CORRELATING DEPOSITIONAL ENVIRONMENTS IN PENNSYLVANIAN BLACK SHALES WITH HIGH-RESOLUTION STUDIES OF ORGANIC-TRACE METAL INTERACTIONS

The redox state of natural waters provides the first-order control on metal fixation in sediments through the variation in mineral solubility with valence state of the metal in question (e.g., Fe, Mo, U). Aside from simple considerations of oxic/anoxic conditions, other factors such as sedimentation rate, the presence of sulfide, and the type and concentration of organic matter will affect the magnitude of enrichments of different metals. Specifically, previous work in these shales demonstrates that trace metals have different correlations with the concentration of organic carbon. Thus, while the Hushpuckney shale was deposited under anoxic/euxinic conditions across the basin from northern Oklahoma to Iowa, variations in the amount and type of organic matter appears to control the absolute level of enrichment of metals such as Mo and U. Given these observations, empirical evidence from studies of other black shales and observations from modern environments, we suggest that trace metals fixed within the organic fraction will serve as a more quantitative proxy for paleoenvironmental conditions than bulk authigenic metal concentrations alone. As the first step to test this hypothesis we have undertaken an organic geochemical analysis of the extractable organic matter in the Hushpuckney Shale to characterize the organic reservoir and assess the variations in organic-bound metals across the basin.

We have extracted bitumen from core samples of the Hushpuckney Shale in Oklahoma, Iowa and Missouri, fractionated it with Si-Al gel chromatography, and will use gas chromatography to assess the relative amounts of terrestrial and marine organic matter preserved in each core. By comparing this ratio to bulk authigenic metal enrichments, we will determine the extent to which organic matter type controls metal fixation. Within this framework, we will integrate variations in the organic geochemistry with inorganic geochemical proxies, to provide additional tools to characterize and correlate synchronous environments as preserved in black shales—rocks that are visually very similar.