Geomechanical characterization of the Miocene Cuitzeo ignimbrites, Michoacán, Central Mexico

• We examine physical, mechanical, and hydric behaviour of different ignimbrite lithofacies.
• We construct a general stratigraphic section and geological map of the study area.
• We report mechanical evolution with respect to textural characteristics.
• We discuss porosity and strength data in a global context.

Volcanic rocks, especially ignimbrites, exhibit complex mechanical behaviours due to their large variation in welding, porosity, textural and granulometrical characteristics. Moreover, there is a lack of knowledge of how alteration processes affect their intrinsic properties. Main objective of this research is to characterize the physical, mechanical, and hydraulic behaviour of different lithofacies of the Cuitzeo ignimbrites (Mexico). These volcaniclastic mixtures of lithics and pumice supported by a fine-rich matrix are characterized by different textural and granulometric features. A general lithostratigraphic section and geological mapping of the study area were accomplished to attain the temporal and spatial distribution of the ignimbrites. Strength, deformation and the failure mode of selected lithofacies were directly related to micro-textural and granulometric characteristics by a series of uniaxial and pre- and post-failure non-destructive analyses (e.g. ultrasonic pulse velocity and X-ray image tomographies). Results for four different lithofacies were compared, allowing us to explore the variation in the mechanical properties associated with characteristics derived from diagenesis. Petrophysical properties show that there is a large dependence on the textural characteristics and particularly on the pumice fragments content. Results derived from laboratory observations and X-ray image reconstruction analysis, show that the main differences in the average values of porosity are associated to the geometry and morphology of the grains and pore network. These are closely related to the stress – strength relationship and the mode of failure of each specimen. In general, the strength for all lithofacies tends to increase linearly with several of the physical properties (e.g. ultrasonic pulse velocity, bulk density). Slake durability tests demonstrate that a large part of the mechanical degradation could be attributed to swelling during wetting-drying cycles. This decay is significant in lithofacies with groundmass rich in expansible clays.