Vertical Distributions of Carbonite Lithologies, Bedding Features, and Macrofossils in the Broken Rib Member of the Upper Devonian Dyer Formation Near Monument Lake, Garfield County, Colorado


The Broken Rib Member of the Upper Devonian Dyer Formation is a series of fossiliferous limestone beds deposited during the Famennian stage of the Late Devonian. The limestone units are well-exposed near Monument Lake in the White River National Forest of west-central Colorado, where they weather into a series of step-like, laterally continuous surfaces. A graphic log, recorded in the field, reveals that lithologies vary from carbonate mudstones to wackestones and reflect the fluctuating water depth associated with a transgressing and regressing sea. The fossil assemblages consist of well-preserved, identifiable brachiopods, bryozoans, pelmatozoans, and other minor taxa that occur in a micrite and fossil fragment matrix. These taxa indicate the Broken Rib Member formed in a sublittoral, marine environment with well-oxygenated water of normal salinity, with a slow accumulation of lime mud. Well-preserved identifiable fossils are uncommon in micritic limestones near the bottom of the section and were likely deposited during a peak transgression during the Famennian stage under storm wave base. Identifiable fossils are more abundant at the top of the section where storms periodically winnowed away mud and deposited fossil fragments. Further investigations that identify marker beds would correlate the findings of this study with other exposures of the Broken Rib Member. Additional analysis of patterns of faunal diversity through the section would allow a more precise reconstruction of the conditions during deposition of the Broken Rib Member

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University of Northern Colorado

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