Cedar Mountain Formation - Stratigraphy

Stratigraphy

The Cedar Mountain Formation is sandwiched between the Morrison Formation below and the Naturita Formation and Mancos Shale above. The youngest date for Morrison just below the Cedar Mountain Formation is 148.1 ± 0.5 Ma . or lower Tithonian. Typically, the Jurassic-Cretaceous boundary in western North America is marked by an unconformity of variable length, and typically signifies 10-49 million years of missing geologic time. This boundary between the Morrison and Cedar Mountain is commonly marked by a horizon of carbonate nodules or by highly polished pebbles that are allegedly gastroliths.

• The Buckhorn Conglomerate is considered the lowermost member of the Cedar Mountain Formation in the region of the San Raphael Swell by Stokes. It is named for exposures near Buckhorn Reservoir near Cedar Mountain. Its position immediately below the Ruby Ranch Member suggests that it may be equivalent to the channel sandstones in the Yellow Cat Member and the Poison Strip Sandstone farther to the east. This idea is strengthened by the similar composition of the gravels in these members, but a direct correlation has not yet been established.

• The Yellow Cat Member is named for exposures near the Yellow Cat mining area north of Arches National Park. It is limited to the eastern portions of the formation and is thickest near Arches National Monument. The member is composed of drab greyish mudstones and some lenses of sandstone. The mudstones were deposited on flood plains, and show evidence of ancient soil development called paleosols. The mudstones originated as flood deposits from river channels that are marked by the sandstone lenses. A recent radiometric date of 126 ± 2.5 Ma places Yellow Cat Member in the Barremian,

• The Poison Strip Sandstone was named for prominent, cliff-forming sandstones in the Poison Strip uranium district north of Arches National Monument. It is actually a series of sandstones that were deposited in river channels, and lesser amounts of mudstones and limestones that were deposited on the flood plain and small ponds. The Poison Strip Sandstone may represent a meandering river complex. Based on the position of the Poison Strip between the Yellow Cat and Ruby Ranch members, it probably was latest Barremian to earliest Aptian.
  • Volunteers from the Denver Museum of Natural History discovered Tony's Bone Bed in 1998 in the Poison Strip Member. Prior to that, Poison Strip dinosaurs were only known from possible Sauropelta and isolated theropod and sauropod bones. Tony's bone bed is 3.75 m below the top of the poison strip. Quality of the bones are "highly variable." Ends of bones may have been damaged by scavenging. Some of the bones may have been trampled. Some of the vertebrae preserve "delicate lamina". None of the bones were articulated so the was probably not buried quickly after death. Tony's Bonebed probably accumulated over time when the water in the river channel was low during the dry season. Carbonate growths appear on bones in the quarry from which Venenosaurus was extracted.

• The Ruby Ranch Member is the most widespread and distinctive member of the Cedar Mountain. It was named for exposures on the Ruby Ranch located southeast of Green River, Utah. The member is composed of maroon mudstones with irregular spheres of carbonate nodules. The nodules formed in ancient soils that developed in the mud deposited on the flood plain in a strongly seasonal, semiarid climate. Evaporation of groundwater during the dry season concentrated calcium carbonate and other minerals in the upper parts of the soil horizon. Radiometric dates place the upper portions of the Ruby Ranch in the late Aptian. Exhumed river channels in the Ruby Ranch indicate that stream flow during the Aptian was towards the northeast, the direction of the encrouching Western Interior Seaway.

• The Mussentuchit Member is the uppermost member of the Cedar Mountain Formation. It was named for exposures along Mussentuchit Wash southwest of the San Rafael Swell. It is predominantly composed of grey mudstones high in organic carbon from fossil plant material, as well as volcanic ash. The mudstones were originally deposited on a broad coastal plain with a high water table or with abundant rainfall. Thus, carbonate nodules are rare. A radiometric date of 98.37 ± 0.07 Ma places the upper part of the member in the Lower Cenomanian, while lower portions of the member have been dated to 104.46 ± 0.95 Ma, in the Albian stage.

• Although not part of the Cedar Mountain Formation, the Naturita Formation immediately overlies the Cedar Mountain and marks the encroaching Western Interior Seaway. The Naturita is not uniformly distributed and was eroded away in places by the advancing Seaway so that the marine shales of the Mancos Formation lay directly on the Mussentuchit or its equivalent. The name Dakota Formation has been improperly used for these strata.