GEOLOGY OF THE SOUTHEASTERN ACAMBAY GRABEN, CENTRAL MEXICAN VOLCANIC BELT

The Acambay graben, Mexico, is a late Cenozoic intra-arc basin of the Mexican Volcanic Belt that contains fossil-bearing sediments of latest Pliocene-Pleistocene age. Detailed geologic mapping in the Acambay graben provides important data for understanding the stratigraphy of the study area. The oldest rocks in the study area are folded fluvial and alluvial sediments (>30 m thick) that are interbedded with ashy volcanic sediments and ash flow tuffs. Only two teeth from Equus sp. were collected in situ from this section. Ashy, diatomaceous lacustrine sediments (2-15 m thick) with interbedded sandstones unconformably overlie the folded section and have yielded Equus sp., cf. Rhynchotherium, and ?Camelops. Unconformably overlying the fossiliferous lacustrine sediments are ashy fluvial sediments (1-4 m thick), basalt flows, scoria cones, intermediate volcanic rocks, and Quaternary alluvium (0.5-5 m thick). One ash bed from this upper section has yielded a zircon fission-track age of 1.20 ± 0.13 Ma. Volcanic ash layers and volcanic rocks are currently being analyzed by X-ray fluorescence and electron microprobe to assist in petrogenetic interpretations. The vertebrate fossil record of the Acambay graben contributes to mammalian biostratigraphy of central Mexico and understanding of the Great American Biotic Interchange. Stratigraphic study of this area enhances the understanding of tectonic and climatic controls on deposition in the Acambay graben during the late Cenozoic.