CRETACEOUS ANOXIC EVENTS IN THE SOUTH ATLANTIC

Abstract

To characterize the anoxic event s in Cretaceous series in the South Atlantic, detailed reinvestigations of biostratlgraphy, organic geochemistry, and lithology were necessary. More than 2,000 samples from different DSDP sites were analyzed (Angola Basin: 364; Walvis Ridge: 363 , 530; Rio Grande Rise: 356, 357; Cape Basin: 36 1; Maurice Ewing Ridge: 327, 330 , 511 ). The chronostratlgraphy was established elsewhere, and geochemical analyses were made by the Institut Francais du Petrole (to tal organic content, nature of organic matter, petroleum potential, maturity). The South Atlantic was affected by several events having global significance. Organic rich sediments were deposited on the Maurice Ewing Ridge as well as in the Cape Basin up to Early Albian and extended north of Walvis Ridge up to the Middle Albian. This period of anoxia also spread farther in the Magallanes Basin (Springhill formation of Hauterivian to Aptian age). Then oxidizing conditions were established from the Middle Albian and during Late Albian. This event, caused by a paleoceanographic change, could be equivalent to event E 1 in th e North Atlantic (transition from Black Bahamas to Hatteras formations). Around the Cenomanian-Turonian boundary the oxic environment was interrupted by anoxic conditions which developed especially north of the Walvis Ridge and Rio Grande Rise. The Cenomanian-Turonian boundary event (CTBE), well known in the North Atlantic and the Tethys (event E 2), has a widespread distribution and is characterized by good preservation of marine organic matter, especially north of the Equatorial Fractures zone (Cape Verde Basin : Site 367). The presence of anoxia at Site 356 (Rio Grande Rise) shows that the CTBE occurred on the western as well as on the eastern margins of the South Atlantic. During Senonian times the environment again became oxic. Only minor organic matter-rich sediments were found in the Coniacian. The first anoxic period was probably a consequence of a restricted environment at the beginning of the opening of the South Atlantic. After an oxic period, the second anoxic event (CTBE) involved processes on a larger scale going beyond the framework of the only South Atlantic.