Near-surface Hydrologic Investigations of Pleistocene lakes within the Valles Caldera, New Mexico

The Valles Caldera lies within the Jemez Mountains along the western margin of the Rio Grande Rift, New Mexico, USA. Following caldera formation at 1.2 Ma, Pleistocene lakes periodically filled parts of the caldera. At least three lakes spanned all or part of two valleys within the caldera, Valle Grande and Valle San Antonio. Two drill holes near the edge of Valle Grande indicate 80-90m of lacustrine mud and silt atop at least 250m of pumiceous sand and gravel. Drilling in Valle Toledo encountered 24m of clay-rich sediments atop 60m of sand and gravel.

As part of the 2019 Summer of Applied Geophysical Experience (SAGE) program, geophysical field investigations were carried out to investigate the structure and hydrology of these valleys. Within Valle San Antonio, electrical resistivity tomography (ERT), ground penetrating radar (GPR), and shallow seismic experiments focused on a series of low and marshy areas of land called fens, while transient electromagnetics (TEM) data were acquired in Valle Grande.

Within Valle San Antonio, GPR proved effective to a depth of 3m, mapping the configuration of paleo-stream channels surrounding nearby San Antonio creek. A seismic refraction model across a volcanic constriction indicates a shallow water table and a depth to fractured rhyolite of only a few meters. ERT-derived resistivity cross sections across two fens offer structural clues as to their origin. In one case, groundwater is interpreted to be deflected upward along the edge of a rhyolite flow, while in the other case, a layer of conductive lacustrine sediments, cut by stream incision, allows deeper groundwater to ascend through an otherwise low-permeable clay layer.

TEM-derived resistivity models in Valle Grande are in close agreement with drill hole lithology, imaging a conductive (clay) unit that is remarkably uniform throughout Valle Grande, nowhere exceeding 100m in thickness. A moderately resistive unit underlies the clay, thickening to at least 500m in the basin center. Near the valley edge, a second 300-400m deep could reflect clay deposits within an older lake bed or sediments or volcanic rocks hydrothermally altered during or just after caldera collapse. Along the northern edge of Valle Grande, resistive rhyolite flows from a nearby rhyolite vent (Cerro del Medio) are imaged extending into the basin in the subsurface.