A geophysical study of the San Luis Basin
— G. Randy Keller, Lindrith Cordell, G. H. Davis, W. J. Peeples, and G. White

The San Luis Basin (Fig. 1) is an elongate intermontane valley located in south-central Colorado and north-central New Mexico. Stretching approximately 240 km in a north—south direction, it is bordered on the west by the San Juan and Tusas Mountains and on the east by the Sangre de Cristo Mountains. At its northern terminus near Poncha Pass, the valley is only 10-12 km wide, but it widens to approximately 75 km near Alamosa, Colorado. The valley floor, although appearing relatively flat, is built from the coalescence of alluvial fans which slope gently from the mountain flanks toward the center of the valley. Fifteen kilometers south of Alamosa, this sloping terrain is broken by the San Luis Hills, a series of flat-topped, low-lying volcanic hills extending along a north—south trend to the Colorado—New Mexico border. Geographically, the valley has been arbitrarily defined as terminating 15 mi south of the state line (Siebenthal, 1910). However, Upson (1939) noted that the valley is not a geological entity unto itself and instead merges southward into the Taos Plateau which is underlain by a Holocene volcanic field (Lipman and Mehnert, 1979). In this study, we have adopted the structural definition of the San Luis Basin (Kelley, 1956) and extended it to include the Taos Plateau with a southern termination at the Embudo constriction just southwest of Taos.

Since the early studies of Siebenthal (1910), Upson (1939), and Kelley (1956), the San Luis Basin has been recognized as being one of the major structural elements of the Rio Grande rift (Chapin, 1971). However, its subsurface configuration is poorly known, and it is thus a good target for geophysical studies. Early gravity studies (Gaca and Karig, 1965) were hampered by the lack of topographic maps. Cordell (1978) conducted a gravity study of the Taos Plateau region, and Davis (1979) conducted a gravity study of the Colorado portion of the San Luis Basin. These surveys became the major source of gravity data for this study and were merged into a consistent data base which was used to create Bouguer anomaly maps of the Rio Grande rift (Cordell et al., 1982) and New Mexico (Keller and Cordell, 1983). A Bouguer anomaly map of the San Luis Basin area is shown in Figure 2. Zietz and Kirby (1972) published a regional aeromagnetic map of Colorado, but the flight-line spacing was sparse in the San Luis Basin area. An aeromagnetic map of New Mexico has recently been completed (Cordell, 1983) and a portion of this map, which includes the southern San Luis Basin, is presented elsewhere in this volume. Seismic data of sufficient resolution to be of use in this study are very sparse. In fact, the reflection profiles of Davis and Stoughton (1979) are the only publicly available data. Although quite limited in areal extent, these data provide valuable constraints on the subsurface structure in the northernmost San Luis Basin.

Reviews of the geologic setting and history of the San Luis Basin area can be found in Baltz (1965, 1978), Tweto (1979), Lipman and Mehnert (1979), and Woodward and Ingersoll (1979). The brief discussion which follows is based on these studies and documents the main aspect of the geologic history impacting geophysical interpretations, which is the fact that little or no pre-Eocene Phanerozoic rock is present beneath the basin because the region has been high standing throughout most of the Phanerozoic. During the early Paleozoic, the area was located on the Transcontinental arch. The Uncomphagre—San Luis highland extended through the area in the Pennsylvanian, but its eastern margin may have been located in the eastern portion of the area presently occupied by the San Luis Basin. The entire region was uplifted again during the Laramide orogeny as part of the Sangre de Cristo—Brazos uplift and virtually all pre-existing sediments were eroded away at that time. Some Eocene alluvium was deposited at least locally (Tweto, 1979), and then rocks of the San Juan volcanic field covered the area in the Oligocene (Steven, 1975). Drilling data and outcrops in the Taos Plateau area, San Luis Hills, and Sangre de Cristo Mountains confirm that these volcanic rocks still underlie the basin. The formation of the Rio Grande rift caused block faulting which has preserved these volcanic rocks in the resulting grabens. Thus, for the purpose of geophysical interpretations, the basin can be considered to consist of graben fill overlying Oligocene volcanic rocks which lie on the Precambrian basement.