Organic Geochemistry of Cretaceous Black Shales from Deep Sea Drilling Project Site 530, Leg 75, Eastern South Atlantic

Abstract

Five-hundred ten meters of Cretaceous sediments were drilled north of the Walvis escarpment in Hole 53OA during Leg 75. An immature stage of evolution for organic matter can be assigned to all the samples studied. Black shales are interbedded with red and green claystone in the bottom sedimentary unit, Unit 8, which is of Coniacian to late Albian age. The richest organic carbon contents and petroleum potentials occur in the black shales. Detrital organic matter is present throughout the various members of a sequence, mixed with largely oxidized organic matter in the gray and green claystone or marlstone members on both sides. Detrital organic matter also characterizes the black streaks observed in the claystones. Vertical discontinuities in organic matter distribution are assigned to slumping. Several types of black shales can be identified, according to their content of detrital organic matter, the more detrital black levels corresponding to the Albian-Cenomanian period. Cyclic variations of organic matter observed for a sequence can occur for a set of sequences and even for some consecutive sets of sequences. Climatic factors are proposed to account for the cyclic sedimentation and distribution of organic matter for every sequence that includes a black bed. 10° N 0° r 10° S 40 30° 20° 10 W 0°F igure 1. Location map of Leg 75 and Sites 530, 531, and 532. 10° E 30°p urpose three steps were followed during the study, according to three different scales. The first scale is a sequence some 40-cm thick, centered on an individual black bed 8-cm thick, with green claystones on both sides. The second corresponds to a bundle of five consecutive black-shale beds with interbedded green claystones beds along an interval 105-cm thick. The third applies to 35 other black beds or sequences selected from among a total of 260 black-shale beds detected within the 163 m of lithologic Unit 8 (Fig. 3). METHODS Organic carbon was determined in acid-treated samples with a Leco apparatus, except for the closely spaced sampling of Sections 101-1 and 101-2 (Table 1) along which 79 samples were collected over a 40-cm interval. The small size of these samples required a specific analytical procedure to be used. Organic carbon was analyzed in carbonaceous residue issuing from Rock-Eval analysis. The method is based on combustion in a constant oxygen flow at a temperature programmed from 250 to 600°C at a linear rate of 25°C/min. The CO 2 was measured by a nondispersive infrared detector. Because of the small size of the samples (about 30 mg), significant LECO analyses could not be performed so as to compare the two methods. Since data of 0.20 wt.% and less are questionable as a rule, their use and meaning are doubtful. Pyrolysis assays were performed for all 194 samples with a Rock-Eval (Espitalié et al., 1977). Three intervals were then resampled in the black-shale bed in Core 101 to collect 5 to 6 g of rock per sample. The three samples obtained were chloroform extracted. Then the extracts were fractionated. A quantitative detection for s...

Astronomically Driven Variations in Depositional Environments in the South Atlantic During the Early Cretaceous

“…Although our results are specific to Site 364 and the margins of northern South Atlantic, periodic black shales of similar age (Early Cretaceous) have been reported elsewhere in the proto‐Atlantic, for example, at Site 530 in the Angola basin (e.g., Deroo et al, ; Katz, ; Meyers et al, ; Rullkötter et al, ; Stow & Dean, ); Site 511 in the Falkland Plateau (e.g., Deroo et al, ), and Site 367 in Gambia Abyssal Plain (e.g., Hofmann et al, ; Wagner et al, ). There are also numerous publications on the astronomically controlled formation of black shales in the Late Cretaceous, for example, from Site 959 in the Ivorian Basin (Beckmann et al, ; Holbourn et al, ), Leg 207 (Sites 1257–1261) at Demerara Rise (Hofmann & Wagner, ; Meyers et al, ), Site 530 in the Angola basin (e.g., Arthur et al, ; Deroo et al, ; A. Forster et al, ; Stow & Dean, ), the Western Interior Seaway (Eldrett et al, ), and ODP Site 1138 in the Indian Ocean (Dickson et al, ). These results suggest that the proposed depositional mechanism for Site 364 of a hydrographically restricted basin that was preconditioned to astronomically controlled variations in terrestrial runoff was probably a common feature of the proto‐Atlantic region during the Cretaceous.…”

“…This observation suggests that rhythmically paced changes in the depositional environment occurred in the basin during the Early Cretaceous. This cyclicity bears a strong resemblance to the astronomically paced formation of organic‐rich sapropels at Deep Ivorian Basin (ODP Site 959) during the Coniacian (Beckmann et al, ); at Demerara Rise (Leg 207) during the Cenomanian–Turonian (Flögel et al, ; Hofmann & Wagner, ; Meyers et al, ); at Site 530 during the Cenomanian–Turonian (Arthur et al, ; Deroo et al, ; A. Forster et al, ; Stow & Dean, ); and at Site 1138 during the Cenomanian–Turonian (Dickson et al, ).…”

Site 742

“…Generally, all of the samples from Site 742 have low hydrogen and oxygen indices with no preference for maturity near the origin. Samples with low T max and hydrogen and oxygen indices were probably an admixture of several kerogens deposited in an oxygenated environment (Deroo et al, 1984).…”

Site 741

“…One means of generating carbon with low HI, OI, and T max would be deposition in an oxygenated environment, along with the admixture of reworked carbon of high maturity. The reworked carbon would depress both HI and OI without significantly affecting the T max of the fresh, immature carbon (Deroo et al, 1984).…”

“…Values of Samples 119-741A-1R-1, 109-110 cm, and 119-741A-4R-2, 140-150 cm, which have only 0.15% and 0.17% TOC, respectively, are plotted within the type I/type II regime of the modified van Krevelen plot. Deroo et al (1984) estimated that Rock-Eval instrument function becomes erratic near these levels, so these points are possibly spurious.…”