Westwater Canyon Great Unconformity
The black inner gorge of Westwater — ~1.7-billion-year-old Vishnu Schist and Zoroaster Granite beneath a Great Unconformity that erases ~1.5 billion years, the only river-level Precambrian on the Colorado outside the Grand Canyon.
Also known as: The Black Gorge, Westwater Inner Gorge, Westwater Precambrian Basement
In the inner gorge of Westwater Canyon the Colorado River cuts down out of red Mesozoic sandstone and into the continental basement — polished black Vishnu Schist and pink Zoroaster Granite roughly 1.7 billion years old. The surface where that basement meets the overlying Triassic Chinle Formation is a Great Unconformity: a single contact across which on the order of ~1.5 billion years of rock record is missing — a larger gap here than the ~1.2-billion-year version exposed in the Grand Canyon's Inner Gorge. Westwater (with neighboring Ruby Canyon) is one of the only places on the Colorado River outside the Grand Canyon where Precambrian basement is exposed at river level, and it is reachable on a one-day run. The black gorge corresponds to the canyon's whitewater core — the hard, non-erodible basement constricts the channel and concentrates the rapids from roughly Marble Canyon through Last Chance.
Geology
Formation: Paleoproterozoic metamorphic and igneous basement (Vishnu Schist + Zoroaster Granite, ~1.7 Ga) was deeply buried, then exhumed and beveled by erosion before Triassic sediments of the Chinle Formation were deposited directly on top of it. The contact between the two records the Great Unconformity — a hiatus on the order of ~1.5 billion years. Later Colorado River incision re-exposed the basement, cutting the narrow Black Gorge whose hard, smooth walls constrict the river and create the canyon's rapids.
Westwater Canyon is one of only two reaches on the Colorado River outside the Grand Canyon (with adjacent Ruby Canyon) where Precambrian crystalline basement is exposed at river level. The Great Unconformity here is of greater duration than the Grand Canyon example (~1.5 Gy vs ~1.2 Gy of missing record), and the basement-to-Chinle contact plus the overlying Wingate cliff make the full unconformity legible from a boat in a single day's run.
Primary formation: Vishnu Schist
Age: Paleoproterozoic (1700 Mya)
Coordinate is the Westwater Canyon centroid as published on the Wikipedia 'Westwater Canyon' article (38°58′20″N 109°13′40″W) and is used as a representative anchor for a corridor-scale exposure rather than a single outcrop — the basement gorge spans roughly river mile 6 to ~13 of the ~17-mile run. Mile references conflict between sources and are not normalized away: the Westwater section entity's mileMarkers place the entry into black rock near river mile 6, while the Whitewater Guidebook notes a schist-canyon vista point near river mile 12.4 on river right. An exact gorge-entrance coordinate is not field-verified. Age and unconformity-magnitude figures (~1.7 Ga basement; ~1.5 Gy missing, vs ~1.2 Gy in the Grand Canyon) are widely published regional geology, corroborated by the Wikipedia 'Westwater Canyon' article and consistent with usgs-stratigraphy-canyonlands; upgrade to a primary geologic reference before promoting status.
History
Informal — 'the Black Gorge' / 'inner gorge' are the boater names for the Precambrian narrows; 'Great Unconformity' is the regional geologic term for the basement-to-Chinle contact exposed here.
Regional Precambrian exposure documented in Colorado Plateau / Canyonlands stratigraphic work (see usgs-stratigraphy-canyonlands) and summarized for this reach on the Wikipedia 'Westwater Canyon' article.