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

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.

Middle Cambrian to Lower Ordovician faunas from the Chingiz Mountain Range, central Kazakhstan

“…Cysts of the prasinophyte Tasmanites are relatively common in similar faunal assemblages, e.g. in black shale associations, and have been reported from many localities in the world from the Proterozoic to Holocene (Schieber & Baird 2001). In the Lower Ordovician Caryocaris Shale in Nevada, they have been found in an almost identical fossil assemblage with caryocarid arthropods, small obolid and acrotretid brachiopods, conodonts and unidentified fragments of phosphatic plates (Ethington 1981).…”

Palaeoenvironmental significance of middle Oxfordian deep marine deposits from La Manga Formation, Neuquén Basin, Argentina

“…High organic carbon contents and the presence of pyrite framboids also indicate anoxic deposition conditions (Wilkin and Barnes, 1997;Schieber and Baird, 2001). …”

“…It should be noted, however, that precise intervals of oxygenation were always restricted to the dominant anoxic to sub-oxic conditions (Wilkin and Barnes, 1997;Schieber and Baird, 2001) as revealed by their high organic carbon contents, and the occurrence of pyrite framboids and fossil microbial remains. On the other hand, coccoid bacterial-like textures and micritic peloids could be interpreted as having a bacterial origin (Tribovillard, 1998;Perri and Tucker, 2007).…”

“…Similar facies have been reported by Stefani and Burchell (1990) and Longman et al (1998). The high organic carbon content and occurrence of pyrite indicate anoxic deposition conditions at least in interstitial waters (Wilkin and Barnes, 1997;Schieber and Baird, 2001;Tribovillard et al, 1994).…”

Caractérisations géochimique et pétrographique :

“…Le mot mudrock en anglais ou pélite est utilisé pour définir l'ensemble des roches détritiques consolidées à grains fins et très fins (silt et argile), plus petits que 1/16 mm (62.5 um), contenant de 30 à 75% de minéraux argileux (Foucault et Raoult, 2000: 260;Lundegard and Samuels, 1980;Boulvain et Dumont, 2007 (Figure 8). (Schieber, 2001(Schieber, , 2002Schieber et Riciputi, 2004, 2005, D'hulst, 2007. Elle se situe dans le Nord-est américain en masse lenticulaire incrustée dans des schistes foliés (Klein et Hurlbut, 1993: 365-366 (Klein et Hurlbut, 1993 : 366).…”