PLIOCENE-PLEISTOCENE SEDIMENTATION IN THE SOUTHEASTERN ACAMBAY GRABEN, CENTRAL MEXICAN VOLCANIC BELT

Pliocene-Pleistocene sediments in the southeastern Acambay graben, central Mexican Volcanic Belt show a period of lacustrine sedimentation in the late Pliocene that interrupts fluvial and alluvial sedimentation during the early Pliocene and Pleistocene. Climatic, volcanic, and tectonic fluctuations in the Acambay graben during Pliocene-Pleistocene time had important effects on sedimentation. The Lagunita beds are the oldest sedimentary rocks that outcrop in the study area and are early Pliocene in age. The Lagunita beds record fluvial and alluvial sedimentation of volcaniclastic sediments (>30 m thick) that are interbedded with thin ash flow tuffs and silicic volcanic breccias. Only two teeth from Equus sp. were collected in situ from the Lagunita beds. The Tierras Blancas beds unconformably overlie the Lagunita beds and are late Pliocene-early Pleistocene in age. The Tierras Blancas beds are ashy, diatomaceous lacustrine sediments (2-15 m thick) with interbedded volcaniclastic sandstones and have yielded Equus sp., cf. Rhynchotherium, and ?Camelops. Vertebrate burrows, interbedded sandstones and mudstones, and diatom variations within the Tierras Blancas beds indicate lake level fluctuations. Unconformably overlying the Tierras Blancas beds are ashy fluvial sediments (1-4 m thick), lava flows, scoria cones, and Quaternary alluvium and colluvium (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. Initial results of XRF analysis of volcanic rocks in the study area show calc-alkaline and medium- to high-K basaltic andesites, andesites, dacites, and rhyolites. The vertebrate fossil record of the Acambay graben contributes to mammalian biostratigraphy of central Mexico and understanding of the Great American Biotic Interchange.