| Summary: | Large amounts of organic carbon were stored as black shales in pelagic sediments during the Lower and Middle Cretaceous, especially within the Tethyan and North Atlantic oceans and their marginal basins (Schlanger & Jenkyns, 1976; Fischer &Arthur, 1977; Ryan & Cita, 1977; Thiede & van Andel, 1977; Arthur, 1979; Jenkyns, 1980; Scholle &Arthur, 1980; Veizer, Rolser & Wilgus, 1980; Weissert, 1981). Black shales (syn. biopelite, sapropelite) are thinly laminated carbonaceous clayey and marly pelagic sediments without traces of burrowing and benthonic life, mostly rich in organic matter (some up to 20 %) and in certain trace elements. Formation of modern black shales occurs in quiet oxygen-depleted waters, such as stagnant basins (fjords, Black Sea), or below zones of high primary production (e.g. upwelling zones off Peru). In all cases, anoxity results from the fact that renewal of the oxygen stock is insufficient for the oxidation of the sinking dead organic matter. Although black shale deposits probably have been formed during every period of the Earth's history, they clearly have a greater incidence in distinct intervals. It has been argued that the amount of organic carbon (defined as non-carbonate-carbon) stored in pelagic sediments during the Cretaceous is larger than during any other time interval of that length (Ryan & Cita, 1977). Irving, North & Couillard (1974) state that about 60 %of the total proven oil reserves has been derived from source rocks with an age of 110 - 80 Ma b.p. Tissot (1979) estimates Middle Cretaceous source rocks to have supplied per time unit more than 5 times as much oil to present day reservoirs than the average over the whole Phanerozoic.
|
|---|