Barite Mobilization in the Upper Devonian Succession of Western New York – Evidence for Anaerobic Methane Oxidation and Methanogenesis during Quasi-Steady State Burial
The complexity of source rocks reflects the interrelationship among such factors as organic matter type, quantity, and quality, thermal maturity level, and diagenetic and burial history. Sulphate reduction reactions are among the first diagenetic modifications to a potential source rock. Subsequent reactions, notably anaerobic methane oxidation (AMO) and methanogenesis, may result in concretionary growth. Small (cm-scale) barite nodules and rosettes concentrated in an ~ 3-m-thick interval at the bottom of the Upper Devonian Hanover Shale of western New York provide insight into what appears to be a quasi-steady state burial history of the Rhinestreet-Hanover shale succession. The nodules, composed principally of barium and sulfur, but also enriched in such trace metals as V, Au, Ni, and Co, formed later than associated carbonate concretions but during the same diagenetic event. Both barite nodules and carbonate concretions precipitated as a consequence of AMO at the sulphate-methane transition zone (SMTZ). Episodic reductions in burial rate stabilized the SMTZ enabling the growth of concretions and nodules. Renewal of burial, however, moved the newly formed concretionary horizon into the zone of methanogenesis causing dissolution of authigenic barite and subsequent upward diffusion of barium to the SMTZ where authigenic barite and carbonate formed again. Repeated dissolution-precipitation events, each one recorded by an interval of insoluble carbonate concretions, offers an explanation for the observed elemental enrichment patterns of the nodules. Preservation of barite nodules at the bottom of the Hanover Shale may reflect the downward shift of the SMTZ induced by accumulation of organic-lean sediment of the middle Hanover Shale (reduced sulphate gradient) and/or the depletion of labile organic matter of the Rhinestreet Shale (reduced methane flux). Ongoing work has recognized similar barium-rich intervals in the upper part of the Marcellus Shale that probably reflect a comparable burial history.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012