chalk

chalk In geology the word ‘chalk’ is used in two senses: for a type of limestone; and (with a capital C) for a stratigraphical formation of Upper Cretaceous age that is notably exposed in the white cliffs of south-east England. (Blackboard ‘chalk’ is nowadays not chalk at all: it is in fact gypsum, calcium sulphate.) Chalk in the first of these senses is a very pure, white, fine-grained silty carbonate mudstone. It is a rock-type that is particularly well developed in the upper Cretaceous of north- western Europe and the western USA, but is also found elsewhere.

Maurice Black of Cambridge showed that chalk consists mainly of two sizes of particles, 0.5–4 micrometres (µm), and 10–100 µm. The finer size consists mainly of tablet-shaped plates of calcite, which are the remains of marine algae. In life, the alga consisted of a sphere (coccosphere) of overlapping rings (coccoliths) or rings with spines (rhabdoliths) of small plates. Although complete coccoliths are sometimes preserved, they have usually disintegrated into their constituent plates. The coarser fraction of chalk consists mainly of the skeletal debris of foraminifera, calcispheres, bivalve fragments, echinoid plates, and bryozoan, ostracod, and coral debris. Chalk contains a benthonic macrofauna of sponges, brachiopods, molluscs, and echinoids, as well as necktonic ammonites and belemnites.

Chalk is composed dominantly of calcite and was deposited originally as this mineral. It thus underwent much less change during diagenesis than limestones originally composed of aragonite and high-magnesium calcite. Chalk has subsequently retained much of its original porosity and typically has porosities of 35 to 47 per cent; even at depths of 1500–2000 m porosities are still 2–25 per cent. Chalk has a characteristically low permeability (6–12 millidarcies) owing to the very small intraparticle pores. It sometimes contains fine-grained clays which are important in the Lower Chalk, where marls or alternations of marl and chalk are common.

Chalk is well stratified, although its homogeneity makes this less obvious than in other rocks. It is more visible when there are beds rich in clays, shell layers, or flints nodules. In some instances chalk has accumulated in biohermal mounds (metres high and kilometres long) and shows evidence of contemporary slumping and deposition by dense gravity flows. It is commonly intensely bioturbated (disturbed by organisms) and has complex burrow patterns mainly infilled by precipitated silica (flint). Prominent well-cemented layers, termed hardgrounds, occur in the otherwise weakly cemented rock; these are named Chalk Rock, Top Rock, etc. They are the result of early cementation on the sea floor and are analogous to cemented crusts on the floor of modern carbonate seas. They mark diastems or pauses in sedimentation when the sea-floor sediment was cemented, bored, encrusted with epifauna, and commonly coated with phosphate and glauconite. Courses of flint nodules, parallel to the bedding, developed around original concentrations of siliceous remains or organic matter or were formed by precipitation in burrows.

The Chalk was deposited in a marine environment at water depths between 100 and 600 m, according to the evidence of the faunas and their state of preservation. Modern analogues are rare, although coccolith muds are known to be forming on the shelves of Yucatán. Sea-level was high in the late Cretaceous, and the relatively small land areas supplied little siliciclastic detritus. Because of the scarcity of marginal deposits there is little definite proof of the actual climate during deposition.

In spite of its low permeability chalk is a good aquifer, for water moves through an intense network of joints; and it sometimes acts as a petroleum reservoir. The Chalk is, in its lower part, extensively quarried for cement.

G. Evans

Bibliography

Hancock, J. M. (1975) The petrology of the Chalk. Proceedings of the Geologists' Association. 86, (4), 499–536.