INSIGHTS INTO 8-7 MA RIO GRANDE INTEGRATION FROM STRATIGRAPHY AND DETRITAL-SANIDINE PROVENANCE OF LAKE SOCORRO, AN UPPER MIOCENE PLAYA-LAKE IN THE CENTRAL RIO GRANDE RIFT, NEW MEXICO

Stratigraphic-sedimentologic study of paleolake Socorro, an upper Miocene playa-lake in the central Rio Grande rift, yields important clues regarding early integration of the rift’s namesake axial river through the Socorro basin. Integration of ⁴⁰Ar/³⁹Ar dates from lava flows and detrital sanidine (DS) elucidate three stages of lake development since 11 Ma. Because of concurrent rift volcanism, especially in the Socorro basin due to inferred deep magmatic intrusions, many DS ages approximate depositional ages. Lacustrine strata in the Lower-stage (11 to 9 Ma) contain 12% green claystone with sparse (2-3%) thin sandstone and 1% gypsum that were deposited in a persistently wet playa lake recharged by sulfate-rich groundwater and local ephemeral runoff. Middle-stage (9-8.4 Ma) lake strata has <1% gypsum and <3% sandstone. Late-stage (8.4-7.2 Ma) deposits lack gypsum and green claystone and contains 10-30% sand tongues with southward paleoflow indicators. For 9-7.2 Ma, we interpret upward-freshening and axially aligned paleocurrents to record linkage with the terminal Rio Grande and increased water discharge. DS geochronology indicates that the bulk of middle- to late-stage sands came from nearby volcanic rocks and the southeastern edge of the Colorado Plateau. Earliest axial fluvial deposits, containing much clay-rip-up clasts and desiccation features, indicate integration of the Rio Grande through the Socorro basin by 7.2-6.9 Ma. The fluvial sediment is associated with an abrupt increase of DS sourced from northern New Mexico, likely reflecting sediment-spillover of an upstream playa-lake system in the Albuquerque basin. The increased sand flux, in turn, promoted 7.2-6.9 Ma fluvial integration in the Socorro basin.

We interpret that Rio Grande integration ca. 8.5-6.9 Ma was influenced by both local and upstream tectonic factors. After 8.5 Ma, concentration of extensional strain onto an intrabasinal fault system lowered a paleotopographic high at the south end of Lake Socorro. Late Miocene integration of the ancestral Rio Grande was likely facilitated by increased sediment bypass through the Española Basin associated with a widespread unconformity dated between 8.0-5.5 Ma, possibly related to deep magmatic injection near the Jemez volcanic field.