Organic Geochemistry of Lower Cretaceous Sediments from Site 535, Leg 77, Florida Straits

At Site 535, the four lithologic units of Cretaceous age are controlled by two types of sedimentologic facies: (1) the massive light-colored limestones or marly limestones in which the total organic carbon (TOC) content is low and the organic matter more or less oxidized and (2) laminated dark facies in which the TOC content is higher and associated with a well-preserved organic matter of Type II origin. Very little typical Type III organic matter occurs in the whole series from late Berriasian to Aptian and Cenomanian. Fluctuations from oxidizing to reducing environments of deposition are proposed to account for the variations in properties of the Type II organic matter between the different facies. Dark laminated layers are good but immature potential source rocks: petroleum potential is often higher than 2 kg HC/t of rock. INTRODUCTION Site 535, drilled during Leg 77 and located in the Straits of Florida at a water depth of 3455 m (Fig. 1), allows the study of the Lower Cretaceous formations in the eastern Gulf of Mexico. Five lithologic units were described on board (Fig. 2) with few postcruise modifications in their age. Because the sediments are mainly calcareous, with generally more than 50% and sometimes up to 99% CaCO3, the lithologic basis for the division of Cretaceous into Units II, III, IV, and V are relatively subtle. The main characteristics of the lithology consist of rhythmic alternations of carbonates: light bioturbated limestones, dark and more or less laminated limestones, and dark marly limestones, which were interpreted on board as ". fluctuations of the anaerobic bottom conditions coupled with plankton productivity. " (see site chapter, Site 535, this volume). During the Berriasian to Cenomanian, the Blake-Bahama and the Hatteras formations were deposited in the North Atlantic Ocean. Many similarities exist between the alternations of the Blake Bahama Formation as they were defined at Sites 105 and 391 and those of the Units III, IV, and V at Site 535. The lithologic change between a calcareous set and a noncalcareous one, which in central North Atlantic makes the difference between the Blake-Bahama and the Hatteras formations, doesn't exist at Site 535. This dissimilarity could be partly due to a relatively shallow paleobathymetry above the calcite compensation depth (CCD) at Site 535 (presently Site 535 is 3455 m deep) compared to the depths at which the Blake-Bahama and the other formations were deposited: Sites 105, 367, and 391 (where the present bathymetry is approximately 4500-5000 m). The geological "El Event" (de Graciansky et al., 1982), which in the central North Atlantic corresponds to the change from Blake-Bahama Formation to Hat1 Buffler, R. T. , Schlager, W. , et al., Init. Repts. DSDP, 77: Washington (U.S. Govt. Printing Office). teras Formation, is not indicated at Site 535 by a variation of lithofacies. However, near the top of Unit III, a great condensation (perhaps a hiatus) occurs. Only four of eight Aptian foraminiferal zones were recovered, including two of late Aptian and two of early Aptian age, but none from the middle Aptian; most if not all of the Barremian is missing (see Site 535 report, this volume). SAMPLING AND METHODS More than 120 samples were collected, 71 of them representative of the Cretaceous lithofacies in Cores 18 to 75 and 55 others in a detailed sequence along a single core (Sections 535-64-3 to 535-64-5). Organic carbon was determined in acid-treated samples with a LECO apparatus, after which Rock-Eval pyrolysis assays were performed for all 126 samples (Espitaliéetal., 1977). Espitalié and others (1980) showed that the mineral matrix of rocks, especially clays, induced retention of hydrocarbons during pyrolysis. Accordingly, the hydrogen index (HI) of a rock is lower than the HI obtained for the demineralized fraction (kerogen) of the same rock. In order to estimate the petroleum potential of organic matter, we must know the quantity of hydrocarbons held on the mineral matrix during the pyrolysis of the studied rock. The activity of the mineral matrix is determined from mixing the carbonate-free sample (previously pyrolyzed under O2) and the standard rock. The comparison between the quantity of hydrocarbons released during a Rock-Eval pyrolysis from this mixture with those of the standard rock alone allows determination of the activity of the mineral matrix of the studied rock (Espitalié et al., in press). Thus we obtain the "corrected hydrogen index" (HI'), which is more representative of the organic matter than the uncorrected index (HI). Such a correction was carried out for samples from Site 535 whenever there was ambiguity about the type of organic matter. Elemental analysis of the kerogen was done for 12 samples, and 20 samples were extracted with chloro-