On the Origin and Significance of Pyrite Spheres in Devonian Black Shales of North America

Abstract

ABSTRACT Devonian black shales deposited on the North American craton contain abundant Tasmanites cysts that are typically preserved as flattened circular discs on bedding planes. Work by the present authors shows that cysts can be preserved as pyrite-infill casts that are expressed as sand-size whole and geopetal half-spheres of pyrite. At the bases of thin black shale layers these 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 equivalent black shales in Tennessee and Kentucky. Reworked pyrite casts, usually dominated by whole spheres, form lenticular lag accumulations and hydraulic placers, together with plant debris and phosphatic particles (bone debris, conodonts). An earlier model for the formation of pyrite spheres in gas bubbles is rejected in favor of formation within uncompressed Tasmanites cysts. Direct observation of cyst cuticle in association with pyrite spheres suggests that localized bacterial sulfate reduction in Tasmanites interior voids led to formation of localized pyrite deposition, in a manner similar to that described from certain ammonoid chamber settings. Cyst fill commenced with formation of framboidal pyrite, followed by later diagenetic pyrite cementation between framboids. These fills show geopetal features and appear to have formed 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 sediments. They probably occur throughout the sedimentary record, and in mudstone successions they may prove to be an important source of sand-size grains in areas far removed from the basin margins. As such they may be important for detection of erosive events and strong bottom currents, and provide valuable information about the depositional history of mudstone successions.