SUMMARY

Four beds in the middle and upper parts of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group at Durlston Bay, Dorset, contain acid insoluble residues that suggest a penecontemporaneous volcanogenic origin, probably as sub-aerial ashfalls. Their particle size distribution patterns, mineralogy, geochemistry and K–Ar signatures indicate post-depositional alteration of fine-grained volcanic glass by pore waters. Major and trace element chemistry indicates that: (1) two beds in the Cherty Freshwater Beds (Lulworth Formation) were derived from ashes of trachyandesite-andesite-rhyodacite/dacite composition that were argillized in saline pore waters enriched in Mg2+; (2) a bed in the Unio Beds (Durlston Formation) developed from an ash of trachytic composition; and (3) a bed in the Viviparus Shales and Clays (Durlston Formation) originated from an ash of trachyandesite-andesite-rhyodacite/dacite composition within the suboxic zone of brackish pore waters. Comparison of the four Purbeck volcanogenic beds with bentonite beds in the lower part of the Speeton Clay on the Yorkshire coast and offshore in the North Sea suggests that those from the Unio Beds and Viviparus Shales and Clays are equivalent respectively to Beds D7D and D6J at Speeton.

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