USING SIZE DISTRIBUTION OF PYRITE FRAMBOIDS TO EVALUATE PALEO-ANOXIA DURING DEPOSITION OF THE DEVONIAN NEW ALBANY SHALE IN THE ILLINOIS BASIN

The question of the role of anoxia in the formation of the Devonian New Albany Shale has been explored through a detailed study of pyrite framboids. Size distribution, mean diameter, and maximum diameter of pyrite framboids were measured on 41 thin sections that span the entire thickness (40 meters) of the New Albany Shale in south-western Indiana. Mean framboid diameters range between 4.12 and 5.51 µm. Overall, 77% of the measured framboids are less than 5 µm in diameter. Of the remaining framboids, 22% range from 6 to 10 µm in diameter; the maximum framboid diameter was 28 µm.

Mean framboids diameters of the range observed in the New Albany Shale have previously been interpreted as an indication of dominantly anoxic bottom waters (Wilkin et al., 1996; Wignall and Newton, 1998). By extension, this would suggest that throughout its depositional history, the New Albany Shale was deposited under largely anoxic conditions. However, 23% of the framboids are larger than 5 µm. Such larger framboids are thought to form when the redox boundary is below the sediment-water interface (Wilkin et al., 1996). Thus, these larger framboids in the New Albany Shale imply bottom water oxygenation episodes during shale formation.

To further assess the role of anoxia during New Albany Shale deposition, geochemical data and derived paleo-redox proxies (i.e., degree-of-pyritization (DOP), V/Cr, V/(V+Ni), Ni/Co, U/Th) were obtained on the same 41 samples used for thin section preparation. These geochemical proxies were examined and correlated with variations in framboid size distributions, lithology, sedimentary structures, and degree of bioturbation. This integrated approach revealed that anoxia indicated by framboid properties or paleo-redox proxies such as DOP contrast with oxygenated conditions indicated by other paleo-redox proxies such as V/Cr and by observations of bioturbation in several of the samples examined. This study shows that an improved interpretation of the paleo-redox conditions in which fine-grained sediments deposited is reached when measurements of pyrite framboid size distribution are used in combination with multiple geochemical proxies and rock detailed examination.