Progress report on the late Cenozoic geologic evolution of the lower Rio Puerco
— David W. Love and John D. Young

The geologic evolution of the lower Rio Puerco has been studied during the past two years as part of a long-term New Mexico Bureau of Mines and Mineral Resources project in the Albuquerque Basin, and as part of two specific projects on the Rio Puerco related to archeology (Love and others, 1982) and possible mine-generated radionuclides in sediments (Popp and others, 1983). These two studies concentrated on deposits of the modern Rio Puerco and the valley fill, but we studied the adjacent valley borders as well to determine sediment sources and to provide a longer time framework. Observations of deposits exposed along the valley borders (Young, 1982) suggested that evolution of the tributary drainages was more complicated than previously described (Denny, 1941, 1967; Wright, 1946; Kelley, 1977). Subsequent reconnaissance of deposits along the tributaries draining the Ladron and Lucero Mountains south of Mesas Mohinas (fig. 1) revealed some details of the development of this corner of the Albuquerque Basin. These investigations, coupled with studies in nearby areas by Machette (1978a, 1978b, 1982; Machette and McGimsey, 1983) document considerable variations in the tectonic and depositional development of the Albuquerque Basin in late Cenozoic time.

The physiography and hydrology of the Rio Puerco drainage basin are described by Love and others (1982) and by Heath (this guidebook). This paper concentrates on the area between Mesas Mohinas, the Ladron Mountains, the foothills of southern Sierra Lucero, and the Llano de Albuquerque (fig. 1). The relatively low-relief valley border west of the Rio Puerco in this area is known as Sabinas Solas (Wright, 1946) or Llanos del Rio Puerco (Titus, 1963). A topographically lower part of Llanos del Rio Puerco, known as Montano Flats, occurs approxi-mately 40 m above the present valley floor west of the Rio Puerco and slopes gently to the east. Valleys of major tributaries of the Rio Puerco, such as Comanche Arroyo and Coyote Draw, are incised from the foothills of the Lucero uplift to the Rio Puerco valley. Valleys of Arroyo Monte Belen, Alamito Arroyo, and Mariano Draw become incised midway across the Llanos del Rio Puerco.

Methods of study included detailed measurements of stratigraphic sections with standardized pebble counts (Young, 1982) and reconnaissance of surficial deposits and exposed sections with qualitative estimates of clast size, rounding, and composition. Complications are related to poor exposures, downslope movement of loose clasts, differential cementation, and possible tectonic juxtaposition. We sketched exposures of valley fill in greater detail to help interpret valley-fill stratigraphy for archeological purposes. Valley fill beneath present exposures was augered to a depth of 41 m (see Heath, this guidebook). We located fault scarps using aerial photographs and traced some scarps during reconnaissance field studies (fig. 2).