Cenozoic environmental evolution of the San Juan Raya Basin, south central Mexico

This thesis was carried out to gain a better understanding of the Cenozoic environmental evolution of the San Juan Raya Basin, which is a sub-system of the Tehuacan Valley in central Mexico. The likely roles of tectonics and climate in the geomorphic processes of the basin were examined by analysing the geology, geomorphology, depositional environments, as well as other palaeo-records: palaeosols, fossils and geochemical deposits. The correlation of 11 sedimentary sections was established on the basis of 12 radiocarbon dates. A glyptodont (Xenarthra) fossil discovered as part of this study provided important palaeoenvironmental information. The first hypothesis on the Neogene evolution of the basin is put forward, indicating that this system was formed by NE faults probably since the late Miocene or later. The asymmetry of the alluvial landforms indicates that the main factor controlling the development of Pleistocene fans from the north slope was tectonics. Late Pleistocene-Holocene deposits are more widely spread and represent environmental changes since 28.5 kyr ago. Macrofossil and stable isotopes suggest a mixed vegetation and high evaporative conditions under a poorly drained basin before the Last Glacial Maximum (LGM), probably under a colder than present climate. No records are available between the LGM and the end of the Pleistocene. Higher moisture availability was inferred in the early Holocene on the basis of high deposition rates and in situ tufa, followed by periods of intense deposition during the middle Holocene. A discrete alluvial fan reveals a combination of climate and tectonics. Alluvial incision since 2.3 kyr BP coincides with the establishment of current climatic and geomorphic conditions. Late Quaternary erosion and deposition suggest that climate and tectonics have played a dominant role in controlling the geomorphic processes of this basin.