Clay mineralogy and diagenesis of the Recent-Pleistocene volcanogenic sedimentary sequence of the Mexican Basin

Abstract

Diagenesis of the Recent-Pleistocene volcanogenic depositional sequence of the Mexican Basin developed, in the gravel and sand strata, kaolinite, interstratified kaolinite/2H₂O-smectite, opalC, 1H₂O and 2H₂O-smectite and Si-contaminated smectite platelets, intimately associated with fine volcanic ash. The diagenesis initiated by silicification, sapropelic, in an environment of high permeability and hydraulic conductivity. At progressively higher depths, with decreasing permeability and flow, and higher alkalinity, kaolinite was transformed, via interstratified kaolinite/smectite, to 2H₂Osmectite platelets, and volcanic glass was directly transformed to smectite lamella. The mudstones are constituted by 2H₂O-smectite. In the smectite, cation replacement was in the tetraand octahedral sheets, but the site of the charge is largely in the octahedral one. Layer charges vary between 0.2-0.9, some within the range of vermiculite. The clay minerals form across the sedimentary sequence a continuous non-uniform phase of heterogeneous composition and variable physical behavior that would influence the stability of the bulk sediments.