Sedimentary rocks: Chalk

Chalk is a familiar rock type, particularly amongst Europeans, forming spectacular white cliffs along coastlines flanking the North Sea, the English Channel and the Baltic Sea. The essential characteristic of a true chalk is its microscopic composition – being composed predominantly of the skeletal remains of tiny calcareous marine algae known as coccolithophorids (Figure 1). Following their appearance in the Jurassic, these haptophycean algae became a common constituent of marine sediments and remain important components of the marine ecosystem today. Only during specific periods of Earth history and in certain palaeogeographic areas, however, were conditions such that pure carbonate oozes accumulated and were preserved on continental shelves and in vast epeiric seas. Chalk is thus most characteristic of the Upper Cretaceous (and in places the Danian) of north-west Europe and North America. This review focuses on these typical chalks, particularly from north-west Europe (Figure 2).