QUANTIFYING PALEO-ENVIRONMENTS IN THE EAGLE FORMATION, NORTHERN BIGHORN BASIN, USING TRACE ELEMENT CONCENTRATIONS

In the past decade, our understanding of paleoenvironmental and paleoclimate records in sedimentary strata has been enhanced with equipment such as portable X-ray Fluorescence (XRF), gamma-ray spectroscopy (GR), and, most recently, portable Laser-Induced Breakdown Spectroscopy (LIBS). The identification and correlation of trace element concentrations (i.e., trace element chemostratigraphy) using these tools provide valuable indicators of changing environments and climate over time, as well as the effects on sedimentary systems.

To date, most chemostratigraphic trace element studies have focused on fine-grained shale and carbonate units in shallow-marine environments. This study expands this technique to coarse-grained clastic units found in terrestrial and deltaic settings, thereby increasing the application of trace element correlations to broader sedimentary environments.

Trace element concentrations were examined in the Late Cretaceous Eagle Formation, exposed in the Elk Basin Oil Field, northwest Wyoming. The Eagle Formation is a light gray to tan coarse-grained sandstone with iron concretions, Diplocraterion, and Ophiomorpha structures. Preliminary analysis of the XRF and GR data shows distinct trends in terrestrial and marine element accumulations occurring on the western edge of the Cretaceous Inland Sea. Additional examination of LIBS data is ongoing.

The analysis of samples through XRF and GR techniques reveals chemical trends that indicate potential changes in depositional facies. These changes may be attributed to migration from the Western interior seaway and volcanism from the Absarokas. The terrestrial facies were recognized from Fe:Mn ratio, whereas the marine facies were observed with Si:Ca ratio. The marine and terrestrial ratios demonstrate inverse peaks and troughs representing times when the Eagle Formation was partly to fully submerged and when it existed as exposed coastal deposits. These discernible patterns suggest that trace element ratios can be used to detect environmental variation. Further examination of elemental ratios, including LIBS, will further distinguish paleoenvironmental changes through the Eagle Formation.