Subsurface Stratigraphy in the Southern San Luis Basin, New Mexico

Abstract

A BSTRACT .— Subsurface lithologic and geophysical data from a series of municipal, exploratory, subdivision, and domestic wells are used to delineate variations in thickness and extent of Tertiary through Quaternary sediments and Pliocene basalt flow sequences in the southern San Luis Basin. Servilleta basalts increase in thickness from south to north within the Taos Valley; they are absent in BOR1, present as a single thin flow in BOR3, and present as three multiple-flow sequences at BOR4, K3, RP2500, and the Airport well. The Ojo Caliente Sandstone Member varies in apparent thickness from thin or absent at BOR1 and BOR2/3 to greater than 1000 ft (300 m) at RP2500 to 340 ft (100 m) at the Airport well. The apparent thickness of the Ojo Caliente is greatest along the Rio Pueblo, suggesting deposition in a depression, possibly an ancestral Rio Hondo or Rio Pueblo drainage, during mid-to-late-Miocene time. The Ojo Caliente sand may have been derived from the west, accumulating against Chama-El Rito alluvial fans building off the eastern mountain front. Data from the drilling program are also used to identify and constrain offset on several intrabasin faults and a mountain front graben bounded on the west by the Seco Fault, across which the Servilleta Formation is offset 950 ft (290 m), and on the east by the Town Yard fault. The average subsidence rate within the graben is estimated as approximately 0.1mm/yr (0.3 ft/1000yr). Down-to-the west offset across the Town Yard Fault is > 1390 ft (420 m).