Redox Conditions during deposition and early diagenesis of the U. Ord. Point Pleasant Ls of southwestern PA and northern WV: Insights from pyrite framboids and trace elements

Abstract

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 16 samples retrieved from three 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. Two models can explain this apparent paradox: 1) Anoxia developing within marine snow aggregates suspended in the water column could have produced a micro-environment conducive to the precipitation of framboids in an otherwise dysoxic water column, or 2) the occurrence of small framboids may be explained by a lack of reactants necessary to sustain pyrite growth in anoxic pore waters. Indeed, the latter model is consistent with low production of H2S inferred from low total organic carbon (TOC) content of the Point Pleasant. Further, total iron content below average shale values in the Point Pleasant indicates low delivery of reactive iron to the seafloor. Both models are consistent with the Point Pleasant accumulating under a dysoxic water column where TOC preservation was accomplished by its burial and removal from zones of oxidation and biologic degradation.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016