Evolution of the Rio Grande rift in the Socorro and Las Cruces areas
— Charles E. Chapin and William R. Seager

Much controversy in recent years surrounding the Rio Grande rift has been caused by failure to look beyond its present geomorphic expression at its past history. Specifically, when did rifting begin? ... What were the early basins and uplifts like? ... Where were they? ... How big? ... How much volcanism and of what kind? ... What were the main source areas for the basin-fill sediments? ... Under what conditions were the sediments transported and deposited? ... Was there a culmination of uplift and faulting? ... If so, when? ... Has the type or rates of sedimentation and volcanism changed with time? ... When did integrated drainage begin and how has it evolved? ... If the rift was different in the past, what might it be like in the future? The answers to these questions, and to others unperceived, are recorded in the basin-fill deposits. To read this record requires knowledge of the sequence and distribution of rock types on the uplifts in order to work out the sequence and facies of deposits in the basins which, in turn, is necessary to unravel the structural history of the up-lifts. In other words, a circuitous and often repetitious path of investigation, from basin margin to basin fill and back again, is necessary to understand the whole.

 

The purpose of this paper is to document the sequential development of the Rio Grande rift in two areas where the basin-fill deposits are well-exposed and where extensive field studies have provided the necessary stratigraphic detail. The similar geologic histories of these two areas (interpreted independently until our collaboration on this paper) gives us confidence that the data may be of wider applicability. We have used time subdivisions for the Cenozoic as given by Harland and others (1964); they are plotted on Figure 9.

 

We wish to acknowledge the work of our colleagues without whom this paper could not have been written. Russell E. Clemons, John W. Hawley, and Frank E. Kottlowski have pro-vided major contributions to our understanding of the Las Cruces area. Thesis investigations by R. B. Blakestad, D. M. Brown, J. E. Bruning, C. Burton, R. M. Chamberlin, E. G. Deal, D. A. Krewedl, D. B. Simon, E. J. Spradlin, W. H. Wilkinson, and T. M. Woodward aided greatly in unraveling the late Cenozoic history of the Socorro-Magdalena area. G. 0. Bach-man and J. W. Hawley provided stimulating discussions of the late Cenozoic sedimentary and geomorphic features and improved the manuscript. W. E. Elston read parts of the manuscript and provided data and helpful advice on the Datil-Mogollon volcanic field. P. E. Damon was a catalyst for the section on Transitional Volcanism from a discussion at the 1974 Geological Society of America Meeting in Flagstaff and through recent discussions of unpublished data. We are espe cially indebted to J. W. Stinnett, Jr., for permission to use unpublished chemical and isotopic data. To all of these people we express our gratitude; any errors in interpretation, how ever, are solely the responsibility of the authors.