PALEOENVIRONMENTS OF ANCHOR MINE TONGUE AND UPPER SEGO EQUIVALENT STRATA IN THE RANGELY AREA OF COLORADO: TIDALLY-INFLUENCED DEPOSITS AND VARIABLE ICHNOLOGY ALONG A COMPLEX COASTLINE

Bluffs near Rangely, CO contain a Cretaceous interval found above previously identified Sego equivalent stratigraphy which correlates to strata found in the Book Cliffs, UT. This interval contains complex sandbody and shale geometries and ichnology. We interpret this interval, based on regional subsurface correlations, to be Anchor Mine Tongue and Upper Sego equivalent. Previous research on the Sego in the Book Cliffs typically focuses on the sandier intervals, with less emphasis placed on the finer-grained intervals, like the Anchor Mine Tongue. The interval of interest contains relatively fine-grained strata which include a maximum flooding surface between the lower and upper Sego and highly variable sandbody and shale geometries found along a complex coastline.

Five measured sections in the Rangely area contain a succession of facies that include trough cross-stratified to combined flow or current ripple laminated sands with abundant organic matter, mud drapes, double mud drapes, mud balls, mud rip ups, oyster beds, and mud lags. Flaser, wavy, and lenticular bedding is common. Overlying the strata are channel-form trough-cross stratified sandbodies containing dinosaur bone and footprints interbedded with coal, carbonaceous shale, and paleosols. These observations suggest the interval consists of predominantly tidally influenced paleoenvironments capped by fluvial-coastal plain deposits. Ichnology was critical to identify continental vs. shallow-marine deposits. Trace fossils include Asthenopodichnium, Crossopodia, Diplocraterion, Helminthopsis, Macaronichnus, Ophiomorpha, Rhizocorallium, Rhizoliths, Sagittichnus, Schaubcylindrichnus, Teichichnus, Teredolites, and dinosaur footprints. The majority of trace fossils suggest a full marine to brackishwater setting. Asthenopodichnium, Rhizoliths, and dinosaur footprints are the only definitive continental trace fossils and are associated with the fluvial systems and floodplains.

The combination of ichnology and sedimentology includes paleoenvironments like tidal channels, tidal bars and flats, estuarine to lagoonal muds, fluvial channels, levees, splays, swamps, and soil forming environments. The entire succession records the progradation of the fluvial-coastal plain over a complex, tidally influenced coastline.