FRAMBOID SIZE DISTRIBUTION IN BLACK SHALE OF THE UPPER DEVONIAN HANOVER SHALE, WESTERN NEW YORK: EVIDENCE FOR FLUCTUATIONS IN THE REDOX BOUNDARY

The Upper Devonian (Frasnian) Hanover shale comprises several intervals of interbedded gray and black shale. We conducted a microfabric analysis of a sequence of four black shale beds (12 to 16 cm thick) and interbedded gray shale (32 to 62 cm thick) roughly 2 m above the Hanover shale-Pipe Creek shale contact near Silver Creek, New York. The organic-rich layers, though typically laminated, are bioturbated toward their contacts with overlying poorly laminated (bioturbated) gray shale. Pyrite framboids in the black shale beds occur in two modes: 1) isolated spheres and 2) polyframbroidal masses of as many as 65 component framboids. The former define a bimodal size distribution with mean sizes of 5.5μm (+/-2.6μm) and 25.1μm (+/-6.9μm). The smaller framboids probably grew within a euxinic water column and settled to the substrate; however, the larger spheres appear to have grown in the sediment. Component framboids of the polyframboids are characterized by a mean size of 7.8μm (+/-4.4μm). The generally large size of these framboids suggests that the polyframboids grew in situ. Indeed, microscopic observations of the polyframbroids reveal them to be completely wrapped by clay grains suggesting that the masses grew into water-rich flocculated sediment. We suggest that the alternating black and gray shale beds reflect repeated lowering of the redox boundary. At the outset, small framboids that had formed near the redox boundary within a euxinic water column settled to the ocean floor with flocculated organic clay. At length, the redox boundary dropped very close to the sediment-water interface resulting in the diagenetic growth of the large isolated framboids and polyframboids. The redox boundary continued its descent moving perhaps 5 to 10 cm into the sediment. This resulted in the accumulation of the gray shale in a dysoxic or oxic water column and establishment of bottom-dwelling organisms that homogenized the gray shale and burrowed into the black shale. The cycle repeated itself when the redox boundary reestablished itself in the water column.