Stratigraphy and Depositional Environments of the Bridge Creek Limestone Member of the Greenhorn Limestone at Rock Canyon Anticline Near Pueblo, Colorado

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Abstract
The Bridge Creek Limestone Member of the Greenhorn Formation records maximum eustatic rise of the Greenhorn Marine Cycle, and the maximum aerial extent reached by the Western Interior Seaway. Limestone-shale couplets reflecting Milankovitch-style climatic cycles, and abundant laterally persistent bentonites, within a highly refined biostratigraphic framework based primarily on rapidly evolving cosmopolitan molluscan taxa, allows the development of a very detailed system of event-isochron stratigraphy divided into intervals of 100,000 yr or less. The Bridge Creek Limestone Member can be divided into three informal subdivisions (lower, middle, and upper) based on overall lithologic characteristics and slope forming profile. These precisely correlate to the upper Hartland, Jetmore, and Pfeifer members in central Kansas and reflect changes in marine environments and sedimentation style corresponding to the periods just prior to, during, and after maximum transgression respectively. The development and preservation of cyclic sedimentation during deposition of the Bridge Creek Member is largely the result of slow rock accumulation rates of 0.5 to 1.0 cm/1000 years across the central Western Interior. The combination of widespread deposition, numerous chronostratigraphic marker-beds, and a highly refined regional and global biostratigraphy make the Bridge Creek Limestone Member a magnificent natural laboratory for detailed ecologic, evolutionary, biogeographic, and oceanographic studies.

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