Depositional Environments as Ore Controls in Salt Wash Member of Morrison Formation (Upper Jurassic), Carrizo Mountains, Arizona and New Mexico: ABSTRACT

Abstract

End_Page 689------------------------------Uranium deposits in the Salt Wash Member of the Morrison Formation in the Carrizo Mountains area appear to be closely related to depositional facies. In the vicinity of the Eastside mines, southeastern Carrizo Mountains, the Salt Wash consists of a lower part, 10 to 15 m thick, and an upper part, 55 to 60 m thick. The lower part contains mudstone and silty sandstone interpreted as overbank and partially abandoned channel-fill deposits. It also contains a few large lenticular channel sandstones deposited by meandering and possibly braided streams. Uranium deposits are uncommon in the lower part. The upper part of the Salt Wash contains a much greater percentage of braided-stream-deposited channel sandstones, many of which coalesce to form prominent continuous ledges. The finer grained low-energy deposits are very limited in extent, commonly being less than 200 m long, 20 m wide, and 2 m thick. They have a lenticular cross section and a scour base. They consist of interbedded mudstones, claystones, and sandstones and are interpreted as abandoned and partially abandoned channel fills. Subsequent scouring of these beds has resulted in clay-clast conglomerates which were incorporated as lag deposits in the bases of overlying channel sandstones. Detrital organic debris is uncommon but is present in some channel-lag deposits as well as in some of the bedded mudstones. Uranium depos ts in the Carrizo Mountains area are associated with abandoned and partially abandoned channel fill and with clay-clast lag conglomerates adjacent to major channel sandstone systems in the upper part of the Salt Wash. End_of_Article - Last_Page 690------------