Oxidation of detrital pyrite as a cause for Marcasite Formation in marine lag deposits from the Devonian of the eastern US

Late Devonian black shale successions in the eastern US contain numerous discontinuities that are characterized by lags containing variable amounts of quartz sand mixed with reworked pyritic and phosphatic debris. These lags can be partially cemented by iron sulfides with a radial fibrous to massive morphology. Examination of iron sulfide cements via electron backscatter diffraction (EBSD) shows that these cements can consist entirely or partially of marcasite. It is proposed here that oxidation and dissolution of reworked pyrite debris led to favorable conditions for intermittent rapid marcasite formation by forcing a substantial pH drop in the surface sediment and by raising dissolved iron concentrations in the pore waters.