Dox Formation
The Dox Formation, also known as the Dox Sandstone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The strata of the Dox Formation, except for some more resistant sandstone beds, are relatively susceptible to erosion and weathering. The lower member of the Dox Formation consists of silty-sandstone and sandstone, and some interbedded argillaceous beds, that form stair-stepped, cliff-slope topography. The bulk of the Dox Formation typically forms rounded and sloping hill topography that occupies an unusually broad section of the canyon.
| Dox Formation | |
|---|---|
| Stratigraphic range: | |
Dox Formation at Tanner Graben-(photo left, on river) and upstream on Colorado River (from Comanche Point, East Rim) | |
| Type | Geological formation |
| Unit of | Unkar Group |
| Sub-units | Escalante Creek Member, Solomon Temple Member, Comanche Point Member, and Ochoa Point Member |
| Underlies | Cardenas Basalt (unit 5) |
| Overlies | Shinumo Quartzite (unit 3) |
| Thickness | 1,150 to 1,345 feet (351 to 410 m) |
| Lithology | |
| Primary | sandstone |
| Other | mudstone, dolomite, shale; interbedded basalt |
| Location | |
| Region | Arizona, Grand Canyon Isis Temple region, southwest Bright Angel Canyon, at north side, Granite Gorge, and along Colorado River |
| Country | United States of America |
| Type section | |
| Named for | Dox Castle, north side of Colorado River, Shinumo quadrangle, Coconino County, Arizona. |
| Named by | Noble (1914) |
The Dox Formation comprises the bulk of the Unkar Group, the base section of the Grand Canyon Supergroup. The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed of, in ascending order, the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. The Unkar Group is overlain in ascending order by the Nankoweap Formation, about 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. The entire Grand Canyon Supergroup overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.
In general, the Dox Formation and associated strata of the Unkar Group rocks dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim). Elsewhere, within the central Grand Canyon, these Unkar strata (Bass, Hakatai, and Shinumo), occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this part of the Grand Canyon, the Unkar Group is incomplete because pre-Tapeats Sandstone erosion has removed strata above the level of the middle part of the Dox Formation. The missing part of the Dox Formation and overlying Cardenas Basalt and Chuar Group are preserved in a prominent syncline and fault block that is exposed in the eastern Grand Canyon.
The Dox Formation contains thick basaltic sills and a number of small, dark dikes. In the area of Desert View and west of Palisades of the Desert, the basaltic sills form very prominent, dark gray cliffs.
The Dox Formation takes its name from frontier educator Virginia Dox, the first white woman to explore the Grand Canyon, for whom the Dox Castle butte was named.