ABSTRACT

Field studies of the Fossil Hill Formation across Manitoulin Island indicate that chert nodules and lenses are concentrated in an interbiohermal facies of calcareous sands and muds which includes subordinate biostromal carbonates. Thin-section examination of the chert reveals microfabrics consisting of silica void-infilling and replacement fabrics. These imply that the chert originated early in the diagenetic history of the host carbonate before lithification. The microfabrics are intimately associated with organic matter. The lowered pH, resulting from decay of incorporated organic remains, caused dissolution of calcite and simultaneous precipitation of silica as the solution-bearing interstitial waters passed through the highly porous sediments.

The occurrence in the chert of abundant sponge spicules suggests that the source of the silica was a biogenous one. Adjacent to present-day Manitoulin Island are the Precambrian La Cloche Mountains which were islands in the Silurian Sea. This "quartzite" terrain may have been the ultimate source of silica, stimulating "blooms" of silica-secreting organisms.

Microfossils recovered from the cherts include acritarchs which are represented by the Subgroups Sphaeromorphitae, Acanthomorphitae and Herkomorphitae, comprising 45%, 33% and 22% respectively of the total assemblage.