Sedimentological, organic petrographic and organic chemical characteristics of Albian and Cenomanian/Turonian black shales from the North Altlantic (Tab. 3, 4)

Large parts of the Cretaceous Atlantic Ocean are characterized by the wide-spread deposition of organic-carbon-rich sediments ("black shales“), with a maximum occurrence during Aptian/Albian and Cenomanian/Turonian times. In order to explain the origin of these black shales, changes in the amount and type of organic matter in relation to changes in paleogeography, paleoceanic circulation, and paleoclimate have been investigated in detail. The study is mainly based on calculation of accumulation rates of marine and terrigenous organic carbon, organic carbon/sedimentation rate relationships, and estimates of paleoproductivity. Because both the amount and type of organic matter in the black shales change markedly with location and age, different mechanisms must have caused the accumulation of these organic-carbon-rich sediments: (1) Terrigenous organic matter supply was the dominant factor for black shale formation during Hauterivian and Albian times in the Northwest Atlantic.(2) Increased oceanic productivity was important for black shale deposition in the Northeast Atlantic during Albian times.(3) Increased preservation of (marine) organic matter in anoxic environments due to extremely reduced deep-water circulation was probably the main cause for the formation of Cenomanian/Turonian black shales, rather than increased supply of organic matter due to higher oceanic productivity.(4) Locally, rapid burial by turbidites may have been an important mechanism for increased preservation of organic matter.