Sandstones from the Black Island member of the Winnipeg Formation (Ordovician) contain a variety of early diagenetic iron sulfide morphologies, including elongate concretions of variable length. Examination of sandstone samples with X-radiography reveals that these concretions, now consisting of pyrite, are associated with burrows.

Detailed textural examination of the pyrite shows an anomalously loose packing of sand grains within these concretions, and also suggests that iron sulfide nucleation commenced at multiple sites in what must have been a stiff matrix. Compositional data acquired by electron microprobe indicate that the iron sulfides did not replace fecal matter left behind by the burrower. Sulfur isotope data point to bacterial sulfate reduction as a sulfide source, corroborated by fossilized bacterial remains within the concretionary pyrite.

Mucus and slime trails of marine benthos were probably important for early diagenetic pyrite production in these sediments; they can be considered a favorable “culture” medium for sulfate reducing bacteria. Because the mucus seems to mineralize very early in burial history, it also provides for enhanced preservation of bacterial remains.