Cretaceous Shelf-Sea Chalk Deposits: A Global Synthesis

Donald E. Hattin

The word "chalk" is linked etymologically to the Cretaceous, but chalky facies neither dominate that system nor are confined to it. As used commonly, the term "chalk" refers to a variety of marine limestone that is white to light gray, very fine grained, soft and friable, porous, and composed predominantly of calcitic skeletal remains, especially those derived from coccolithophores. No simple definition suffices to embrace all Cretaceous chalks, which include sandy, marly, shelly, phosphatic, glauconitic, dolomitic, pyritic, and organic-rich lithotypes. Most of the world's exposed Cretaceous chalk deposits were formed at shelf depths rather than in the deep sea. Cretaceous shelf-sea chalks are developed most extensively in northern Europe, the U.S. Gulf Coastal Plain and estern Interior, and the Middle East, with lesser occurrences also in Australia. Most Cretaceous shelf-sea chalks formed in the temperate zones, and in relatively deep water. Cretaceous chalks deposited on well-oxygenated sea floors are bioturbated and massive where deficient in terrigenous detritus, or bioturbated and rhythmically interbedded with argillaceous units where influx of terrigenous detritus varied systematically with climatic changes. Cretaceous chalks deposited on poorly oxygenated sea floors tend to be laminated, organic-rich, and contain abundant fecal pellets. Cretaceous chalk sequences commonly commence abruptly above unconformities, suggesting association with rapid rises of sea level.

Accumulation of sufficient pelagic mud to form vast deposits of Cretaceous shelf-sea chalk required (1) sustained high productivity of calcareous plankton, (2) extensive development of stable shelf and continental platform environments, (3) highstands of sea level, (4) deficiency of aragonitic skeletal material in chalk-forming sediments, and (5) low rates of terrigenous detrital influx. These conditions were met at different times in different places, even within the same general region.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.