The Chuska Mountains are an elongate range on the southeast Colorado Plateau and within the Navajo Nation whose highest elevations approach 10,000 feet. The range is about 80 by 15 km (50 by 10 miles), and it trends north-northwest and is crossed by the state line between Arizona and New Mexico. The highlands are a dissected plateau, with an average elevation of about 2,740 m (8,990 ft), and subdued topography. The highest point is Roof Butte (36.4601° N, 109.0929° W) at 2,994 m (9,823 ft), near the northern end of the range in Arizona. Other high points include the satellite Beautiful Mountain at 2,861 m (9,386 ft) and Lukachukai Mountains at 2,885 m (9,465 ft), both also near the northern end, and Matthews Peak at 2,911 m (9,551 ft). The San Juan Basin borders the Chuskas on the east, and typical elevations in nearby parts of that basin are near 1,800 m (5,900 ft). The eastern escarpment of the mountains is marked by slumps and landslides that extend out onto the western margin of the San Juan Basin. To the north, the Chuskas are separated from the Carrizo Mountains by Red Rock Valley, which is today commonly referred to as Red Valley.
Geology
The Chuska Mountains and the Defiance Uplift immediately to the southwest form one of the prominent uplifted highlands of the Colorado Plateau. The uplifted region is separated from the San Juan Basin to the east by the Defiance and associated monoclines. Relative uplift, basin subsidence, and monocline formation began in the early stages of the Laramide orogeny about 75 to 80 million years ago. Although the Chuska Mountains can be considered part of the Defiance Uplift, they stand higher. They are capped by an erosional remnant of Chuska Sandstone, a unit locally more than 500 meters thick. The flat-lying Chuska Sandstone rests unconformably on Mesozoic rocks deformed in the Defiance monocline. Biotite in layers of altered volcanic ash within the Chuska Sandstone has yielded radiometric ages of 35 and 33 million years by argon-argon dating. The Chuska Sandstone is formed of sand dune deposits, and it appears to be a remnant of a huge Oligocene sand sea, the Chuska erg. The erg hypothesis is consistent with major exhumation of the central Colorado Plateau in the late Oligocene and early Miocene (e.g., from about 26 to 16 million years ago). If so, then major uplift of the central Colorado Plateau may postdate the Laramide orogeny.
