Diagenesis and shear rheology of a Recent-Pleistocene volcanogenic sequence, Mexican Basin. Implications to swelling and stability

Abstract

Low-grade diagenesis in the Recent-Pleistocene volcanogenic sequence of the Mexican Basin formed, in the gravel and sand strata, kaolinite, smectite and interstratified kaolinite/smectite, intimately associated with fine ash containing volcanic glass, opal-C, plagioclase, K-feldspar and pyroxene, in an environment of high permeability, high hydraulic conductivity and acid fluids. In the mud, transformation was total to 1H₂O- and 2H₂O-smectite, in an environment of low permeability, low hydraulic conductivity, and alkaline fluids. At depth, kaolinite transforms to smectite via interstratified kaolinite/smectite. The clay minerals are the continuos phase within the sequence. A suspension prepared with 15.45 wt. % clay separated from the <2 m fraction of 26 m deep sediment in the gravel strata, containing kaolinite, smectite and interstratified minerals, shows non-Newtonian viscoelastic behavior, shear rate <0.02 s-1, shear stress <1.5 Pa, strain <0.8, instantaneous viscosity 190 Pals at a stress of 1.03 Pa, low-energy particle association. A suspension prepared with 14.02 wt. % of clay separated from 60 m deep sediment, containing smectite and low kaolinite, shows viscoelastic behavior, shear rate <0.004 s-1, shear stress 2-10 Pa, strain <0.2, instantaneous viscosity 3800 Pals at a stress of 5.91 Pa, high-energy face-to-face particle association. The 60 m clay, subjected to a deformation stress vibrating at 1 hertz, within the viscoelastic range, develops cinematic viscosity <0.8 Pals, in-phase shear modulus higher than the out-of-phase shear modulus. When vibration is raised to 5 hertz the cinematic viscosity is <4 Pa s, the out-of-phase shear modulus is higher than the in-phase one, more energy is dissipated than stored. Differences between the clay minerals from the 26 and 60 m deep sediments sustain that differences on the abundance, type and distribution of clay minerals, and consequently in the fine fraction, result in a non-uniform differential behavior of the sediments across the sedimentary sequence.