BLACK SHALES IN THE NORTHERN EUROPEAN CHALK

During the Late Cretaceous, a sedimentary environment that favored the deposition of chalk was prevalent over vast areas of the earth’s surface. Chalk is an extremely pure coccolith limestone with dark chert nodules. The extent of Late Cretaceous landmasses in northwest Europe is conjectural, but as sea level rose, land area decreased, clastic material was trapped in nearshore environments, and pelagic calcareous facies became widespread on continental shelves. The same environment favored the deposition of another unusual lithology, namely organic matter-rich mud, during oceanic anoxic event 2 (OAE 2) in the late Cenomanian–early Turonian. We carried out a multiproxy study of the OAE 2 interval in North Lincolnshire and in the Danish Central Graben. At both these localities, the expression of OAE 2 is a c. 1–2 m-thick, dark, discrete horizon of black shale, which in northern England is known as the ‘Black Band,’ embedded in an organic matter-lean chalk succession. The Black Band is very similar to the Bonarelli Level, which is the expression of OAE 2 in the more oceanic Tethyan realm, both in its lithological characteristics and in its stratigraphic context. In environments characterized by a higher terrigenous to biogenous ratio, e.g. the Western Interior, Vocontian, and Lower Saxony Basins, OAE 2 is typically expressed as a relatively thick alternation of marls/shales and limestones. This suggests that the development of thin, discrete black shale horizons has some relation to the conditions that favor the deposition of chalk. Our data show that the paleogeographic and paleoöceanographic conditions that favor the deposition of chalk, i.e. low siliciclastic input and oligotrophic conditions, are the same that favor the deposition of organic matter-rich mud. We propose that a small increase in the amount of organic matter that reached the sediment/water interface during OAE 2 led to more organic matter being remineralized in the sediment which, in turn, resulted in the shoaling of the redoxcline and in the dissolution of carbonate. The opposing chemical requirements for the preservation of carbonate and organic matter generate discrete black shale horizons in chalk successions without the need for significant changes in environmental conditions.