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Abstract

The Greenhorn Limestone is a remarkable pe1agic/hemipe1agic carbonate-rich unit that marks late Cenomanian-midd1e Turonian peak and near-peak transgressional conditions in the Western Interior Sea. Parts of the section consisting of or equivalent to the Hart-land and Jetmore Members comprise a sequence of generally laminated chalky, marly, or calcareous shale beds that alternate with bioturbated chalk, chalky limestone, or hard brittle limestone beds. Progressive westward lithologic change to less calcareous facies reflects influence of the Sevier orogenic belt as a major source for Greenhorn terrigenous detritus. Limestone beds of the Hartland-Jetmore sequence are time parallel and most can be traced across vast expanses of the study area. Any two of these marker beds serve to define a 1ithochronozone within which, or within a succession of which, regional variations of biota, facies, strati-graphic thickness, remanent magnetism, and geochemistry can be investigated with chronologic precision not afforded by other methods. Few such analyses have been carried out but the marker beds have been useful in demonstrating the widespread effects of short-term events, an example being that which produced the foraminiferal calcarenite that lies between marker beds JT-1 and JT-2 at many Greenhorn localities between north-central Kansas and western New Mexico. Analysis of thickness variations between marker beds has shown the presence of condensed Greenhorn sections, commonly rich in calcarenite, that apparently were deposited over topographic highs on the sea floor. General correspondence of condensed sections with structural features strongly suggests that Greenhorn deposition was influenced by contemporaneous tectonism. Precise regional correlation of time-parallel Greenhorn limestone beds also makes possible detailed regional comparisons of geochemical data not only for a given lithochronozone but also through a succession of precisely correlated stratigraphic intervals. This approach will permit rigorous testing of models that have been proposed to account for the strati-graphic rhythmicity that characterizes much of the Greenhorn section in the central Great Plains and Southern Rocky Mountains.

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