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Abstract

High-resolution event stratigraphy of the Niobrara Formation in Colorado at Pueblo and along the northern Colorado Front Range provides a more precise and detailed framework for correlation than traditional lithologic and biostratigraphic methods. Bentonites and shale-limestone bedding couplets can be successfully correlated regionally. The bedding couplets are believed to be reflective of Milankovitch-style climatic cycles. Difficulties in correlation of bedding couplets in the Fort Hays Limestone Member between Pueblo and the northern Front Range are attributed to disconformities within this member in the latter area. Detailed lithostratigraphic, paleoecologic and geochemical studies of the lower Niobrara Formation (Fort Hays Limestone Member, shale and limestone, lower shale and lower limestone units of the Smoky Hill Shale Member) show significant contrast in paleoecologic and geochemical data between the lower shale unit and bounding units. Decreased bioturbation and increased organic carbon content in the lower shale unit suggest a change in bottom water conditions from a well-oxygenated environment in the bounding units to poorly oxygenated and possibly intermittently anoxic environments during deposition of the lower shale unit. A negative excursion in oxygen isotopic values in the lower shale unit suggests a model of a brackish water cap and salinity stratified seaway which resulted in poor vertical mixing of the water column to explain the change in bottom water conditions. The observed change in bottom water environments can be placed in the context of transgressive-regressive (fourth-order) cycles which are superimposed on the third-order Niobrara marine cycle; early to middle transgression, as represented by the lower shale unit, is associated with poorly oxygenated to anoxic intervals.

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