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Abstract

Pelagic carbonate oozes have been major sediment of continental slopes and the abyssal plain above calcite compensation depth since at least Jurassic times. The deposit owes its properties to the domination of nannofossil and microfossil calcite skeletons and a paucity of terrigenous material. In shallower, shelf settings, normally the pelagic constituents are diluted by land-derived muds and coarser materials and the purer carbonate oozes are restricted to local importance.

During the late Cretaceous, however, world wide transgression drowned large areas of shelf and land under epicontinental seas and extensive pelagic oozes were deposited under unusually shallower conditions to produce the well known on-shore occurrences of Cretaceous chalk. It is these rocks that form the basis of the present chapter. Their importance as reservoir rocks in several areas, not least in the North Sea, has led to the intensive investigation of the properties of chalks in recent years (e.g., Scholle, 1977a; 1977b); detailed ichnologic studies of chalks, however, have rather lagged behind.

We define “chalk” in more or less the same terms as in general use in the Deep Sea Drilling Project reports (Gealy, Winterer and Moberly, 1971, p. 17; Benson, Sheridan et al., 1978, p. 14), as a firm, partly indurated calcareous ooze or friable limestone. Its fully lithified equivalent has been termed “chalkstone” (Bromley and Gale, 1982; Scholle, Arthur and Ekdale, 1983, p. 623), in harmony with the nomenclature of terrigenous rocks (clay, claystone; silt, siltstone; etc.).

At first sight, chalks represent a monotonous lithology comprising firm, pale-colored micrites.

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