PYRITIZATION OF A STIGMARIA FROM THE BOSS POINT FORMATION (PENNSYLVANIAN), NOVA SCOTIA

In this study we describe and interpret a uniquely preserved Stigmaria from the type section of the Boss Point Formation (Pennsylvanian). This unit records numerous alternations between braided fluvial, floodplain, and lacustrine deposits. Thin coals and organic-rich horizons are locally developed atop stigmarian roots of lycopsid trees. The specimen described in this study was recovered from an interval dominated by laminated lacustrine shales and clastic floodplain deposits ~940 m above the base of the formation.

The Stigmaria is preserved as a sandstone cast that is surrounded by a sulfide rind and an outer, poorly preserved periderm. The sandstone fill is composed of a homogeneous, fine-grained feldspathic arenite. Calcite cement is abundant in the sandstone; isolated patches of pyrite cement are present locally. A 5 mm thick sulfide rind is present beneath a thin carbonaceous film formed from the compressed periderm. EDX and polished thin section analysis confirmed that the rind is composed of pyrite. Scanning electron microscopy analysis of fracture sections showed that the rind is microcrystalline and that no pyrite framboids are present. Microscopic analysis also revealed that no periderm cell structures were preserved by the pyrite and that crystallization occurred in the interstices of the sediment fill rather than in the periderm itself. Stable sulfur isotope analysis yielded a d³⁴S value of 30.9 per mil.

This type of preservation could have occurred early through the activity of sulfate-reducing bacteria in an environment with marine influence or by mobilization of sulfur-rich fluids during diagenesis. The recent discovery of marginal marine fossils in associated strata suggests that the bacterial scenario is the most likely. High d³⁴S values indicate that pyrite crystallization occurred in a closed system that experienced Rayleigh fractionation. Burial of the Stigmaria, in conjunction with the periderm encasing the sediment, created barriers that restricted sulfate input which resulted in high d³⁴S values and the microcrystalline texture of the pyrite cement. Although Stigmaria and pyrite are both common in Carboniferous coal fields, this may be the first study that documents this unique type of pyritized preservation.