Print this pageAbstract: Maestrichtian Conglomerates in Southwestern Piceance Creek Basin, Colorado
An unnamed 100 m thick chert pebble-conglomeratic sandstone at the top of the Mesaverde Formation in the southwestern Piceance Creek basin was dated, using pollen, as Late Cretaceous-Maestrichtian in age. The pollen assemblage is similar to that of the Hell Creek Formation of Montana. This unit has been called Ohio Creek Formation by some previous workers and has been considered to be Paleocene in age. The base of the unit appears to be conformable with the underlying Mesaverde Formation, and a major regional unconformity occurs at the top. The Wasatch Formation, which is above the unconformity, consists of red, purple, gray, and black claystones; a discontinuous conglomeratic sandstone as much as 10 m thick is at its base. A sample collected about 10 m above the unconfor ity yielded late Paleocene pollen. A paleoweathering profile as much as 120 m thick occurs below the unconformity. Here, feldspars have been altered to kaolinite. Root casts and paleosols are preserved locally in the upper few meters. At least 100 m of Wasatch Formation wedges out on the unconformity surface on the south and west toward the ancient highlands of the Uncompahgre uplift and the Douglas Creek arch. This unconformity should be recognizable in samples from drill holes on the north in the Uinta and Piceance Creek basins by the deeply weathered kaolinized zone present below. Detection of this and similar unconformities having extensive alteration zones is critical to the assessment and exploration of hydrocarbons within these rocks.
AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah