Mesozoic and Tertiary argillaceous units: distribution and composition

Summary The generalized distribution and mineralogy of major Mesozoic and Cenozoic mud formations in England can be illustrated using isopach maps and composite cross-sections. Burial diagenesis has only affected the clay minerals in the deeper parts of the Wessex-Weald area onshore. Early diagenetic clay minerals are only significant in the Triassic where sepiolite, palygorskite and corrensite formed in strongly saline environments. Authigenic clays formed sporadically in the Jurassic, Lower Cretaceous and Tertiary when smectite formed by halmrolytic alteration of mafic volcanic ashes. For most of the Mesozoic and Cenozoic the clay mineralogy reflects weathering in the source areas.