New Mexico Geological Society Annual Spring Meeting — Abstracts

Fluvial systems are sensitive, yet complex gauges of active tectonic processes. The Rio Grande through northern New Mexico provides an ideal location to evaluate neotectonic influences on river integration and incision. Rio Grande axial rivers did not reach southern rift basins until 4-5 Ma and did not reach the Gulf of Mexico until after 1 Ma; instead, upstream drainage was divided among several intermittently connected, internally drained basins. The processes that influenced hydrologic integration of Rio Grande rift basins, and timing of integration of sub-basins, remain incompletely understood. This study aims to reconstruct the birth of this continental scale river system, understand the time evolution of its integration, and study differential incision of the northern Rio Grande/Chama system over the last ~5 Ma.
Detrital zircon and detrital sanidine dating of modern and ancestral Rio Grande river sands, combined with ⁴⁰Ar/³⁹Ar dating of adjacent basalt flows, is underway to add new data to ongoing studies of the provenance for various paleoriver deposits. Thirteen detrital grain samples have been collected from paleoriver deposits that range in age from 4.5 Ma to modern sands. Progress to-date to constrain the ages of the paleoriver deposits and associated paleoprofiles includes eight new ⁴⁰Ar/³⁹Ar ages on basalts that overlie or underlie the deposits. Ages are reported at 2σ uncertainty relative to FC-2 at 28.201 Ma and 40K total decay constant of 5.543e-10/a. At the Red River confluence 4 new basalt ages listed in stratigraphic order (bottom to top) are 4.93± 0.03 Ma (lower Servilleta flow), 4.85± 0.03 Ma and 4.79± 0.08 Ma (middle Servilleta flow), and 3.43± 0.08 Ma (upper Servilleta flow at this location). The distinctly older ages for the middle/lower Servilleta basalts indicates a ~1.4 Ma time gap between the lower/middle and upper Servilleta flows. The middle/lower Servilleta ages at the Red River confluence are nearly analytically identical and likely represent several flows emplaced over a ~100 ka interval. Pilar Mesa basalts have stratigraphically constrained ages (bottom to top) of 3.48± 0.12 Ma, 3.36± 0.06 Ma, and 3.09 ±0.10 Ma, which correspond to the age of the upper Servilleta flow at the Red River confluence.
A new basalt age at Black Mesa of 4.51±0.03 Ma, interpreted with respect to existing ages of ~3.5 (Ar-Ar) Ma and a 2.85 Ma age (K-Ar) leads to a testable hypothesis that Black Mesa basalt flows record a longer history of basalt flows than previously known. The observed river deposits under basalts of 4.5 and 3.5 Ma, suggest that basalts likely flowed down ancestral paleorivers (e.g. possible paleo Rio Grande, paleo Rio Ojo, or paleo Rio Embudo) at several times during the overall aggradational accumulation of Servilleta basalts on the Taos Plateau from 4.8 to 2.8 Ma. Possible local vent sources for the flows needs to be investigated, and provenance of underlying deposits clarified, but the indication of multiple ages of basalt flows at Black Mesa raises the possibility of elucidating fluvial connectivity between the Espanola basin and upstream headwaters from 4.5 to perhaps 2.85.