Stratigraphy and Sedimentology of the Upper Pleistocene to Holocene Lake Chalco Drill Cores (Mexico Basin)

Abstract

The Basin of Mexico is a high elevation (2240 m asl), large (9540 km2), tectonic endorheic basin developed in the central-eastern Trans-Mexican Volcanic Belt. In 2016, the ICDP MexiDrill project recovered a total of 1152 m of sediments from a maximum depth of 520 m in Lake Chalco, in the SW of the Basin of Mexico. The upper 309.15 m (composite sequence, mc) are composed of fine-grained lacustrine sediments alternating with discrete visible tephra layers, and the lower 200 m are primarily volcaniclastic facies and basaltic lavas with some intercalated fluvial and alluvial facies. Initial lacustrine deposition started in the Chalco Basin, after the deposition of a thick volcaniclastic sequence, and continued until the lake was drained in recent centuries. Lake Chalco has remained as a shallow lake until the present day.Five main lithotypes throughout the core have been defined: (1) Organic, with (i) organic-rich silty clay, (ii) sapropelic silty clay, and (iii) peat; (2) Diatomaceous, with laminated, banded, and mottled diatomaceous silty clay; (3) Calcareous, with banded or massive carbonate silty clay and ostracod-rich layers; (4) Clastic, with silty clay, sand, and gravel, both clast and matrix-supported; and (5) Volcanic, as (i) volcaniclastic layers (tephras and clastic deposits) and (ii) lavas. Eighteen lithological units have been defined based on lithotypes and magnetic susceptibility values. Laminated diatomaceous facies occurred during phases with deeper depositional environments. Carbonate deposition marked more alkaline, shallower phases. Peat and organic-rich silts deposited during periods with better development of wetlands in the basin. The preliminary chronological framework suggests the lake sequence spans at least the last 367 ky. More saline diatom assemblages in diatomaceous facies and the occurrence of carbonate lithotypes and peat layers suggest relatively lower lake levels during interglacials; higher lake levels during glacial stages are indicated by more frequent freshwater diatom assemblages and the occurrence of finely laminated diatomaceous facies. The detailed stratigraphy and facies descriptions from the MexiDrill cores provide the basis for subsequent development of chronology and paleoenviromental studies of the Mexico basin record. Furthermore, the depositional model for Chalco is applicable to lakes developed in active tectonic and volcanic settings.