New Mexico Geological Society Annual Spring Meeting — Abstracts

Spectacular soft-sediment deformation in mid- to upper Eocene (40-37 Ma) rocks of the Sawtooth Mountains of west-central New Mexico records gigantic slumping event(s), and provides geologists with an opportunity to study similar processes that may pose future hazards to human lives and property. The Sawtooth Mountains have been mapped in reconnaissance, but this study is the first detailed analysis of their deformation.

To better understand the geometry of deformation, 1:6000-scale geologic mapping and cliff mapping were done. Cliff mapping involved drawing bedding and fault planes on cliff photos while observing them from different perspectives in the field. Riedel shear and detachment fault orientations were collected and used to determine slip direction(s) along the detachment. Samples were collected for thin section petrographic and microprobe analysis. Petrography serves two purposes, (1) estimating permeability differences between the lower and upper plate to test whether the upper plate or cataclastic detachment surface acted as an impermeable seal to the lower plate, preventing dewatering, reducing effective normal stress, and aiding deformation, and (2) describing microstructures within the fault zone and deformed sections of the plates to determine slip direction(s) and whether deformation occurred once or multiple times and catastrophically or slowly.

In the field area, chaotically-bedded argillaceous sandstones of the volcaniclastic unit of Largo Creek (VLC) conformably overlie the undeformed fluvio-lacustrine Baca Formation. The lower plate VLC is separated from the upper plate Dog Springs debris-flow breccias by an extensive detachment fault. Detachment faults may also be present within the VLC and at the Baca-VLC contact in some areas. Geologic mapping reveals different bedding orientations in the upper plate. Western peaks display sub-horizontal bedding that is sub-parallel to the detachment whereas eastern peaks have sub-vertical bedding that strike north. A large ENE-trending anticline is present in one of the western peaks. Cliff mapping has revealed VLC plastic deformation expressed as chaotic, folded bedding and Dog Springs brittle deformation expressed as coherent blocks with faults and fractures. One of the eastern peaks displays fault drag and Riedel shears that imply eastward slip of the upper plate Dog Springs. However, the angle between upper plate bedding and the detachment on the same peak implies westward slip of the upper plate assuming original detachment faulting was part of a slump event. Hand samples are currently being prepared for petrographic and microprobe analyses.

In the eastern peaks, the upper plate possibly underwent two slip events in opposite directions, an initial slip to the west followed by slip to the east. Differences in upper plate bedding orientations suggest that deformation occurred as multiple events or is more complex than currently understood. Preliminary observations of clastic dikes and lithologic differences between the lower and upper plate suggest liquefaction helped to enable deformation.