New Mexico Geological Society Annual Spring Meeting — Abstracts

Previous and new age control indicates coarse sedimentation in the Rio Grande rift ca. 5 Ma. We describe three sites illustrating this coarse sedimentation and discuss how it could be due to tectonics, climate, and inter-basin fluvial spillover.

The first site lies in northern New Mexico at the structural high between the Española and San Luis basins. Here, a 1-25 m thick package of 4.7 to 5.0(?) Ma sand and gravel unconformably overlies Late Miocene sandy strata with 1-5% pebbles. The former consists primarily of medium- to very coarse-grained sand, pebbles, and subordinate cobbles that were deposited by three west- to southwest flowing paleodrainages sourced in the southern Taos Range and Picuris Mountains. The northern two paleodrainages merged westward to form the ancestral Rio Grande (ARG), whose associated clasts were reworked in 4.0-4.6 Ma maar deposits 5 km east of Ojo Caliente.

The second site consists of a 1200 ft-deep well in the west-central Socorro basin, which penetrated the entire Plio-Pleistocene ARG sedimentary package and bottomed in 30 ft of clayey playa deposits. There is a coarsening-upward trend from 1170 to 1050 ft, and sediment is particularly coarse between 802 and 1050 ft (common well-rounded quartzite and granite clasts). We suggest that the 1050-1170 ft interval reflects initial Rio Grande sedimentation in a semi-closed basin.

The third site lies at the structural high between the Palomas and Engle basins near downtown Truth or Consequences (TRC). Here, the basal scoured erosion surface of ARG sediment overlies finer, locally derived piedmont sediment. The lower 20-25 m of the ARG is markedly coarse, containing many cobbly-bouldery beds that are lacking or very sparse in higher strata. An ⁴⁰Ar/³⁹Ar age of 4.8 Ma (from cryptomelene) projects 9-13 m above the base of the unit. The lowermost 3 m of the ARG deposit consists largely of limestone pebbles-boulders from the Mud Springs Mountains, and elsewhere in the lower coarse package are abundant cobbly beds whose gravel were derived from mountains to the west or northwest.

We agree with Gragg et al. (this vol.) that a decrease in rift tectonic extension rates occurred near the Mio-Pliocene boundary, which would promote fluvial spillover and southward expansion of the Rio Grande (Connell et al., 2005, 2012). At the first site, a decrease in tectonic subsidence rates would be conducive to progradation of coarse, footwall-derived sediment across the southern San Luis basin. However, hanging wall sediment also coarsened in the eastern Española basin half graben between 5 and 7 Ma, which is is not readily explained solely by decreased tilt rates. Previous studies indicate climatic changes at about this time, which can account for the increased competency of streams irrespective of tectono-structural position as well as suggestions of increased spring discharges in the TRC area ca. 5 Ma. High initial slopes on the downstream side of inter-basin paleotopographic highs would promote temporarily high stream power and coarse sedimentation associated with fluvial spillover, but increased stream competency of tributaries at the third site is probably best explained by climatic factors.