Comparative paleoenvironmental and architectural analyses of the Anchor Mine Tongue to Upper Sego interval along the Book Cliffs of Utah and the Rangely Anticline of Colorado




Jones, Rebecca Helen

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A recent, revised regional sequence stratigraphic correlation indicates that outcrop belts along the Rangely Anticline, Colorado contains strata that correlate with the Anchor Mine Tongue and upper Sego of the Book Cliffs, Utah. A detailed sedimentologic and ichnologic investigation of the Anchor Mine Tongue and upper Sego (Campanian) equivalent strata exposed the Rangely Anticline in the Piceance Creek Basin of Colorado revealed that these deposits record the evolution of a mixed fluvio-tidal deltaic system with a delta front significantly modified by tides. This system prograded over lower Sego tidal bars. Predominant structures that support this interpretation include combined flow ripples, trough cross-stratification, low-angle planar-lamination, mud drapes, mud rip-up clasts, mud balls, and trace fossils within sands that are interbedded with finer-grained bioturbated sediments. Ichnology was utilized to refine paleoenvironmental interpretations, and to distinguish between: tidally modified distal and proximal delta fronts and river-dominated deltas; subaqueous tidal channels and distributary channels; interdistributary bays and tidal flats; and marine mudstones and mud-rich paleosols. The Anchor Mine Tongue and upper Sego stratigraphy that is exposed along the Book Cliffs in the Uinta and Piceance Creek was also investigated using a combined sedimentologic and ichnologic approach. Deposits record a marine transgression followed by renewed progradation of tide-dominated deltas with wave-influence along the distal delta front. Paleoenvironments were similar to those of the Rangely area with the addition of marine shelf, wave-modified proximal prodelta, and tidally influenced, wave-modified distal delta front in the Book Cliffs region. The Anchor Mine Tongue and upper Sego in the Book Cliffs preserves the proximal and distal expressions of tide-dominated systems. The Anchor Mine Tongue and the upper Sego both along the Rangely Anticline and along the Book Cliffs are compared-contrasted. Deposits along the Rangely Anticline exhibit more fluvial-flood dominance than deposits along the Book Cliffs. The Rangely stratigraphy represents a mixed fluvio-tidal system whereas the Book Cliffs stratigraphy is tide-dominated. The distribution and variation of paleoenvironments supports a northwest-southwest trending shoreline during the Campanian.


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