Open-file Report - 611
Geology of the Eastern Plains of San Agustin and Upper Alamosa Creek

This report gives a detailed treatment of the geology of the eastern Plains of San Agustin and the upper (northern) reaches of Alamosa Creek (aka Alamosa basin). We synthesize previous studies of geology and provide new detail on basin fill stratigraphy and sedimentology. This report is a complement to a hydrogeologic study (due to be released 2021; summarized in OFR-615) regarding groundwater movement, age, and storage under the Plains of San Agustin and the upper Alamosa Creek watershed.

The Plains of San Agustin is a 55 mi long, 10–20 mile wide geographic feature located about 120–150 miles southwest of Albuquerque. It is comprised of a series of hydraulically closed basins that collectively form a northeast-southwest topographic depression. A set of low hills south of Datil divide the topographic depression into the West and East San Agustin basins. This report focuses on the East San Agustin basin, which hosts the Very Large Array (VLA) radio telescope observatory. It is also where the Augustin Plains Ranch, LLC, has proposed a contentious, large-scale groundwater extraction and artificial recharge project. Three grabens have been identified in the East San Agustin basin, where the Earth’s crust has subsided along fault lines and the sedimentary basin fill is relatively thick. The Augustin Plains Ranch project is located in the north-south trending North graben, which has a bedrock floor and overlying strata that are tilted westwards towards faults at the base of the eastern Datil Mountains. Basin fill thickens westwards towards these faults, where it is at least 3,500 ft thick.

The upper reaches of Alamosa Creek flows 15 miles south in a ~9 mi wide, north-south trending topographic low that we call the upper Alamosa basin. This basin coincides with the northern Winston graben. This geologic structure is tilted westwards towards faults at the eastern foot of the Black Range. Basin fill thus thickens westward, as it does for the North graben, and is estimated to be as thick as 4,600 ft.

The geology of the study area has four main components: older sedimentary bedrock, volcanic bedrock, basin fill, and valley-fill alluvium The bedrock consists of volcanic and volcaniclastic rocks of the Mogollon-Datil volcanic field that overlie 750–1,900 ft of sandstones and mudstones of the Eocene-age Baca Formation. The lower Spears Group, composed mainly of tuffaceous debris flows and mud flows, comprises the lower 1,500–2,000 ft of the Mogollon-Datil field volcanic package and is interpreted to be an aquitard. Above the lower Spears Group lies 1,300–5,900 ft-thick sequence of interbedded ash-flow tuffs (ignimbrites), lava flows, volcaniclastic sediment (assigned to the middle-upper Spears Group) and minor tongues of non-volcanic, windblown sand (eolianites). The volcaniclastic sediments of the middle to upper Spears Group probably are poor aquifers. Eolianites are inferred to have both intergranular and fracture flow, and likely have higher permeability than the volcaniclastic sediments.
In addition, fracture networks may be sufficiently open and connected in the lava flows and tuffs, particularly the regionally extensive and thick ignimbrites, to allow appreciable groundwater movement.

Basin fill strata of the Santa Fe Group can be subdivided into the following facies: proximal to medial piedmont, distal piedmont, basin floor (or alluvial flat), and playa. In the southern part of the North graben, we recognize piedmont versus basin floor facies using geophysical-well-log and cuttings criteria from three wells with detailed wireline logs and/or cuttings records. Here, we interpret a western piedmont facies interfingering eastward with a finer-grained basin floor facies, which in turn interfingers eastward with an eastern piedmont facies. Two to three, “layer-cake” stratigraphic intervals can be locally recognized using inferred disconformities in wireline data or abrupt lithologic changes, but mapping their lateral extent away from wells awaits further study.

Overall, the texture of the basin fill in the southern North graben, the location of the proposed well field by Augustin Plains Ranch, is sand and clayey-silty sand that is capped by a 300–350 ft-thick, upper gravel-bearing interval with a notably high proportion of clay. This upper gravel-bearing interval experienced a dramatic eastward progradation in the Pleistocene that could be related to a slow down of faulting or paleoclimatic changes. Analyses of sonic and density-porosity well-log data from the 3,500 ft-deep SA-221 well indicates a general range in sand porosity of 5–25%, decreasing with depth. The notable clay content of subsurface sediment here—and also inferred for the northern part of the North graben and most of the C-N embayment—makes these areas susceptible to groundwater-related subsidence if there is large-scale pumping. In addition, the high clay content in the coarse, uppermost western piedmont unit of the southern North graben may impede infiltration from the surface to the water table. In the North graben, piedmont deposits of the lower part of the upper Santa Fe Group appear to have the highest proportion of high-porosity sands.

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