New Mexico Geological Society Annual Spring Meeting — Abstracts

Constrained within a classic continental rift zone, the Rio Grande is a major throughflowing drainage system within the southwestern United States. Although rifting began in the Oligocene, the Rio Grande was not an established axial river system until early Pliocene. Furthermore, the onset of river incision did not occur until around the middle Pleistocene. The history of Rio Grande downcutting is an important component of the evolution of continental rift zones. Although the rift zone, as well as the Rio Grande itself, has received a lot of attention, detailed incision histories have had limited attention.

This study assesses the timing of Rio Grande incision within and around the Socorro Basin in central New Mexico. The approach of this study is to use tributary terraces to reconstruct paleo-Rio Grande elevations within the rift zone. Tributary terraces are the main focus of this study because axial Rio Grande terraces are not well preserved and are often complex in that they are interfingered with alluvial fan deposits. It is assumed that the tributary terraces form in response to Rio Grande base level change. Thus, they provide a record of Rio Grande elevations. Terrace profiling will provide a means to correlate the tributary terraces to remnant Rio Grande terraces, as well as reconstruct paleo-Rio Grande profiles. Four tributary terrace sequences within the Socorro Basin will be used in this study. Soil development will be used to correlate terrace surfaces between tributaries and to establish relative ages. One numeric age has been determined for a terrace surface in Socorro Canyon. The numeric age and the carbonate profile mass of this surface will be used to calibrate carbonate accumulation rates in the study area and will provide a basis for relative age determination.

Although the timing of downcutting is a major focus of this study, the ultimate goal is to provide an explanation of the external causes of incision. That is, is dowcutting of the Rio Grande a result of climatic conditions, tectonic conditions, or both?