English: Uranium roll front in sandstone in the Cretaceous of Colorado, USA.

This is a remarkable example of a uranium roll front deposit, located at the Turkey Creek Watergap southeast of the town of Morrison, Colorado. It is hosted in sandstones of the J Sandstone of the South Platte Formation (Dakota Group, Lower Cretaceous). The uranium is ultimately coming from nearby pegmatitic granites. The black rim is composed of uraninite (UO2 - uranium dioxide). The larger outcrop has abundant yellowish and yellow-brown staining - some of that is native sulfur (S) and some is disseminated iron oxide. I have not heard or read any specific information regarding the mineral identity of the yellowish core of the uranium roll front seen here (I've been told that carnotite is not present at this site, but tyuyamunite is).

Just off the right margin of this photo is a uranium-mineralized fault zone. Uranium compounds are relatively mobile and move through permeable rocks while dissolved in fluids. Groundwater has moved through the fault, picked up dissolved uranium compounds, and moved through adjacent porous sandstones. Uraninite then precipitated around a tongue of groundwater, resulting in a roll front. Uranium mining has occurred on both sides of the Turkey Creek Watergap and targeted uraninite in the fault zone.

Here are some scintillometer readings that I observed being taken from this site, starting from the parking spot along the road, up to the black uraninite rim of the roll front: 900 counts per minute (cpm), 1500 cpm, 1600 cpm, 8000 cpm, 10,000 cpm (the maximum readout for the instrument used). Other equipment brought here previously has recorded 33,000 cpm radiation from this roll front.

Molybdenum roll fronts are also present at the site - "moly blue" (= ilsemannite, Mo3O8·nH2O - hydrous molybdenum oxide) is present immediately east of the roll front (= to the left of the viewer). Pyrite-rich zones are common in the sandstones here, often closely associated with yellow-colored native sulfur (S).

This outcrop is also a site of hydrocarbon seepage - crude oil has emerged from several areas in the sandstone, resulting in dark staining and buildups of tar/asphalt. The nearby fault zone contains uraniniferous asphalt. Just down the road (Turkey Creek Road) is a former small oil field - the Johnson # 1 well and the Great Basin # 1 well produced crude oil from the Dakota Sandstone, which occurs at about 5000 feet depth, compared with it being at the surface at this outcrop.

Locality: roadcut along the southern side of Turkey Creek Road through Turkey Creek Watergap, ~2.9 km southeast of the town of Morrison, Jefferson County, north-central Colorado, USA (39° 38' 05.67" North latitude, 105° 10' 07.44" West longitude)