Chuska Sandstone
The Oligocene Chuska Sandstone is a geologic formation that crops out in the Chuska Mountains of northeastern Arizona and northwestern New Mexico. The formation is a remnant of a great sand sea, or erg, that once covered an area of reaching from the present locations of the Chuska Mountains to near Albuquerque and to the southwest. This erg deposited a succession of sandstone beds exceeded in thickness only by the Navajo Sandstone on the Colorado Plateau. Description The Chuska Sandstone is up to thick and is divided into two members. The Deza Member, to which the lowermost beds of the formation are assigned, is up to thick and consists mostly of pale orange to yellow-gray sandstone (66%), claystone (16%) and sandy siltstone (16%). Sedimentary structures are present that indicate deposition by running water, and the member fills shallow paleovalleys eroded in the underlying Mesozoic beds. The Deza Member is not always present, and when present, it grades into the overlying Narbona Pass Member without a sharp contact. The Narbona Pass Member makes up most of the Chuska Sandstone, and consists of as much as of wind-deposited (eolian) sandstone. The sandstone is pinkish-gray to yellowish-gray and is crossbedded and arkosic (it