EVOLUTION OF THE CHUSKA MOUNTAIN FRONT, NAVAJO NATION, NEW MEXICO: CONSTRAINTS FROM GEOMORPHIC RELATIONSHIPS, MAPPING, AND GEOCHRONOLOGY

The Chuska Mountains (Níłtsą́ Dził) of the Navajo Nation are a topographically prominent uplifted highland of the central Colorado Plateau. This study aims to constrain the timing, style, and tectonic-geomorphic relationships of mountain-front retreat of the eastern Chuska Mountains through mapping and dating of mass movements and pediment-terrace surfaces and deposits. The Chuska Mountains in this study area comprise gently deformed Cretaceous and Paleogene siliciclastic sedimentary rocks and rise ~1 km above adjacent plains of the San Juan basin. Recent geologic mapping of the Coyote Canyon 15-minute quadrangle identified four generations of mass movements within a massive landslide complex, each with an associated suite of products and presumed processes. We correspondingly identified and mapped eight unique geomorphic surfaces east of the Chuska Mountain front in the Chaco River drainage at heights from 70 to 8 m above adjacent modern streambeds. Each surface represents an episode of pedimentation followed by deposition of coarse alluvium derived from the Chuska Mountains, including from mass movement deposits in the study area. The gradients of these surfaces and the composition of clasts in their associated deposits leads to high confidence in their provenance in the Chuska Mountains, despite geographic separation of up to 22 km from the mountain front. Age, slope, and landscape position of pediment-terrace surfaces indicate that progressive beveling throughout the Quaternary led to a lower-gradient landscape during retreat of the Chuska Mountain front, with the youngest set of pediment-terraces being essentially at the same gradient as modern streams in the study area. The tectonic-climatic drivers of alternating episodes of pedimentation and deposition are poorly understood. Pediment-terrace deposits crosscut tectonic deformation structures in the study, yielding a minimum age for the timing of folding and faulting. There likely exists a relationship between the pediment-terrace deposits along the Chuska Mountain front and the seven mapped terraces of the Chaco River identified by previous researchers approximately 30 km downstream, but further mapping and geochronologic work is needed to verify these potential relationships.