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High-Resolution Carbon Isotope and Elemental Chemostratigraphy of the Greenhorn, Carlile, and Niobrara Formations, Denver Basin, Colorado

Abstract

Recent drilling and production activity in the Niobrara Formation has inspired renewed interest in such unconventional carbonate mudrock systems. With the majority of Niobrara activity taking place in the Denver Basin, CO, high-resolution chemostratigraphic analysis has the potential to enhance exploration strategies in this area. This study explores the application of carbon isotope and elemental chemostratigraphy to the Greenhorn, Carlile, and Niobrara Formations in the Denver Basin by analysis of seven cores. Carbon isotope data from this study are used in an attempt to determine the nature and scope of potential for a high-resolution data set as a chronostratigraphic correlation tool. Application of carbon isotope stratigraphy toward a sequence stratigraphic framework will also be examined. Carbon isotope stratigraphic profiles for four cores are complete. Results show varying trends across the Denver Basin, with clearly identifiable carbon isotope excursions that can be used for unequivocal correlations. Regional and global correlation potential is evident, with carbon isotope excursions related to specific global oceanic events, such as oceanic anoxic events II and III. Newly developed handheld energy-dispersive x-ray fluorescence (ED-XRF) analyzers are capable of providing laboratory quality elemental data sets (for elements of atomic weight greater than sodium) with rapid analysis times (from tens of seconds to minutes). High-resolution ED-XRF analysis of the study interval attempts to provide insight into various aspects of the depositional system, with applications including geochemical proxies for mineralogy/lithology, provenance, diagenetic history, and water column conditions at the time of deposition. Major element analysis shows a direct relationship with mineralogy/lithology (e.g. calcium as a proxy for calcium carbonate). Paleoredox indicators, such as molybdenum and vanadium, show much greater values in the Niobrara Formation as compared with the Carlile Formation and Greenhorn Formation.