Reevaluation of Depositional Environments of Salt Wash Member of Morrison Formation, Uravan Mineral Belt, Southwest Colorado: ABSTRACT

Abstract

The Uravan mineral belt of southwestern Colorado has proved to be a significant source of uranium and vanadium. Since 1948, 63 million lb of U3O8 and 332 million lb of vanadium have been produced. About 90% of the ore has come from the upper sandstone ledge of the Salt Wash Sandstone Member of the Morrison Formation. It has long been recognized that the Salt Wash was the product of fluvial sedimentation. More recently most authors have concluded that the Salt Wash was deposited as a braided stream system and some have proposed that the system was part of a large alluvial fan complex because of the arcuate pattern of the belt. However, results of a detailed sedimentologic analysis of this sequence in the Slick Rock district suggests that the entire Salt Wash including the uraniferous upper ledge was deposited in a fine-grained meander belt system. Evidence for this interpretation is based on the high percentage (up to 54%) of fine-grained bioturbated and/or rooted flood-plain sediments; associated coarsening-upward crevasse splay or overbank splay deposits; and the abundance of fining-upward point bar sequences. The fine to medium-grained sandstones of the point bar deposits crop out as a series of discontinuous ledges numbering between 3 and 6 throughout the district. Each ledge consists of a number of abbreviated and complete point bar sequences ranging in thickness from a few feet to over 25 ft. These point bar deposits grade laterally and vertically into levee, abandoned channel, and crevasse splay assemblages and are interbedded with thick sequences of overbank mudstones, siltstones, and thin sandstones. End_of_Article - Last_Page 796------------