Facies analysis of the lower cycles of the Mesaverde Group (Upper Cretaceous) in northwestern Colorado

The uppermost 180 m of the Mancos Shale and overlying 390 m of the Iles Formation of the Mesaverde Group in northwestern Colorado were deposited at the west margin of an epicontinental seaway that spanned North America 66-98 million years ago from the circumboreal seaway on the north to the Gulf of Mexico on the south. Sedimentation in the seaway was controlled by uplift in source areas to the west, subsidence in the basin, and eustatic sea level changes. Sedimentation rates were relatively high, averaging about 220m/million years; marine cycles of deposition occurred in northwestern Colorado that correspond to the Claggett, Judith River, and Bearpaw regressions of Montana.

The upper Mancos shale and lower Iles Formation can be subdivided into ten facies based on five criteria: (1) Lithology, texture, and thickness relations (geometry); (2) sedimentary structures and contacts; (3) trace and body fossils; (4) paleocurrent data; and (5) adjoining facies (underlying, overlying, and laterally). Each facies has distinct attributes which characterize a specific depositional environment. Environments represented by facies include (A) offshore shoreface transition (prodelta); (B) shallow marine sand bars; (C) destructional delta front; (D) constructional delta front (sheet sands and mouth bars); (E) beach foreshore-shoreface deposits; (F) interdistributary marsh and swamp deposits and fluvial floodplain; (G) distributary channels; (H) crevasse splays; (J) fluvial streams; and (K) tidally influenced distributary channel.

Facies analysis of outcrop data shows that transgressions and regressions occurred, some of which are related to major (eustatic) sea level changes and others that reflect only local causes. These events in northwestern Colorado are summarized in following chapters, and an attempt is made to show their relationship to broad regional patterns of sedimentation.