IMPORTANCE OF ICHNOLOGY IN DEFINING PALEOENVIRONMENTS ALONG AN ANCIENT COMPLEX COASTLINE: THE ANCHOR MINE TONGUE AND UPPER SEGO EQUIVALENT OF RANGELY, COLORADO

A new, revised regional sequence stratigraphic correlation indicates that the outcrop belts along the Rangely Anticline, CO, contain strata that correlate with the Anchor Mine Tongue and Upper Sego of the Book Cliffs, UT. These strata contain complex sandbody-shale geometries and diverse ichnology, and have not been previously documented. Combining sedimentologic, ichnologic, and architectural analyses clarify paleoenvironments and reconstruct the paleoenvironmental evolution of this system.

Facies and ichnologic analyses evidenced a mixed fluvio-tidal coastline that includes deposits of a tidally influenced distal to proximal delta front, tidal bars, subaqueous tidal channels, river-flood-dominated deltas, interdistributary bays, tidal flats, and distributary channels and floodplain. Tidal influence is evidenced by strata containing double mud drapes, mud balls, sigmoidal bedding, combined flow ripples, and a high abundance, high diversity marine ichnologic assemblage. River-flood influence is evidenced by strata with current ripples, low angle planar laminations, and a low-abundance, low diversity ichnologic assemblage. Coastal plain deposits contain purely continental traces or lack trace fossils.

A total of 30 trace fossils were grouped based on dominant salinity and include 13 fully marine; 13 marine-brackish-freshwater; 1 freshwater; and 3 continental traces. Subaqueous tidal channels differ from fluvial distributary channels in that 1) tidal channels contain Ophiomorpha, Thalassinoides, and Gyrochorte, whereas 2) distributary channels lack trace fossils or contain Asthenopedichnium. Tidal flats differ from interdistributary bays such that tidal flats contain flaser, wavy, and lenticular bedding along with a higher abundance, higher diversity trace fossil assemblage including Asterosoma, Ophiomorpha, Schaubcylindrichnus, and Teichichnus. Alternatively, interdistributary bays have a low abundance, low diversity assemblage and few marine traces.

Strata of the Anchor Mine Tongue and Upper Sego near Rangely record the evolution from distal and proximal delta front deposits to interdistributary bays, tidal channels, and tidal flats. The succession terminated with fluvial-coastal plain deposits prograding over the shallow marine system.