Structural analysis of spectacular late Eocene soft-sediment deformation in the lower Spears Group, Sawtooth Mountains, western New Mexico

Spectacular soft-sediment deformation (?150-m thick) in the upper Eocene lower Spears Group is exposed along the northern periphery of the Mogollon Datil volcanic field (MDVF) in the Sawtooth, Datil, and Gallinas Mountains of western New Mexico. Despite detailed studies of the stratigraphy, geochronology, and geochemistry of volcanic and volcaniclastic rocks in the MDVF, no thorough structural analysis of widespread soft-sediment deformation has been undertaken. The present study exploits excellent exposures in the Sawtooth Mountains to reconstruct the late Eocene deformational history of the northern MDVF.;Four methods were used to analyze lower Spears Group deformation: (1) 1:6000 scale geologic mapping on enlarged 1:24,000 USGS topographic quadrangle base maps, (2) mapping fault and bedding traces on oblique photos of cliff exposures, (3) measuring and describing slip and separation indicators such as Riedel shears, fault-plane striations, drag folds, and displaced contacts, and (4) analyzing thin sections of cataclasite and undeformed rock samples.;A structural stack with five distinct levels is present throughout the study area. The lowest level exposed is the middle to late Eocene Baca Formation with gently east-dipping bedding. A detachment fault sub-parallel to underlying bedding is present in some locations at the upper Baca contact and in others less than five meters above a depositional contact. Above the Baca, the volcaniclastic unit of Largo Creek (VLC) of the lower Spears Group forms a sheet (0--150 m thickness) of variably contorted sandstones. In thicker sections, localized listric faults in lower exposures are truncated above by a zone of up to five gently- to moderately-dipping faults (15°--35°) bounding deformed VLC sandstones. The upper VLC contact is an extensive gently- to moderately- dipping (5°--30°) detachment fault with indicators suggesting the upper plate slipped east or southeast. Overlying Dog Springs Formation (lower Spears Group) debris-flow deposits are sub-horizontal and parallel to the subjacent detachment in western peaks but, in eastern peaks, steeply east-dipping (75°--85°) and east-facing strata form a hanging-wall ramp. South- and southwest-striking reverse-dextral(?) faults offset the stack down to the east.;Analyzing the structural stack and incorporating previous investigators' findings results in multiple spatial and temporal interpretations. The detachment fault separating the Dog Springs from underlying units formed during one or more mass movements, possibly landsliding or block spreading events. The upper plate translated east or southeast, presumably from a topographically high area west of the Hickman fault zone. The hanging-wall ramp is interpreted to have formed where the detachment cut up- section eastward across an inferred growth fold adjacent to that fault. The unusually thick nature of soft-sediment deformation and common granular fault zones lacking sharp slip surfaces is attributed to shear of saturated and poorly-consolidated VLC sandstones by a thick (~375 m minimum) rigid overriding plate that may have slipped multiple times, as suggested by grain-size reduction and multiple slip surfaces in the detachment core. In lower Spears Group exposures east of the study area, exotic non-volcanic blocks were likely derived from southern sources. Crosscutting relationships between faults provide a timeline of deformation events. Following deposition of the lower Spears from 37--40 Ma, localized slumping and thrusting formed listric faults in the VLC. One or more large blocks of Dog Springs deposits detached from a western source and moved eastward over VLC deposits. Reverse-dextral faults offset the detachment fault down to the east, probably during late-Laramide transpression, which ended approximately 36 Ma, providing a minimum age constraint on deformation in the Sawtooth Mountains.