Holocene and Upper Pleistocene stratigraphy of Xochimilco lake, central Mexico

Abstract

The lacustrine system developed in the basin of Mexico has been of great interest in studies of various geological and social disciplines. Its sediments preserve the history of the environmental changes that occurred during the late Quaternary. Between 2016 and 2018, a set of lacustrine sediment cores were collected in the Xochimilco sub-basin, located south of the basin of Mexico. From these sedimentary sequences, a master sequence was constructed that represents the upper 66.5 m of the lacustrine deposits. For the establishment of its stratigraphic column, 15 lithotypes were defined from their sedimentary components in which the clastic components made up of silicates dominate, mostly silt and clay in size. The most abundant biogenic fraction is diatoms that frequently form oozes, and ostracods are a common component in most of the sequence, which locally form hashes. Interbedded with the clastic and biogenic lithotypes are volcaniclastic deposits. Eight lithological units were defined based on their dominant lithotypes. The time scale of the sequence is still uncertain for the oldest sediments; however, it is estimated that it covers the last 300000 to 350000 years. From this temporality, environmental oscillations of millennial scales are inferred. Diatom-rich laminated sediments represent the deepest phases in the lake. The deposits with higher biogenic and authigenic calcareous components indicate the phases of higher ionic concentration during which the lake was shallower. The detailed analysis of the stratigraphy and the description of the sedimentary components in Xochimilco allows to establish an initial interpretation of the variations in the depositional environments, and provides the base from which the detailed studies of paleoclimatic indicators will be supported.