46 SEDIMENTARY ROCKS/Chalk Figure Upper Maastrichtian Danian chalks exposed at Stevns Klint, eastern Denmark The lighter coloured, lower third of the cliff (up to the prominent overhang) is the uppermost Maas trichtian The K/T boundary (arrow) is gently undulating and the boundary clay layer is only preserved in the depressions The Danian bryozoan rich succession above shows well developed mounds Height of cliff ca 40 m Photo: F Surlyk structures such as isoclinal folds and stratigraphically inverted successions Chalk debrites, comprising chalk pebbles or slabs supported in a fine-grained chalk matrix, form a significant part of the Maastrichtian– Danian succession in the North Sea Central Graben Most resedimented chalk clasts are plastically deformed, implying that they were poorly lithified at the time of deposition However, the presence of angular clasts in some debrites indicates that some of the material originated from lithified chalk, either from penecontemporaneous firmgrounds/hardgrounds or from exhumed more deeply buried chalks, for example at fault scarps Sand-grade ‘classical’ turbidites are uncommon in the chalk, most likely due to the scarcity of sand- and silt-sized material, although dilute low-density turbidity currents were important in the redistribution of mud-grade sediment The Chalk Sea In the Late Cretaceous, pelagic carbonate oozes extended far onto the European craton and formed the dominant facies for tens of millions of years This was the result of a unique coincidence of global and regional factors The chalk sedimentary record attests both to such long-term controlling factors as eustatic sea-level and regional tectonics and to the influence of short-term climatic variation controlled by orbital forcing mechanisms Palaeogeography Figure Photograph of slabbed core (Maastrichtian, Denmark) in which the ichnofabric is enhanced by oil staining Note the complex cross cutting relationships recording the overprinting of successive tiers (see Figure 6) Cl, large Chondrites; Cs, small Chondrites; T, Thalassinoides; Z, Zoophycos Photo courtesy of RG Bromley The chalk sea of north-west Europe existed for more than 35 Ma, from the Cenomanian to the Danian, at a time when global sea-level was at its highest during the Phanerozoic and relative tectonic stability prevailed in the region Much of the north-west European craton was flooded to depths in excess of 50 m Hinterland relief was low and potential source areas were restricted in extent so siliciclastic supply was limited and a pelagic carbonate drape accumulated, extending from a palaeolatitude of 35 N northwards to 50 N where the carbonates passed into siliciclastic muds (Figure 2) The biogenic components largely belonged to the heterozoan association that today characterizes cool-water, temperate carbonate systems; typical Cretaceous tropical organisms, such as reef corals, large foraminifers and rudist bivalves are absent or rare in the chalk of north-west Europe However, direct latitudinal comparison with presentday seas are invalid since the Cretaceous was one of the ‘greenhouse’ phases of Earth history when equable temperatures extended further poleward than in our present ‘icehouse’ situation The chalk sea is thus probably best characterized as ranging from warm temperate to sub-tropical, despite its mid-latitude