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Preliminary Chemostratigraphic Record for the Devonian Three Forks Formation and Associated Units, North Dakota

Abstract

Traditionally, the Three Forks Formation (Devonian, Williston Basin, ND) has been difficult to analyze for facies-scale chemostratigraphic variability due to the overwhelming concentration of brines emanating from the rock which coat the surface of the core. The brine coating affects the surface-sensitive x-ray fluorescence analysis typically undertaken in our lab by 1) attenuating x-ray energies from the low-energy emitters (e.g., Mg, Al, Si, and P), and 2) impacting inter-element corrections established for non-brine-bearing mudrocks. We have taken tremendous care to wash and immediately analyze the Three Forks succession preserved in a core in order to minimize brine contributions to the error of measurement, and consequently, develop a high-quality chemostratigraphic record of change from this interesting succession of strata. The rocks were soaked with water and rinsed multiple times before scanning for major and trace elements. If was observed that brine would quickly build up on the surface of the rocks, thus the rocks were scanned under incipient dryness, and an evaluation of the brine contamination was made by monitoring the chlorine peak on the x-ray fluorescence software. Raw counts data for the chlorine was evaluated, and samples were reanalyzed until deemed “clean” of chlorine. The Mg/Al ratio, which provides a good estimate of dolomite/clay variability, increases upwards through the succession in a cyclical manner. The %S illustrates that anhydrite is a dominant component in the lower half of the succession, with both Sr and Ba increasing in a punctuated manner in the upper third of the anhydrite maximum. Chemostratigraphic results reveal that significant stratigraphic variations in %Al (clay mineral proxy), %Mg (dolomite proxy), %S (sulfate proxy), and Si/Th (quartz/clay mineral proxy) occur and can be used to break up the succession; however, the use of hierarchical cluster analysis and elemental ranking within the clusters provides a robust method for defining chemofacies that record oscillatory depositional conditions that are potentially linked to changes in basin water chemistry and sediment transport/dilution.