The Utility of X-Radiography for Generation of High-Resolution Paleoclimatic Proxy Records from Black Shales, with Examples from Core Shale Members of Upper Pennsylvanian Midcontinent Cyclothems

Ancient black shales have considerable potential for generation of long, high-resolution paleoclimate proxy records, given the availability of a geochemical proxy that can be easily measured. Owing to substantial differences in density between the siliciclastic matrix of black shales (2.6-2.7 g cm-) and the major non-clastic components, i.e., organic carbon, Fe-sulfide, and phosphate (ca. 1.0-1.2, 5.1-5.2, and 3.1-3.2 g cm-, respectively), rock density may represent a suitable proxy. This paper presents a new procedure for rapid analysis of fine-scale compositional variation in black shales, using X-radiography as a primary tool for measurement of rock density. The procedure entails (1) generation of X-radiographs over the complete stratigraphic interval of interest in laminated black shale cores, (2) geochemical and petrographic analysis of limited intervals within the cores to quantify variance in major rock components, and (3) statistical analysis of compositional controls on variance in X-radiograph gray-scale density (a proxy for rock density). This procedure was applied to eight study units from three offshore black shales (Hushpuckney, Stark, and Muncie Creek) of Upper Pennsylvanian cyclothems of midcontinent North America.