New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed sed/strat study, geologic mapping, and new ⁴⁰Ar/³⁹Ar age data indicate the ancestral Rio Grande integrated through the previously endorheic (closed) Socorro Basin during 7.4-7.0 Ma, heralding the earliest known stage of southward axial river integration in southern New Mexico. An essential component of the geochronologic data is detrital sanidine (DS) analyses, providing maximum depositional ages (MDA) of deltaic and fluvial sand beds as well as provenance markers. Two stages are inferred for the playa-lake system occupying the Socorro Basin (“Lake Socorro”) prior to Rio Grande integration. First-stage lake sedimentation is characterized by pinkish-reddish gray to reddish-brown to pastel-colored (including greenish) clays, minor interbedded sandstones, and local gypsum precipitation. These strata are interbedded with the 9.9 Ma basalt of Kelly Ranch. The sandstones are tabular, fine- to medium-grained, and lack cross stratification. Mud cracks and other dessication features are absent or very sparse, and no signs of microfauna were observed in exposures 5-10 km north of Socorro. There, playa-lake clays intertongue to the northwest with a fan-delta deposited by the ancestral Rio Salado, based on southeast-paleoflow indicators and DS provenance signature (i.e., lack of Jurassic ages and lack of 25.1-25.5 Ma DS from the Latir volcanic field). Paleogeographic reconstruction using the 8.5 Ma basalt of Broken Tank and clast counts indicate a large, southwest-sloping fan formed a basin-wide, paleotopographic high ground near present-day San Antonio. Second-stage lake sediment consists of reddish brown clays interbedded with sand bodies, the latter locally coarsening upwards and exhibiting intricate micro-lamination with clays (horizontal-planar and ripplemarked, with thicker cross-stratification to the north). Gypsum precipitation is notably sparser than in the first stage. Paleoflow indicators in the sands are generally to the south. The reddish brown clay has 0-3% scattered sand grains and desiccation features are not obvious. 25.1-25.5 Ma DS ages from the Latir volcanic field are absent or sparse (<0.5%). Deposition during the closed-to-fluvially integrated basin transition is characterized by floodplain and lake(?) clays being subordinate or subequal to sands, abundant clay rip-ups in the sands, >1% Latir sanidine grains in DS analyses, and channels back-filled by western-sourced sediment or floodplain clays. Clayey sediment commonly exhibits desiccation features (clay brecciation, calcium carbonate nodules, shrink-swell mixing and slickensides).

Two basalts (9.9 and 8.5 Ma) flowed into the playa and both lack pillow features or extensive palagonite that would be expected of deep water. These two lavas, the 7.0 Ma basaltic andesite of Sedillo Hill, plus a coarse ash sourced from the local Grefco rhyolite dome (7.95 Ma) provide age control that supplements fifteen new DS MDAs. Using this age control and the aforementioned sedimentologic observations, we infer: 1) The first-stage playa-lake (>10.6 Ma to 8.0-8.5 Ma) rarely dried up but yet the water levels were quite shallow; the lake water may have been fed by shallow-discharging, sulfate-rich groundwater whose salinity or pH levels were not amenable for aquatic life. 2) The second stage (8.0-8.5 to 7.4 Ma) had slightly(?) deeper water levels, deltaic sands extended southward across the basin, and was possibly less saline and more amenable to aquatic micro fauna -- perhaps due to a higher influx of water from the Rio Grande and/or lake water draining southwards through permeable fan deposits. 3) The closed-to-fluvially integrated basin transition occurred between 7.4 to 7.0 Ma; the 7.0 minimum age is interpreted based on the lack of 6.9-7.0 Ma (Peralta Tuff) DS grains in the stratigraphically lowest fluvial sand tongues. The earlier paleotopographic high on the south end of the basin was progressively lowered by west-down faulting near San Antonio, accompanied by eastward basin-floor tilting, which was essential for allowing spillover of Lake Socorro. Higher inferred sediment and water discharges into the Socorro Basin by the Rio Grande during part of the transition, accompanying the observed post 7.4-Ma increase in 25.1-25.5 DS ages (from the Latir volcanic field), may also have facilitated this early Rio Grande integration.