Abstract

The White Canyon-Monument Valley district of southeastern Utah and northeastern Arizona contains important uranium deposits in the basal portion of the Chinle formation of Middle (?) and Late Triassic age. The basal unit of the Chinle throughout much of the district is a widespread conglomerate, sandstone and mudstone unit resting discomformably on Permian and Early Triassic strata and commonly designated the Shinarump member. In this report the basal unit is subdivided into two units by a second prominent disconformity. The lower portion, which is essentially restricted to channels carved into older rocks, contains all the uranium deposits and is retained as the Shinarump member. The upper part is considered as the basal sandstone of the overlying Monitor Butte member.The concept of a Shinarup channel is somewhat expanded to embrace the larger more continuous erosional troughs within which there may be one or more discontinuous scours, which many previous workers have referred to as "channels." Shinarump channels are widely traceable and join to form a complex paleodrainage system trending generally north-ward in the Monument Valley area but trending westward in the White Canyon area. Channel-fill deposits in Monument Valley are mostly coarse clastics but carbonaceous mudstone becomes more abundant toward the north and in places constitutes the dominant channel-fill.Most Shinarump deposits occur in channel segments in an arcuate belt along the southern and western margin of a paleostructural high. Ore deposits are of the vanadium-uranium and copper-uranium types and occur as irregular tabular lenses roughly parallel to the bedding and enlongated parallel to scour trends. Fine-grained black and yellow uranium minerals fill interstices and replace carbonaceous material in sandstone beds in the deeper scours near channel bends or intersections.It is postulated that the uranium was derived from a source to the south of the district and transported northward through the channel system during or soon after deposition of the Shinarump. Uranium was deposited at numerous scattered locations, by H 2 S produced by sulfate reducing bacteria, in the deeper scours within the channels. An erosional period following Shinarump deposition and prior to Monitor Butte deposition caused removal of most Shinarump deposits from upwarped areas and concentration of the dispersed uranium deposits into a belt of larger ore deposits adjacent to the eroded areas.

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