The Pliocene to lower Pleistocene Ancha Formation, upper Santa Fe Group, is a relatively coarse deposit found south and west of Santa Fe, northern New Mexico. It extends southward from the downdropped southern Española Basin of the Rio Grande rift onto a weakly faulted structural platform that extends to the Rio Galisteo, a distance of approximately 30 km (19 mi). The Ancha Formation is found as far west as the La Bajada escarpment (also ~30 km distance). The Ancha Formation is texturally variable but predominately a sand to gravelly sand, with clayey-silty, fine-grained sand increasing towards the southwest. Examination of well logs indicates that the lower part of the Ancha Formation is commonly gravelly. Due in part to its relative coarseness, the Ancha Formation forms a locally important shallow aquifer for the Santa Fe area. The characteristics of the formation’s base and its thickness are important to regional groundwater studies and are also useful for other studies involving basin stratigraphy, structure, geophysical interpretations, and basin evolution. The base of the Ancha Formation coincides with a Pliocene erosional surface overlying tilted and faulted beds of the Tesuque Formation (upper Oligocene-upper Miocene), the Espinaso Formation (upper Eocene to lower Oligocene), the Galisteo Formation (Eocene), and, locally, older Mesozoic and Paleozoic units. In order to characterize the thickness and the basal contact of the Ancha Formation, three data sets were evaluated: (1) cuttings and geophysical logs of key exploration drill holes and water wells, including monitoring wells; (2) lower resolution, generalized lithologic logs from water wells; and (3) outcrop exposures of the basal contact. This report presents the latest lithologic, thickness, and hydrologic observations for the Ancha Formation in the Santa Fe embayment in the form of four map plates: (1) Plate 1, elevation contour map of the base of the Ancha Formation; (2) Plate 2, isopach map showing thickness of the Ancha Formation; (3) Plate 3, saturated thickness of the Ancha Formation (2000 to 2005 conditions); and (4) Plate 4, subcrop geologic map showing distribution of strata underlying the Ancha Formation. Supporting data are presented in five tables.
