ABSTRACT: Subsidence and Infilling Patterns During Deposition of Upper Cretaceous Mancos Shale, Northwest Colorado and Northeast Utah

Ronald C. Johnson

The Upper Cretaceous Mancos Shale of northwest Colorado and northeast Utah was deposited during the Coniacian through the late Campanian in an offshore environment within a broad U-shaped embayment along the western margin of the Cretaceous epeiric seaway. A detailed study of the Mancos using geophysical logs and surface observations reveals several major and minor shifts in source direction. The Coniacian and Santonian part of the Mancos consists of overlapping lobate shale wedges that generally thin and grade to the east and southeast into calcareous shales equivalent to the Niobrara Formation. The shoreline during this period was about 100 to 150 mi west and northwest of the study area. A southern source was a major influence during the early Campanian, when silty and andy shale sediments, which formed the highly gas-productive Mancos B interval, prograded to the north across the study area. The Mancos B interval contains well-developed clinoforms having 400-600 ft of relief, and this unit may represent a prograding shelf edge contemporaneous with the Point Lookout regression occurring about 100 mi to the south. The Mancos B ends abruptly in the northwest part of the study area against a nonprograding, northwest-thickening shale buildup, which may represent the stationary shelf edge along the northwest margin of the embayment. The sandiest part of the Mancos B occurs adjacent to this shale buildup. The supply of southerly derived sediment decreased near the end of the early Campanian, and the younger Mancos section was apparently derived largely from he northwest. This source area shift corresponds roughly to the onset of the Iles regression along the northwest margin of the embayment and the onset of the Lewis transgression along the southwest margin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990