Pelagic Cretaceous sediments, deposited in a range of palaeotectonic and palaeogeographic settings, from continents to oceans, are commonly black and bituminous. 3 particular time-envelopes define the major occurrences of such facies: late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and, to a lesser extent, the Coniacian-Santonian. These intervals define the duration of so-called Oceanic Anoxic Events during which global marine waters were relatively depleted in oxygen, and deposition of organic matter, derived from both terrestrial and planktonic sources, was widespread. Cretaceous OAEs correlate closely with transgressions, and such a correlation exists throughout the stratigraphical column. Flooding of land-masses is thought to have transported much terrestrial plant material seawards; the progressive increase in shelf-sea area is thought to have stimulated production of marine plankton. Bacterial consumption of this organic matter favoured the development of poorly oxygenated mid-to late Cretaceous waters in which many of the characteristic facies of the Period, including glauconitic sandstones and phosphatic chalks, were deposited.

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