lake facies of the Popotosa Fm. were deposited from 15.2 to 6.9 Ma. A trachyandesite flow at San Acacia (4.87±0.04 Ma) overlies an eastern piedmont facies that may have graded westward into ARG deposits. The basalt of Socorro Canyon (3.73±0.1 Ma) flowed eastward onto ARG deposits. These relationships and ages suggest that the establishment of fluvial deposition became younger from north to south.

The youngest fluvial SFG is constrained by the 1.22-Ma ash from the Tshirege Mbr (Bandelier Tuff) near the top of ARG sections near Tonque Arroyo, Tijeras Arroyo, Hell Canyon Wash, and Socorro basin, and by a 1.05-Ma ash of Valles Dome rhyolite in Hell Canyon. An ARG gravel inset below the top of the SFG along the western margin of the Rio Grande valley contains tephra correlated to the 0.66-Ma Lava Creek B ash. Thus, SFG deposition ceased between 1.0-0.7 Ma south of Santo Domingo. This constraint on the initial incision of the Rio Grande valley and cessation of extensive ARG deposition is similar to events in southern New Mexico dated between 0.7-0.8 Ma. This widespread, relatively contemporaneous, and rapid incision suggests that the development of the Rio Grande valley from Santo Domingo to Las Cruces, NM, was strongly influenced by climate.