Cenozoic alluvial deposits of the Upper Gila River area, New Mexico and Arizona

Abstract

Cenozoic deposits in intermontane basins in the upper Gila River basin have been collectively included in the Gila conglomerate of Gilbert and Ransome. Re-examination of type-section areas shows that the term is unsatisfactory because it includes a substantial proportion of deposits other than conglomerates; it suggests that deposits in separate basins are identical; its use masks sequences of alluvial deposits within individual basins and relationships of deposits between basins; and it oversimplifies a complex Cenozoic history. It is suggested that the term Gila conglomerate be abandoned. The deposits, in the many basins, which heretofore may have been included in the Gila conglomerate, can be separated into two major divisions which may in turn be subdivided into mappable rock units. In each structural trough the upper units are characterized by: (1) Truncation by the highest pediment surfaces; (2) depositional or normal-fault contact with adjacent mountain areas whose composition the deposits reflect; (3) size-gradation relationships that suggest deposition within the boundaries of the present structural troughs; and (4) lack of mineralization. Limited fossil evidence suggests that deposition occurred during Pliocene to Pleistocene (Kansan) time. The lower units crop out in areas of deep dissection or structural uplift. They may be in fault contact with the upper units, underlie them unconformably, or grade into them. The lower units are characterized by: (1) Texture and composition that suggest deposition in basins other than those reflected by the present topography; (2) thrusting or normal faulting more complex than that associated with the upper units; (3) local mineralization; and (4) shallow intrusion. Limited fossil evidence sug gests a Miocene age for some of the older of the lower units, The alluvial deposits in individual basins in this area are amenable to stratigraphic analysis by standard procedures for describing rock units Correlations within or between basins should be on the basis of proper fossil or stratigraphic evidence and should be limited to the particular units involved and not expanded to include all the alluvial deposits in the areas concerned.