Geochemical Expressions of Cyclicity in Cretaceous Pelagic Limestone Sequences: Niobrara Formation, Western Interior Seaway
Walter E. Dean, Michael A. Arthur, 1998. "Geochemical Expressions of Cyclicity in Cretaceous Pelagic Limestone Sequences: Niobrara Formation, Western Interior Seaway", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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Marked cycles in color, carbonate and organic-carbon concentrations, geophysical log characteristics (for example, gamma-ray and sonic velocity), degree of bioturbation, and composition of benthic faunal assemblages characterize the pelagic carbonate sequences of the Upper Cretaceous Niobrara Formation in the Cretaceous Western Interior Seaway ofNorth America and seem to be attributable to Milankovitch orbital forcing (periodicities of ca. 20-400 ky). A hierarchy of cycles is present, starting with decimeter- to meter-scale bedding cycles with nominal 20 ky periodicity that are bundled into larger cycles of perhaps 100 ky, which are, in turn, encompassed by much longer cycles of >1 My. The longer cycle may not be Milankovitch in origin but instead may be related to variations in sea level and tectonism in the Western Interior Seaway instead. Geochemical data constrain interpretations of the depositional mechanisms that led to the cyclicity in that subtle variations in mineralogy and redox conditions are easily seen in major- and minor-element geochemical data. Carbon-isotope and Rock-Eval pyrolysis data on the organic fraction across the cycles suggest that cyclic variations in lithology are in part caused by changes in surface-water biotic productivity and preservation of organic matter at the seafloor.
All cycles in the Niobrara Formation apparently are the product of a combination of changes in dilution of biogenic components by terrigenous detritus, carbonate production in surface waters, and degree of oxygenation of bottom-water. All of these factors are interrelated and ultimately controlled by fresh-water runoff to the seaway from uplifted highlands to the west, and possibly from the low-relief eastern margin, as well as by large-scale transgressive-regressive cycles. Geochemically, the cycles can be expressed in terms of a three-component system of CaCO3, clastic material (represented by aluminum, Al), and organic matter (represented by organic carbon, OC). Within the bedding cycles, the clastic-rich marlstone beds almost always contain the highest concentrations of OC. However, on the scale of the entire formation, variations in clastic material (A1) are not always in phase with those of organic matter (OC), which suggests that factors controlling the production and preservation of organic matter were not always related to factors controlling the deposition of detrital clastic material.
Q-mode factor analyses show that most major, minor, and trace elements (especially Ti, Mg, Na, K, Ce, Li, Nd, Sc, and Y) are associated with Al in the clastic fraction. Strontium is mainly in the carbonate fraction, and most other trace elements (especially Cd, Cu, Mo, Ni, V, and Zn) are associated with the organic fraction. Most iron and sulfur are present as pyrite (FeS2) in these rocks which, like most carbonate-rich rocks and sediments, are iron-limited with respect to pyrite formation. Concentrations of manganese (Mn), which is very sensitive to redox conditions, are highest in the limestone beds of the Fort Hays Limestone Member, which were deposited under oxic bottom-water conditions, and are very low in the Smoky Hill Chalk Member, which was deposited under suboxic to anoxic bottom-water conditions.
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This volume presents the results of a coordinated, multidisciplinary study of Cretaceous carbonate and clastic rocks in cores collected along a transect across the old Cretaceous seaway that extended from the Gulf Coast to the Arctic by a team of academic, industry and U.S. Geological Survey scientists. The overall goal was to construct a subsurface transect of mid-Cretaceous strata that were deposited in the U.S. Western Interior Seaway. In particular, the papers in this volume focus on the Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation and equivalents in cores from six drillholes from western Kansas, southeastern Colorado and eastern Utah.