The Santa Fe Group in the northern Winston graben, southwest New Mexico, and how changing Rio Grande rift tectonism may have influenced its deposition and erosion
— Daniel J. Koning

The northern Winston graben of the Rio Grande rift, located 55-85 km northwest of Truth or Consequences, NM, is asymmetrically tilted westward towards the Black Range fault. This fault and the west-down Red Paint Canyon fault zone along the eastern side of the basin were primarily active in the Miocene, with probable waning activity in the Pliocene and minimal movement in the Quaternary. Fourteen stratigraphic sections and ~200 outcrop descriptions indicate that piedmont facies dominate the Santa Fe Group in the northern Winston graben, with clayey playa facies located west-northwest of the Monticello Box. Although there was a relatively major stream flowing into the playa from the north, there is no evidence it extended into the San Augustin Plains. The upper Santa Fe Group is generally weakly cemented and dips 0-15° west. Its piedmont sediment is differentiated into a proximal-medial, coarser-grained facies and a distal, finer-grained facies. The latter grades laterally into the clayey playa facies. The upper Santa Fe Group unit is interpreted to be middle(?) to late Miocene (possibly earliest Pliocene) in age because it is contiguous with slightly tilted basin-fill strata to the south capped by a 4.9 Ma basalt flow. The middle Santa Fe Group is assigned to steeply tilted (>15° dips) and moderately-strongly cemented strata. Early Miocene in age, the middle unit is locally exposed on the footwall of the Red Paint Canyon fault zone (probably in paleovalleys) and at Ojo Caliente warm springs. The oldest late Cenozoic sedimentary units consist of well-cemented, tuffaceous volcaniclastic sediment of the Beartrap Canyon Formation and unit of East Red Canyon; these are age-equivalent with the lower Santa Fe Group unit in the southern Winston graben. Both paleocurrent data and the spatial distribution of the aforementioned facies indicate a closed basin in the middle(?) to late Miocene, and possibly earliest Pliocene. Tectonic tilt rates are interpreted to have slowed in the latest Miocene and Pliocene, consistent with low dip magnitudes in the upper Santa Fe Group unit. During this decrease of tilt rates, presumably accompanied by a decrease in basin subsidence rates, paleosols developed in the distal piedmont facies and locally the western piedmont prograded eastward over the playa facies. Low subsidence rates probably facilitated spill-over and the transition from a closed to open basin. Latest Miocene-earliest Pliocene pediments along the margins of the basin were onlapped by thin (5-10 m, locally as much as 35 m) upper Santa Fe Group deposits. Widespread erosion of the Santa Fe Group in the Pliocene produced relatively flat, extensive surfaces in the Winston graben, concomitant with >100-130 m of aggradation in the Engle and Palomas basins to the east. A shift of tectonic strain towards the center of the rift likely accounts for this spatial contrast in erosion/deposition.