New Mexico Geological Society Annual Spring Meeting — Abstracts

The Ancha Formation (late Pliocene-early Pleistocene) underlies the Santa Fe embayment between Santa Fe and Galisteo Creek. Where saturated, this relatively coarse deposit constitutes an important aquifer. We use lithologic information from wells and outcrops to evaluate the geometry of its basal contact and spatial variations in its thickness and texture, which can then be used to assess aquifer thicknesses and groundwater flow paths.

The base of the Ancha Formation is a scoured angular unconformity. Bedrock units that lie beneath the Ancha Formation in the southern embaymen include the Espinaso and Galisteo Formations plus Mesozoic and Paleozoic strata. Northward of 35°31.5' N latitude, volcaniclastic Tesuque Fm units are present and then arkosic Tesuque sediment derived from the southernmost Sangre de Cristo Mountains. These older strata are 1 to 4 orders of magnitude less permeable to groundwater than the Ancha Formation, with the Tesuque Formation being more permeable than older strata.

We subdivide the Ancha Formation into two lithofacies assemblages. One was deposited by a southwest-flowing, ancestral Santa Fe River and is consistently coarse-grained in a downstream direction. Santa River deposits are as much as 70 m thick 1-2 km south-southwest of Agua Fria, but thin westward to 19-30 m under the Caja del Rio Plateau. North and south of the ancestral Santa Fe River deposits are alluvial slope assemblages deposited by smaller, west-flowing, ephemeral streams. The northern alluvial slope deposits pinch out northward and northeastwards against the Santa Fe uplands. The southern alluvial slope deposits are generally 15-45 m thick but thicken to 70-90 m in the center of the embayment. There is also a relatively thick (50-70 m), northwest-trending zone near El Dorado and Seton Village. The lower 10-40 m of the southern alluvial slope deposits is generally coarse-grained (sand and gravel). In overlying strata, sandy gravel dominates near the Sangre de Cristo Mountain front but gross texture fines westward to clayey-silty fine sand.

We attribute thickness variability to tectonics and pre-Ancha (late Miocene-early Pliocene) erosion. Footwall unloading along the La Bajada fault to the west may explain the eastward thickening of the Ancha Formation in the western embayment. The Ancha Formation also thickens over paleotopographic lows created by pre-Ancha erosion. The thick zone of Ancha Formation near El Dorado is underlain by Tesuque Formation that was likely preferentially eroded. Erosion also carved 5-15 m-deep paleovalleys (locally as much as 30 m deep). Although locally observed in outcrop, paleovalleys are defined regionally by contouring the elevations of the Ancha Formation base. Two paleovalleys are especially noteworthy. The northern is associated with the ancestral Santa Fe River and trends southwest near Arroyo de los Chamisos. The southern one trends west and then northwest from the Eldorado area. These two paleovalleys join 2 km east of La Cienega, and the downstream paleovalley roughly follows the course of the modern Santa Fe River, deviating to the south ~2.5 km southwest of La Cienega. The paleovalleys are hydrologically important because they may hold thick (up to 30 m) accumulations of saturated Ancha Formation.