Organic Facies of Middle Cretaceous Black Shales in Deep North Atlantic

Abstract

The middle Cretaceous black shales of the deep North Atlantic consist of alternating layers of sediment rich in total organic carbon (TOC) that contains abundant amorphous types of organic matter and sediment poor in TOC that contains mostly woody and coaly types of organic matter. The amorphous materials are derived mostly from marine organisms in the eastern North Atlantic off Africa, in the Caribbean, and on the Demerara Rise off Surinam and from higher land plants in the western North Atlantic and off Europe. Marine amorphous material is important off Africa because this was an area of upwelling and highly productive surface water. Amorphous marine organic matter was deposited across the entire North Atlantic Basin in the Cenomanian in response to an ocean-wide upwell ng event connected with the opening of the equatorial Atlantic. The difference between the TOC-rich and background TOC layers reflects deposition of the former under reducing conditions, which allowed the preservation of labile amorphous materials of marine and terrestrial orgin, and of the latter under oxidizing conditions, when nearly all that was preserved was refractory organic material.