Abstract

Sedimentologic studies of the coarser-grained fluvial units of the Jurassic Morrison Formation, along with their finer-grained interbeds, in the southern San Juan Basin area suggest that the coarser-grained units were deposited by predominantly braided streams while the finer-grained units were deposited predominantly in lacustrine and overbank depositional environments. The coarser-grained, or fluvial units can be divided vertically into three major separate fluvial complexes, each with its own preferred paleotransport direction.

Fluvial deposition in the area began with northeasterly-flowing streams in the Recapture Member. This fluvial transport direction persisted through early Westwater Canyon time. By late Westwater Canyon time, however, the dominant transport direction, throughout the area, was toward the southeast. A return to northeasterly-flowing streams is recorded by the Poison Canyon sandstone (of economic usage) that is only locally present.

The fine-grained units, including much of the Recapture Member, a “K” shale interval above the Westwater Canyon and finer grained parts of the Westwater Canyon Member formed in environments distal and adjacent to the braided stream complexes. The “K” shale exposed in the Poison Canyon area is interpreted as largely lacustrine in origin, whereas the other fine-grained units studied contain both overbank and lacustrine deposits.

Surface ore trends in the Poison Canyon sandstone (of economic usage) coincide with distinct facies in the underlying “K” shale and overlying Brushy Basin Member. Where these units contain thick, greenish-gray, smectitic, pyritic lacustrine claystones and mudstones, the adjacent sandstones of the Poison Canyon are mineralized. Where the “K” shale and Brushy Basin change facies laterally into coarser and more oxidized units, the sandstones in the Poison Canyon are barren of uranium. The greenish-gray mudstones and claystones that accompany ore-bearing sandstone are viewed as convenient, local sources of the humate that impregnates the sandstone; alkaline, reducing, humic-rich pore-waters, expelled from these muds by seepage or compaction may have transported the dissolved humic substances into the nearby sandstone beds, where precipitation of the humate occurred. Uranium, probably derived from ash incorporated in Morrison sands, was carried by ground waters that flowed through the Poison Canyon sandstone, where it was fixed by the humate to form the tabular orebody. The lacustrine-humate model, as outlined here for the genesis of surface primary ore trends in the Poison Canyon sandstone, may also explain the distribution of primary tabular, or trend, ores elsewhere in the Grants mineral belt.