New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande Rift is a classic continental rift system and is located in the southwestern United States. Like many of the sub-basins of the Rio Grande Rift, the Socorro Basin, located in central New Mexico, is structurally controlled. It is characterized by 'domino-style rifting' where the west and east margins are rotated fault blocks.

The Rio Grande became a through flowing axial river system during the late Pliocene, and presently flows through many of the sub-basins of the Rio Grande Rift. Aggradation within the rift began during the early Pliocene and continued until the middle Pleistocene when a major change in base level caused the Rio Grande to incise. Tributary terraces are currently being correlated and used in the evaluation of the Rio Grande's incision history within the Socorro Basin.

Soil development has widely been used as a correlative tool for geomorphic surfaces, and in the deserts of the southwestern United States, soil development is dominated by the accumulation of secondary calcium carbonate (CaCO₃). Many studies have used soil profile mass of CaCO₃ and published accumulation rates to calculate ages and correlate surfaces. However, few estimates of the variability of CaCO₃ accumulation rates have been made to validate regional correlation or age estimates. This study correlates terrace surfaces by using elevation above the modern arroyo, then assesses the regional variability of CaCO₃ accumulation rates by comparing the CaCO₃ profile mass on terraces of the same age.

One tributary terrace surface has been dated using Chlorine-36. This age in conjunction with the carbonate profile mass, yields a CaCO₃ accumulation rate for that surface. Assuming the accumulation rate has been temporally constant, the ages of all terraces within the sequence were determined. Correlated terraces were then assigned ages based on the dated terrace sequence. Using the correlated age and the CaCO₃ profile mass of each terrace, accumulation rates were calculated for each tributary. If similar terrace elevations in different tributaries represent regional geomorphic surfaces, then it is clear that there is spatial variability in the CaCO₃ accumulation rates within the Socorro Basin.