Genetic stratigraphy, provenance, and new age constraints for the Chuska Sandstone (Upper Eocene-Lower Oligocene), New Mexico-Arizona
— Steven M. Cather, Lisa Peters, Nelia W. Dunbar, and William C. McIntosh

The Chuska Sandstone is about 535 m thick and consists of a basal fluvial succession (Deza Member; 0–81 m thick) and overlying eolianites (Narbona Pass Member). Aggradation of the Deza Member occurred in response to the south–southwesterly progradation of the distal part of an extensive (200 km length) piedmont system derived from the San Juan uplift of southwestern Colorado. As it prograded, this system pushed ahead of it a short, northeast–facing piedmont system and a transverse basin-fl oor system as it onlapped a post-Laramide, late Eocene paleogeomorphic surface. Crossbedding in the eolianites of the Narbona Pass Member indicates paleowinds were mainly from the south–southwest, and blew up the paleoslope established during Deza Member deposition. The petrographic similarity of the Narbona Pass Member to the Deza Member indicates the eolianites are mostly recycled Deza-type sands that continued to be derived fluvially from sedimentary, volcanic, and basement terranes to the north–northeast during deposition of the Narbona Pass Member. Stratigraphic and topographic relationships in the Chuska Mountains suggest that nearly one km of middle Eocene–lower Oligocene strata were once present in the axial part of the San Juan Basin, but were subsequently eroded.

New ⁴⁰Ar/³⁹Ar results for volcanic ashes in the Deza Member (34.75 ± 0.20 Ma) and the lower Narbona Pass Member (33.33 ± 0.25 Ma) and for trachybasalts that disconformably overlie the Chuska Sandstone near Narbona Pass (24.83 ± 0.26, 24.97 ± 0.16, 25.05 ± 0.17, 25.24 ± 0.17 Ma) indicate the basal Chuska Sandstone is upper Eocene, and the upper part of the unit is lower Oligocene and possibly lower upper Oligocene. Our radioisotopic results indicate the switch from dominantly fluvial to dominantly eolian sedimentation in the central Colorado Plateau occurred between 34.75 and 33.31 Ma. Age constraints for the beginning of Chuska eolian deposition are broadly compatible with those for other eolian successions in the Colorado Plateau–Rocky Mountains area. The switchover from fl uvial to eolian processes may be a response to major global cooling that occurred ~34 Ma.