On the origin and significance of pyrite spheres in Devonian black shales of North America

Devonian black shales deposited on the North American craton contain abunda nt Tasmanites cysts that are typically preserved as flattened circular disc s on bedding planes. Work by the present authors shows that cysts can be pr eserved as pyrite-infill casts that are expressed as sand-size whole and ge opetal half-spheres of pyrite. At the bases of thin black shale layers thes e occur in situ at many stratigraphic levels in the distal prodelta facies of the Catskill Delta complex of New York, as well as in laterally equivale nt black shales in Tennessee and Kentucky. Reworked pyrite casts, usually d ominated by whole spheres, form lenticular lag accumulations and hydraulic placers, together with plant debris and phosphatic particles (bone debris, conodents). An earlier model for the formation of pyrite spheres in gas bubbles is reje cted in favor of formation within uncompressed Tasmanites cysts. Direct obs ervation of cyst cuticle in association with pyrite spheres suggests that l ocalized bacterial sulfate reduction in Tasmanites interior voids led to fo rmation of localized pyrite deposition, in a manner similar to that describ ed from certain ammonoid chamber settings. Cyst fill commenced with formati on of framboidal pyrite, followed by later diagenetic pyrite cementation be tween framboids, These fills show geopetal features and appear to have form ed within the redox zone below the sediment-water interface, Although described here from the Upper Devonian, comparable pyrite textures are also known from Proterozoic, Cambrian, Ordovician, and Silurian sedime nts. They probably occur throughout the sedimentary record, and in mudstone successions they may prove to be an important source of sand-size grains i n areas far removed from the basin margins. As such they may be important f or detection of erosive events and strong bottom currents, and provide valu able information about the depositional history of mudstone successions.