Age of the Rio Grande Valley in Southern New Mexico

Abstract

On the basis of physiographic and lithologic evidence, previous workers in southern New Mexico believed an ancestral Rio Grande occupied a course through the Jornada del Muerto Basin, across La Mesa, and into northern Mexico, where the waters evaporated in the lake region. Subsequently, a lower river extended headward through the gap at El Paso, Texas, captured the upper stream, and diverted its flow to the Gulf of Mexico. A re-examination of the evidence in the southern Jornada Basin and of La Mesa shows that the postulation of the ancestral course of the river is highly conjectural and cannot be proved. Bordering the Rio Grande Valley are gravels containing a vertebrate fauna including Cuvieronius and Equus, which indicate a Kansan age. Three paleosols separate the gravels from overlying sediments on which the Jornada-La Mesa surface is formed. This surface must date from mid-Pleistocene. Distribution of gravel on La Mesa surface west of the present Rio Grande Valley but derived from rocks in the mountains east of the valley precludes existence of the valley in Jornada-La Mesa time. The Jornada surface occurs in an internally drained basin extending northward toward central New Mexico. The La Mesa surface was an internally drained basin extending southward into Chihuahua, Mexico. The Rio Grande now is entrenched northwest-southeast below the Jornada-La Mesa surface and is bordered by a stepped sequence of geomorphic surfaces, the youngest of which is less than years old by radiocarbon dating. Thus the river valley at a maximum is mid-Pleistocene in age and has been emplaced in the same general axial position since that time.