DEPOSITION AND DIAGENESIS OF THE UPPER ORDOVICIAN POINT PLEASANT LIMESTONE, APPALACHIAN BASIN, USA: INSIGHTS INTO ORGANIC MATTER PRODUCTION, PRESERVATION, AND RESERVOIR DEVELOPMENT

The mode and occurrence of sedimentary pyrite has often been used to assess the redox conditions of bottom and pore waters in ancient sediments. Framboids form rapidly in the zone of iron reduction immediately below the sulfide chemocline while euhedral pyrite grains form at more protracted rates in hydrogen sulfide (H₂S)-bearing water. Sediments accumulating under dysoxic water are characterized by a low occurrence of pyrite which takes the form of euhedral grains with a subordinate occurrence of framboids. However, in anoxic pore waters, morphology shifts to framboidal pyrite of variable and often large (>10 µm) size. Further, sediments accumulating under an anoxic water column illustrate a framboid population that is small in diameter (<5-6 µm) and less variable in size. Pyrite in cuttings and core chips retrieved from vertical and horizontal Point Pleasant wells were analyzed by SEM. Results 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 on average (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. This apparent paradox may be explained by a model where the occurrence of small framboids resulted from a lack of reactants necessary to sustain pyrite growth in anoxic pore waters. 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 total iron/aluminum (Fe/Al) ratio of the Point Pleasant (average Fe/Al 0.45), is ~20% lower than total Fe/Al of average shale values (Fe/Al = 0.55), which would indicate a low delivery of reactive iron to the seafloor during Point Pleasant deposition. Further, primary productivity may have been limited by the lack of bio-limiting iron. Under such conditions in recent sediments it has been observed that diatoms produce more robust siliceous shells. It is possible that the same mechanism was operating during accumulation of the Point Pleasant where biogenic silica accounts for 45-75% of the silica fraction encountered.