The mineralogy of palaeogene sediments in Northeast Kent (Great Britain)

Abstract

The Palaeogene deposits of northeast Kent are approximately 700 ft. thick and consist of unconsolidated clays and loamy sands, which are mainly marine in origin. The mineralogical composition of fine sand (50–250 μ), silt (2–5 μ, 5–20 μ and 20–50 μ) and clay (< 2 μ) fractions from samples representing the main subdivisions of the succession is described, and possible sources of the detritus and mode of origin of some non-detrital minerals are discussed. The detritus was derived ultimately from three main sources: (a) the Chalk; (b) metamorphic rocks (containing staurolite and kyanite) and granites, possibly of Armorican massifs; ( c) metamorphic rocks containing garnet, epidote and amphiboles. Clay fractions in most of the deposits are composed of montmorillonite with subsidiary mica, but kaolinite occurs in beds containing abundant detritus from possible Armorican sources. The main non-detrital minerals are glauconite, jarosite, pyrites, low-temperature tridymite and clinoptilolite, and detrital clay micas in the sandy sediments have authigenic growths of a layer silicate mineral. The main clay-rich subdivision of the succession (the London Clay) is weathered to great depths; mineralogical effects of this weathering include oxidation of pyrites, alteration of detrital biotite and formation of selenite and jarosite.