Diagenesis and Rheology of a Recent-Pleistocene Volcanogenic Sedimentary Sequence, Mexican Basin

Abstract

Diagenesis of the Holocene-Pleistocene volcanogenic sediments of the Mexican Basin produced, in strata of gravel and sand, 1H₂O- and 2H₂O-smectite, kaolinite, R3-2H₂O-smectite (0.75)-kaolinite, R1-2H₂O-smectite (0.75)-kaolinite, R3-kaolinite (0.75)-2H₂O-smectite and R1-1H₂O-smectite (0.75)-kaolinite. Smectite platelets were formed from volcanic glass by loss of Si⁴⁺. Partially-formed platelets have Si⁴⁺ between 4.55−4.10 a.p.f.u., Mg+Mn and the interlayer charge are relatively uniform while ^VIAl+Fe³⁺+Ti varies between 0.98 and 1.63 a.p.f.u. Almost fully transformed platelets have Si⁴⁺ of 4.08−4.04 a.p.f.u.; Mg+Mn and the interlayer charge decrease proportionally to increasing ^VIAl+Fe³⁺+Ti. Smectite-kaolinite mixed layers have octahedral occupancies of 2.01–2.15 a.p.f.u., ^IVAl³⁺ 0.09–0.55 a.p.f.u. and interlayer charges about half that of smectite; structural formulae corresponding to smectite (0.75–0.80)-kaolinite indicate octahedral occupancy of 2.50 a.p.f.u., tetrahedral replacement 0–0.31 a.p.f.u., and interlayer charge 0.24–0.51 equivalents, some indicating interstratifications of beidellite. Kaolinite is presumed to have formed from K-feldspar; smectite-kaolinite interstratifications sustain the transformation of kaolinite to smectite in an increasingly siliceous high-cation environment. In the mudstones of low-hydraulic conductivity and practically stagnant alkaline fluids, glass was transformed to 2H₂O-smectite lamellae of ^IVAl between 0 and 0.47 a.p.f.u., octahedral occupancy 1.70–2.00 a.p.f.u. and interlayer charge of 0.23–1.21 equivalents, some corresponding to beidellite. The interlayer charge increases with ^IVAl and decreasing occupancy of the octahedral sheet; the abundance of Mg+Mn is inverse to that of ^VIAl+Fe³⁺+Ti.

Clay suspensions containing 1H₂O- and 2H₂O-smectite, kaolinite and R3-2H₂O-smectite (0.75)-kaolinite lead to low-energy edge-to-edge particle associations, non-Newtonian pseudoplastic behavior, and maximum apparent viscosity of 180 Pa s at 0.008 s⁻¹ followed by rapid descent. Clay fractions with slightly larger 2H₂O-smectite contents and smaller kaolinite contents reach maximum viscosity of 3611 Pa s at a shear rate of 0.0018 s⁻¹ and of 3300 Pa s at 0.0024 s⁻¹. They denote two high-energy face-to-face particle associations, followed by slow descent of the apparent viscosity under viscous flow. Suspensions change from elastic to viscous behavior at shear stresses of 1.03 and 5.91 Pa, respectively. Clay suspension vibrated at a frequency of 1 Hz develops a shear storage dynamic modulus greater than the shear dynamic loss modulus or the energy is preferentially stored, whereas at 5 Hz more energy is dissipated than stored.