Tertiary lateritic weathering in Devon, England, and the Palaeogene continental environment of South West England

Abstract

The plateau deposits of East Devon contain a variety of residual deposits including non-indurated kaolinitic weathering profiles and silcretes. These reflect a complex pedological, diagenetic and geomorphological history which began with the emergence of the post-Chalk land surface at the end of the Cretaceous. During the Palaeocene, lateritic weathering of the Chalk led to the formation of kaolinitic residual flint gravels which are up to 10 m thick over much of the East Devon tableland. These deposits show a variety of deep weathering profile morphology, but well differentiated lateritic weathering profiles are preserved in irregular deep pockets on Chalk. Where resting directly on Albian sediments the residual gravels are thought to represent the response of earlier profiles developed on Chalk to continued pedogenesis and diagenesis once the Chalk had been completely removed. Locally, late silicification of the weathering profile formed silicretes. Cenomanian-Albian calcarenites and arenites have been decalcified and kaolinised to considerable depths beneath the gravels where a protective Chalk capping is absent. In areas adjacent to the Sticklepath-Lustleigh Fault Zone destruction of the Palaeocene weathering mantle was accomplished by late Middle Eocene times when new weathering profiles were established on newly exposed Upper Palaeozoic rocks. These younger profiles were subsequently eroded and redeposited in deep tectonic basins along major wrench fault zones. This phase of erosion and sedimentation was accompanied by a climatic change which is reflected by changes in profile morphology and clay mineralogy with time. The oldest Palaeocene profiles are mature, took >10 7 years to form and developed in a tropical climate, whereas profiles dating from the Lower Eocene onwards are immature, took ≈10 6 years to form and developed in a sub-tropical to temperate climate.