REDOX CONDITIONS DURING DEPOSITION OF THE UPPER ORDOVICIAN POINT PLEASANT LIMESTONE, APPALACHIAN BASIN, USA: CONTROLS ON THE ACCUMULATION OF ORGANIC CARBON (Invited Presentation)

The mode and occurrence of sedimentary pyrite provides a useful tool to assess the redox conditions of bottom and pore waters in ancient sediments. Sediments accumulating under dysoxic water demonstrate a low occurrence of pyrite, in the form of euhedral grains with a subordinate occurrence of framboids. In anoxic pore waters, morphology shifts to framboidal pyrite of variable and often large (>10 µm) size. Sediments accumulating under an anoxic water column illustrate a population of small (<5-6 µm), more uniform size framboids. SEM analysis of pyrite in cuttings and core chips retrieved from Point Pleasant Limestone gas exploration wells demonstrate a dearth of pyrite in the Point Pleasant (0.02-1.7% of area analyzed). While pyrite morphology is dominated by euhedral grains and masses (~80% of pyrite encountered), the framboids are uniformly small (4.7 µm), with just a few >10 µm. The lack of pyrite and its occurrence as mostly euhedral grains and masses suggest accumulation under a dysoxic water column. Conversely, the size of the framboids suggests they formed in a water column containing free H₂S. Small framboids resulting from a lack of reactants necessary to sustain pyrite growth in anoxic pore waters would explain this apparent paradox. Abundant nucleation sites competing for a finite amount of reactants would result in a population of many small framboids with few large examples. Indeed, the low total iron/aluminum (Fe/Al) content of the Point Pleasant (average Fe/Al 0.45), is ~20% lower than total Fe/Al of average shale values (Fe/Al = 0.55), indicating low delivery of reactive iron to the seafloor during Point Pleasant deposition. This model is consistent with the sediment accumulating under a dysoxic water column, where burial and removal from zones of oxidation and biologic degradation increase the preservation potential of total organic carbon (TOC). Indeed, the Point Pleasant demonstrates an inverse correlation between in-situ shell beds and TOC. These shell beds (~2-3% TOC) represent times of cessation in sedimentation where colonization of the seafloor ensues under oxygenated conditions. Conversely, the absence of these fossil-bearing strata (~4 % TOC) suggest times of continuous sedimentation and elevated potential for organic carbon preservation.