Petrology and Stratigraphy of the Morrison (Upper Jurassic) and Cedar Mountain (Lower Cretaceous) Formations, Emery County, Utah

The Upper Jurassic Morrison and Lower Cretaceous Cedar Mountain formations are well exposed on the west flank of the San Rafael Swell, a large Laramide anticline located in Emery County, Utah. Both formations consist of siltstone compassed predominantly of quartz, illite, kaolinite, and motmorillonite; fine- to medium-grained quartz arenite or sublitharenite; and small amounts of caliche. Because of petrographic and stratigraphic similarities, the two formations are difficult to distinguish from one another. The only distinctive unit is the Buckhorn Conglomerate Member of the Cedar Mountain, a course pebble conglomerate, or sandstone with a conglomeratic base. Minor differences between the Morrison and the unnamed main member of the Cedar Mountain are: 1) Cedar Mountain siltstones contain chlorite, whereas Morrison siltstones have chlorite only in the mixed-layer clays: 2) zircon is common in Morrison sandstones but rare in Cedar Mountain sandstones; 3) Cedar Mountain sandstones contain slightly more quartz and slightly less chert and lithic fragments; 4) the lower Morrison (Salt wash Member) contains more sandstone, 30 to 35%, than the Cedar Mountain, 5 to 15%; and 5) the upper Morrison (Brushy Basin Member) contains more limestone, 6 to 10%, than the Cedar Mountain, 0.3 to 2%. The Morrison and Cedar Mountain formations both are fluvial deposits. The Slat Wash Member of the Morrison was deposited by aggrading, meandering rivers that originated in the Sevier Orogenic Belt to the west and southwest of the study area. A combination of subsidence in the depositional basin and a decrease in gradient resulted in a transition to the deposition of the Brushy Basin Member. The Brushy Basin system included sluggish meandering streams and the thick aggradation of silts and muds on a large convex floodplain dotted with numerous lakes. The Buckhorn Conglomerate Member was, after a debatable length of time, deposited by braided streams flowing across a steep gradient with large loads of pebbles and sand. Concurrently, the Sevier Orogenic Belt was uplifted to supply the large amounts of course sediment. Subsidence again took place along with a transition to meandering channels. The deposition of the main member of the Cedar Mountain began across a low gradient, extensive floodplain dotted with lakes. A fossiliferous locality (Jones l.f., Rough Road Quarry) in the Cedar Mountain Formation is due to the accumulation of organisms in a backswamp environment on the floodplain that was a “dead-end” for occasional floods of small streams. Uranium is not found in the study area, even though it is mined form the Salt Wash Member on the east flank of the San Rafael Swell. The absence of ore in the study area is primarily due to a lack of suitable host sandstones in the Brushy Basin Member and main Cedar Mountain and lack of organic matter in both formations