PYRITE FRAMBOID SIZE AND SIZE DISTRIBUTION IN MARINE BLACK SHALES: A CASE STUDY FROM THE DEVONIAN-MISSISSIPPIAN OF CENTRAL KENTUCKY

Variations in mean pyrite framboid size and in framboid size distribution have been related to redox conditions in modern and ancient environments. Mean framboid diameters of 5.0 +/- 1.7µm have been reported for modern euxinic environments, whereas oxic and dysoxic environments have been associated with framboids that have somewhat larger diameters, 7.7 +/- 4.1µm (Wilkin et al., 1996). Framboids that form within a euxinic water column have been shown to exhibit a narrower size range than those that form within anoxic sediment porewaters beneath an oxygenated water column. Pyrite morphology (framboids, “infilled” or “welded” framboids, euhedral crystals and masses) may also be indicative of redox conditions of pyrite formation.

Three Devonian-Mississippian marine, organic-rich black shale units (the Sunbury Shale, the Cleveland Shale, and the Huron Shale) comprise approximately 40m of the D-6 core (Montgomery County, east-central Kentucky). On-going studies of carbon-sulfur-iron relationships and trace-element contents for these units suggest that anoxic conditions were more persistent during accumulation of the Sunbury and Cleveland Shales than during accumulation of the Huron Shale. These units were compared in terms of pyrite morphology, mean framboid size, and framboid size distribution.

Preliminary results show mean diameters of framboids for samples from these units vary between 4.75 and 6.25µm. Differences occur in the range of diameters and in other distinguishable characteristics, including extent of diagenetic pyritization and presence of euhedral crystals. The small size of many of the framboids in these units is consistent with formation under predominantly anoxic conditions; however, the presence of larger framboids in some parts of the section suggests brief periods of dysoxic bottom-water conditions. Size distributions and modes of occurrence will be discussed in terms of paleo-redox conditions during accumulation of these three units.