JURASSIC DEPOSITIONAL HISTORY AND VANADIUM-URANIUM DEPOSITS, SLICK ROCK DISTRICT, COLORADO PLATEAU

The Jurassic interval studied in the Slick Rock vanadium-uranium district consists of the upper units of the San Rafael Group (the Summerville Formation and the Junction Creek Sandstone) and the Tidwell and Salt Wash Members of the Morrison Formation. The dominantly argillaceous Summerville Formation is interpreted as a lacustrine deposit. A salt cast-bearing seven meter thick arenite sequence in the central portions of the Summerville assemblage is inferred to have been deposited in the nearshore and evaporative foreshore setting during a major lacustrine regression. As the Summerville Lake dried up, sand dunes migrated across the lacustrine plain. These are now preserved as the eolian Junction Creek Sandstone in the southern part of the study area.; A major pulse of fluvial sedimentation succeeded deposition of the San Rafael Group. The floodplain-dominated Tidwell Member is truncated by the Salt Wash Member which consists of laterally persistent sandstones interbedded with mudstones, siltstones and lenticular sandstones deposited in a complex fluvial system. The continuous sandstones were deposited by the aggradation and avulsion of broad, deep, low sinuosity channels which traversed the district from west to east. During flood stages, giant ripples migrated down the channels. Lunate mega-ripples and straight crested dunes were the dominant bedforms during moderate and low water stages, respectively. Lenticular sandstones were deposited in meandering tributaries to the low sinuosity channels while interbedded mudstones and siltstones which average 50 percent of the assemblage, originated as well-drained floodplain deposits.; Examination of the fluvial architecture of the framework sandstones of the Salt Wash reveal two east-trending zones of high sandstone content. These are inferred to be two fluvial axes of the Salt Wash distributary system. Comparison of the paleogeography, architecture and character of the low sinuosity streams of the Salt Wash with Holocene fluvial systems leads to the conclusion that the Slick Rock District is located in the braided-to-meandering transition zone of the Salt Wash.; The Salt Wash Member is the principal ore producing interval in the Slick Rock vanadium-uranium district. Production isopleth maps of vanadium and uranium yield from the Salt Wash prior to 1971 reveal that the larger ore deposits are oriented along east-west or east-northeast trends parallel to the depositional fabric of the host sandstones. Larger ore bodies are preferentially developed within and along the margins of the major low sinuosity stream deposits and within the meandering tributaries near their confluences with the principal low sinuosity channels. On a local scale, ore deposits most commonly occur in low sinuosity channel sandstones. Mineralization is also found in point bar sequences, and thirdly, associated with carbonaceous debris.