Abstract

In southwestern Colorado and southeastern Utah, strata of conglomeratic sandstone are localized at the base of the Brushy Basin Member of the Morrison Formation of Jurassic age. These discrete lithologic units contain sedimentary structures oriented in a prevailing easterly direction. They are believed to cover about one-third of the underlying Salt Wash Member in southwestern Colorado and southeastern Utah, and they locally rest on ore-bearing sandstone in the Salt Wash Member. Eastward-trending planar cross-stratification and trough cross-stratification and a general westward coarsening of sediments in the conglomeratic sandstone strata suggest that they are the products of stream aggradation from westerly source areas.

Uranium-vanadium deposits in the uppermost, almost continuous, layer of sandstone of the Salt Wash Member are classified according to their association with the conglomerate strata. Of the 363 deposits studied in the uppermost sandstone layer of the Salt Wash Member, 101 are known to be directly beneath conglomeratic sandstone strata, 211 are below the projected extension of conglomeratic sandstone strata, 33 are beyond the safe limits of such projection, and 18 are lateral to the margins of conglomeratic sandstone strata. At places, near clusters of deposits in the uppermost sandstone of the underlying Salt Wash Member, conglomeratic sandstone strata of the Brushy Basin Member are also mineralized.

It is postulated that ground-water movement during deposition of the Morrison Formation in Late Jurassic time localized the uranium and vanadium. The direction of this ground-water movement is believed to have been related to streams that deposited the conglomeratic strata, so that in the ore-bearing sandstone metal ions contained in the ground water were localized in places of high transmissibility and in the vicinity of decaying organic debris. According to the age of the ores, precipitation of the metals was much later than their localization, probably during Late Cretaceous or early Tertiary time.

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