Depositional Cycles in the Niobrara Formation, Colorado Front Range
Lisa K. Barlow, Erle G. Kauffman, 1985. "Depositional Cycles in the Niobrara Formation, Colorado Front Range", Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes, Lisa M. Pratt, Erle G. Kauffman, Frederick B. Zelt
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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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Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes
Fine-Grained Deposits and Biofacies of the Cretaceous Western Seaway: Evidence of Cyclic Sedimentary Processes - This volume emphasizes the influence of cyclic sedimentary processes on the distribution of rock types and faunas in the Cretaceous strata in Colorado. As a whole, the volume provides an interdisciplinary view of the causes and consequences of gradual cyclic, periodic, and catastrophic changes in environmental conditions recorded by strata in the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems.