Biogenic chert in the Cow Head Group (Cambro-Ordovician), western Newfoundland

Chert in the Cow Head Group is mainly a replacement of limestone and shale and, to a lesser extent, an interparticle cement. Its field occurrences are distinct as: (1) silicified margins on coarse conglomerates and thinly bedded limestones; (2) nodules within limestone and shale; (3) pervasively silicified beds of limestone and shale; and (4) clasts or partial replacement of clasts within conglomerate. Radiolarians and sponge spicules are composed of microquartz or calcite and are particularly common in the Ordovician part of the succession where most chert occurs. In limestone spatially associated with chert, the use of cathodoluminescence demonstrates that calcite-replaced radiolarians and spicules are volumetrically more important than realized through transmitted-light petrography. Petrographic relations between siliceous and rare pyritized radiolarians further indicate that these particles may be dissolved prior to compaction. No trace of their former existence remains, other than indirectly through the presence of silicified limestone and shale. Crushed grains cemented by chalcedony indicate that chert was precipitated during or after compaction. The history of silicification and the replacement or dissolution of siliceous bioclasts is protracted, ranging from near the sediment-water interface, where it is concomitant with early limestone lithification, to deeper burial, postdating mechanical compaction.