Early lithification and hardgrounds in upper Albian and Cenomanian calcarenites, southwest England

Early diagenetic lithification of calcarenites on the sea floor led to the development of a variety of hardgrounds, intraformational conglomerates, breccias and boulder beds in southwest England during late Albian and Cenomanian time. Cemented nodules commonly developed below the sea floor, and these were avoided by burrowing infauna. In some instances the nodules were exhumed and reworked on the sea floor to form distinctive intraformational conglomerates; exposed nodules frequently became bored and encrusted by organisms and mineralised by glauconite and phosphate. In other cases, sea floor cementation produced true hardgrounds whose upper surfaces were affected by the same processes, but the hardened layers were only a few tens of centimetres thick and were underlain by soft sediment. Fracturing and brecciation of some hardground layers occurred through differential compaction and through undermining and collapse as the result of burrowing and erosion beneath the hardened layer. Reworking of clasts produced in this manner yielded intraformational breccias. Petrographic analysis reveals multiple generations of carbonate cement, commonly beginning with syntaxial overgrowths on echinoderm fragments and “dog tooth” spar on polycrystalline carbonate grains. A progression exists in the Albian-Cenomanian succession of southwest England from relatively simple hardgrounds and intraformational conglomerates low in the sequence up into complex hardgrounds that may record many stages of sediment accretion, cementation, mineralisation and erosion. This progression appears to record increasing water depths and increased sea floor diagenesis.