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The Cutler Formation is composed predominantly of fluvial arkosic sandstones, siltstones, shales, and mudstones deposited by meandering streams that flowed across a flood plain and tidal flat. Sedimentary structures indicate two types of channel deposits: meandering and distributary. The area was occasionally transgressed by a shallow sea from the west, resulting in the deposition of several thin limestones and marine sandstones. The marine sandstones were deposited as longshore bars. Wind transported sand along the shoreline of the shallow sea, forming a coastal dune field. Marine and eolian sandstones are more common in the upper part of the Cutler Formation in the southern part of the area, whereas in the central and northern part the formation is predominantly fluvial Cross-bed orientation indicates that streams flowed S67°W on the average, whereas longshore marine currents moved sediment S36°E and N24°W, and onshore wind transported sand S80°E.
The uranium in the Cutler Formation is found in the central and northern part of the area, in the upper part of the formation, in fluvial sandstone bodies that were deposited in a distributary environment. No uranium is known in the marine or eolian sandstones. Petrographically, the uranium-bearing sandstones are identical to other Cutler fluvial sandstones except that they contain less calcite cement and more clay and are slightly coarser grained. The diagenetic sequence indicates that uranium and vanadium were introduced late in the sequence, after oxidation had formed hematite and before the formation of calcite cement. Ore formation has modified the host sandstones very little.
The uranium and vanadium minerals are finely disseminated and thus difficult to identify but seem to include some uraninite, coffinite, uranophane, and carnotite. Much of the uranium is associated with iron oxide grain coatings and matrix. The uranium and vanadium are present together and independently. Both calcium and iron are depleted, and barium is concentrated in the ore zone. No significant organic carbon was found in the ore zones, and small amounts of selenium are concentrated at the base of the ore zones.
Formation of these orebodies has occurred without any obvious reductant. Perhaps sorption of uranyl by hematite was the concentration mechanism. The time of formation is not known; evidence is present for both a Permian age and a Triassic or younger age.
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