The nature and origin of Chert in the Upper Greensand of Wessex

Abstract

The sporadic distribution of chert, both vertically and laterally, within the Upper Greensand, is described. The chert beds of Devon and the Dorset coast consist of glauconitic sands, usually calcareous but leached free of calcareous material wherever there is no protective capping of Chalk. In contrast the chert beds of Somerset and the Vale of Wardour consist of sparsely glauconitic silty sediments. The chert nodules themselves typically show a massive flinty core surrounded by a porous light-coloured marginal rock. The various theories of formation previously forwarded for these cherts are examined, but appear inadequate to account for all their features. It is postulated that decomposing organic material caused the precipitation of masses of silica gel on the Cretaceous sea-bed; that these dehydrated into nodules of hard but porous stone, which were then converted into true chert by redistribution of the silica of opaline sponge spicules. A complementary migration of calcareous material often produced doggers of limestone in the intervening sands. Slight variations of acidity, or possibly of temperature, within the sediment are though to control the process.