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

Abstract

Summary URTeC Control ID Number: 1619340 This study focuses on the application of carbon isotope and elemental stratigraphy to the Greenhorn – Carlile – Niobrara Formation interval, with analysis of seven cores in the Denver Basin and one core in the Washakie Basin. Carbon isotope stratigraphy has been shown to exhibit lithology independent chronostratigraphic correlative capabilities with the potential to provide local, regional, and global chronostratigraphic correlations. Carbon isotope data gathered from two Niobrara cores, one at six-inch sampling intervals and the other at two-foot intervals by Stout (2012), are used as a high-resolution pilot data set from which to build on. Recently collected carbon isotope data, sampled at two- and three-foot intervals along a 560-foot Greenhorn – Carlile – Niobrara core, present the first continuous carbon isotope profile of this scale and resolution for Late Cretaceous stratigraphy in the Denver Basin. Four more Greenhorn – Carlile – Niobrara cores will be sampled for carbon and oxygen isotopes. The carbon isotope profiles will be used as chronostratigraphic markers for intra- and inter-basinal correlations. Preliminary results clearly show correlative carbon isotope excursions among the cores. Elemental stratigraphy has many applications including chemostratigraphic correlation, a proxy for mineralogy/lithology, and a proxy for water column conditions (e.g. redox potential) during the time of deposition (Tribovillard et al., 2006; Ratcliffe & Wright, 2012). Elemental analysis has particular merit in the study of mudstones such as those of the Greenhorn – Carlile – Niobrara Formation interval, with the ability to reveal geochemical trends in fine-grained, relatively homogeneous lithologies. The handheld energy-dispersive x-ray fluorescence (ED-XRF) analyzer is a modern tool capable of rapid, in situ measurements for elements greater than or equal to magnesium in atomic weight. The handheld ED-XRF analyzer has made high-resolution elemental stratigraphy a viable and even practical consideration. Elemental analysis was conducted by handheld ED-XRF analyzer on the same core suite as those sampled for carbon isotopes, at a sampling interval of one foot. Preliminary results show that particular elemental trends directly correspond to mineralogy/lithology and can be divided into distinct chemostratigraphic packages.