--> --> Chemostratigraphy of the Greenhorn, Carlile, and Niobrara Formations, Denver Basin, CO

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Chemostratigraphy of the Greenhorn, Carlile, and Niobrara Formations, Denver Basin, CO

Abstract

Late Cretaceous calcareous mudrocks of the Western Interior Seaway have recently become prime targets for unconventional hydrocarbon resources. Within the Denver Basin, such mudrocks constitute a 500 – 700 ft. interval of nearly continuous carbonate strata, extending across the Greenhorn, Carlile, and Niobrara Formations (GCN). The fine-grained nature of GCN sediments provides the unique opportunity to apply high-resolution chemostratigraphic (carbon isotope and elemental) analyses over two third-order transgressive-regressive cycles, a span of more than 10 m.y. Eight cores from the Denver Basin were analyzed, of which six cores are composed of the GCN interval and two cores of the Niobrara Formation. Carbon isotope data were collected at intervals varying from six-inches to five-feet, which show new high-resolution correlations within the Denver Basin. Additionally, global correlations with carbon isotope studies from South Texas, Great Britain and Italy are clearly present. When used as a chronostratigraphic proxy, carbon isotope correlations give new perspective to the timing of depositional events. Elemental stratigraphy was conducted at one-foot intervals using a Niton XL3t GOLDD+ handheld energy-dispersive X-ray fluorescence analyzer (ED-XRF). Rapid analysis time (∼1–3 minutes) coupled with large data acquisition (>40 elements) make the handheld ED-XRF an extremely powerful device. Elemental concentrations and ratios prove to be a remarkable tool for mudrock analyses, providing a multitude of data types including but not limited to litho- and mechanical stratigraphic proxies, paleoenvironmental conditions during time of deposition, and diagenetic history. Furthermore, statistical techniques such as factor analysis and time-series analysis can be applied to such large elemental data sets as a means for data filtration, determining a wide variety of correlations, and potential quantitative predictive capabilities.