Using energy dispersive X-ray fluorescence (ED-XRF) to assess the stratigraphic relationships of the middle Jurassic Gypsum Spring and Piper formations, Bighorn Basin, Wyoming and Montana

Abstract

Chemostratigraphy via X-ray fluorescence measures the shifts in elemental properties within a given section of strata. It has proven to be a successful tool in both correlation and paleo-environment reconstruction. Recently, handheld energy-dispersive x-ray fluorescence (ED-XRF) has become a cost effective, reliable, and highly efficient means of obtaining geochemical information from mudstones. So long as there is a working calibration, a handheld ED-XRF can be used to gather data in real time to assess the stratigraphic relationship and equivalency of lithologically similar and subadjacent units. The middle Jurassic shales of Northern Wyoming and Southern Montana are prime for a chemostratigraphic analysis via x-ray fluorescence. The Gypsum Spring, Piper, and overlying Sundance Formation were deposited along the ramp margins of a cratonic forebulge in a retro-arc foreland basin, and are characterized by multiple sequences of primarily shallow marine evaporates, red shales, and carbonates. Previous efforts to correlate these units have yet to establish a universally agreeable set of stratigraphic rules. The challenge is due to 1) a lack of complete type sections representative of the units across much of Wyoming and Montana, 2) a lack of persistent diagnostic fossils, and 3) a pervasive discord regarding the location of erosional surfaces (Parcell and Williams, 2005). Throughout much of the Western Interior, chert lag deposits have been used to locate Triassic and Jurassic unconformities. While the lag deposits have been used with much success in the southwest United States, there is widespread disagreement concerning the existence of these horizons in the Middle Jurassic rocks of Wyoming and Montana. ED-XRF analysis of these formations serves to resolve the correlations concerns and other stratigraphic issues related to depositional history of the Gypsum Spring and Piper Formations.