37. ORGANIC GEOCHEMISTRY OF UPPER JURASSIC-CRETACEOUS SEDIMENTS

Cretaceous and Jurassic sediments 435 m thick were drilled at Site 511, in the basin province of the Falkland Plateau, during DSDP Leg 71. The calcareous Unit 3 and the clayey zeolitic Unit 4, both of Senonian age, revealed poorly preserved organic matter indicative of oxidized environments. The same characteristics prevailed for the clayey Unit 5 of Turonian to Albian age. Strictly reducing environments existed for black facies along Unit 6 of earliest Albian to Late Jurassic age and allowed the preservation of a rich organic material that is marine in origin. Besides the transition from reducing conditions in Unit 6 to oxidizing conditions in Unit 5, there are 20 meters of sediments in Cores 56-58 where detrital, nonmarine and then marine organic matter, both implying more or less reducing environments, are interlain by poorly preserved ma-terial. In the black shales of the bottom Cores 69 and 70, some nonmarine detritus is mixed with the predominantly ma-rine organic material. An immature stage of evolution can be assigned to all of the samples studied. The chapter also undertakes a comparison with contemporaneous lithologies at adjacent Sites 327 and 330 and at-tempts some reconstruction of the geography of the eastern Falkland Plateau during the Mesozoic