CONTACTS, AGES, AND CORRELATION OF LOWER CRETACEOUS STRATA ON THE NORTH SIDE OF THE UINTA BASIN, NORTHEASTERN UTAH

Geologic mapping and the discovery of several dinosaur sites in the Cedar Mountain Formation (CM) in northeastern Utah necessitates a reappraisal of the Lower Cretaceous section in this area in light of recent work and new dating methodologies. The CM unconformably overlies the Upper Jurassic Morrison Fm. with the Buckhorn Conglomerate Mbr. marking the base locally. Elsewhere, a zone of calcrete has been used to mark the base. However, in central Utah, the base of the CM in the interfluvial areas is better defined lower in the section at the first occurrence of chert pebbles in the matrix, associated with chert layers, and extensive iron staining in outcrop. A “porcelainite” at the contact yielded a suite of zircon ages spanning the Precambrian through the Late Jurassic indicating derivation from the Morrison. There is a published minimum age of 104.46 ± 0.95 Ma based on 3 zircons for the Abydosaurus type locality near the middle of the CM at Dinosaur National Monument (DNM). The overlying Dakota Fm. provides another age constraint. We report a new radiometric age from a thick volcanic ash in the middle carbonaceous interval. Extracted zircons yielded 44 of 50 grains with U-Pb ages between 96 and 109 Ma, and 21 grains with ages between 100 and 104 Ma, giving a final age of 101.4 ± 0.4 Ma (2 sigma). This radiometric age is compatible with the late Albian age for this interval as indicated by palynomorphs. The local occurrence of dinoflagellates indicating brackish water at the base of the Dakota suggests the influence of the llate Albian Skull Creek marine cycle in northeastern Utah and the correlation of the Dakota with the Muddy Sandstone, which is sandwiched between the Skull Creek and overlying Mowry shales in Wyoming. A conglomerate is present at the Dakota/Mowry contact west of DNM.

There is little support for applying the nomenclature used for CM members in central Utah to these rocks in the Vernal area. This is particularly true in regards to the Mussentuchit Mbr. at the top of the CM, which has published Ar/Ar dates from volcanic ashes through the member from 98.37 ± 0.07 Ma to 96.7 ± 0.05 Ma in the type area, all younger than our Dakota age and compatible with published ages for the overlying Mowry Shale in Wyoming. Likewise, a discontinuous conglomerate separates the Mussentuchit from the underlying members of CM suggesting a sequence boundary.