2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 13
Presentation Time: 11:15 AM


DRAKOS, Paul1, LAZARUS, Jay1, RIESTERER, Jim1 and SIMS, Kenneth2, (1)Glorieta Geoscience, Inc, P.O. Box 5727, Santa Fe, NM 87502, (2)Dept of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, drakos@glorietageo.com

Subsurface lithologic and geophysical data from a series of municipal, exploratory, subdivision, and domestic wells are used to delineate variations in thickness and extent of Tertiary through Quaternary sediments and Pliocene basalt flow sequences in the southern San Luis Basin near Taos, New Mexico and are utilized to help constrain the location of buried faults. Two deep wells drilled in 2007 are used to refine previously published stratigraphic analyses of basin fill deposits. Analyses of long-term pumping tests conducted in relatively close proximity to faults are used to evaluate the effect of faults on groundwater flow in shallow and deep basin fill aquifers. Selected wells and surface water sources were sampled and analyzed for major anions and cations, trace metals, 87Sr/86Sr, 3H, δ2H, and δ18O. Tritium results indicate that recharge to the shallow aquifer occurs on a time scale of < 5 to 10 years, but that recharge to some deep alluvial wells and wells completed into sediments interbedded with the Servilleta basalts occurs on a time scale of > 50 years. Recharge into the mountain front fractured bedrock aquifers occurs on time scales ranging from < 5 to 10 years to > 50 years. δ2H and δ18O data suggest that some deep basin-fill aquifer wells have received higher-elevation or older (Pleistocene?) recharge, whereas other deep wells have received lower-elevation or modern/Holocene recharge. 87Sr/86Sr varies from 0.7074 to 0.7156, indicating water-rock interaction with variable source rocks. Because these waters have relatively enriched 87Sr/86Sr it is likely that the Pennsylvanian carbonates and granitic basement rocks have contributed significantly to their Sr isotopic compositions. Although faults typically do not act as impermeable boundaries in the shallow alluvial aquifer, the Seco fault and several of the Los Cordovas faults act as impermeable boundaries in the Servilleta Formation and in the deep basin fill aquifer. However, other Los Cordovas faults do not act as significant impermeable boundaries in the deep aquifer, suggesting variable cementation along fault planes at depth.