Lithology of the Chalk Group - Flint

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Flint is composed of silica, in the form of ultramicroscopic quartz crystals, derived from the dissolved skeletons of siliceous sponges and micro-fossils (radiolarians and diatoms) that inhabited the Chalk sea. The complex chemical process of flint formation occurred at some distance below the sea bed whilst the chalk was still being deposited, often as replacements of burrow systems formed by soft-bodied organisms (Clayton, 1986). Such flints typically have irregular nodular or elongate forms. Some flints formed around vertically elongated burrows extending for up to a couple of metres through the sediment, and the resulting columnar flint is known as a paramoudra after the trace fossil, Bathichnus paramoudrae, that determined its morphology. Laterally continuous tabular flints are typical of homogeneous, well-bedded sediments in which burrows are either absent or poorly defined; lack of these preferred sites for silica replacement promoted the formation of more evenly developed flint horizons (Clayton, 1986). Some flint bands have a distinctive appearance and are geographically extensive, making them valuable for correlation. Locally, thin sheet-like flints, with a distinctive hollow centre, are found cross-cutting the stratigraphy. These flints are inferred to have grown along fractures or shear-planes in semi-consolidated chalk.

References[edit]

CLAYTON, C. J.1986. The chemical environment of flint formation in Upper Cretaceous chalks. In SIEVEKING, G. de G. & HART, M. B. (Eds). The scientific study of flint and chert: proceedings of the fourth international flint symposium held at Brighton Polytechnic 10-15 April, 1983. Cambridge. Cambridge University Press.