ABSTRACT

Silica diagenetic fabrics occurring in the Portland and Corallian Beds of southern England are described. On the basis of their morphology, five major fabric types are distinguished: mosaic quartz, chalcedonic overlays, spherulitic chalcedony, lutecite, and microcrystalline granular quartz. Of these, the first three may be cementation fabrics, and all excepting chalcedonic overlays may exhibit replacement features.

The silica fabrics are dated according to their relation to carbonate diagenetic fabrics. Two major stages of silicification are demonstrated, the first being pre-calcite-cement fabrics, and the second post-calcite-cement fabrics; each was probably accompanied by calcitization of sponge spicules. The latter occur in sufficient quantities throughout the sediments to account for the silica fabrics. Lack of silicification in the Corallian Beds, despite their high spicule content, is probably caused by calcite cements effectively reducing porosity early in their diagenetic history. This contrasts with the extensive development of cherts in the Portland Beds, in which high porosities are maintained after deposition to the present day.