ABSTRACT

The diagenetic history of the Lower Limestone Shale Group is discussed with special reference to cementation, crystal enlargement, dolomitisation, silica diagenesis and stylolitization.

Initial high porosity was reduced by cementation to almost nothing. Granular, drusy and rim-cements have been distinguished, together with ferroan and non-ferroan calcite cement. Cementation was an early diagenetic process.

Crystal enlargement was the major neomorphic process; crystals, and their boundaries, in these neomorphic mosaics show typical strained features.

Dolomitization occurred at different times during diagenesis. Dolomite takes the form of micrite sized crystals, granular dolomite, or rhombohedra either isolated or in groups.

Authigenic quartz growth is the commonest form of silica diagenesis although silica replacement of fossils occurs.

Stylolties occur both in early and late diagenesis with the maximum development in late diagenesis.

A chronological series of diagenetic processes, related in the main to tectonic stress and deep burial, is proposed for the carbonate rocks of the Lower Limestone Shale Group in South Wales.