No abstract

Organic matter contents of black shales from the Cretaceous Hatteras and Blake-Bahama formations have been compared to those from surrounding organic-poor strata using C/N ratios, δ 13 C values, and distributions of extractable and nonsolvent-extractable, long-chain hydrocarbons, acids, and alcohols. The proportion of marine and land-derived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbon-poor relative to their organic-carbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this outer continental rise location under generally oxygenated bottom-water conditions.

Palaeozoic strata in the Michigan Basin produce crude oils which, despite their rather small volume and economic value, are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting.Based on n-alkane profiles, biological marker distributions and carbon isotope ratios, the oils belong to three main families of different genetic origin and a few less important mixed types. Silurian oils from Salina and Niagara Limestones have broad n-alkane distributions and abundant isoprenoid hydrocarbons. A strong phytane-over-pristane predominance and the lack of diasteranes indicate a carbonate source for these oils. Chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n-alkanes.Oils found in the calcareous Ordovician Trenton formation contain n-alkane, cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils. Many oils from the Devonian Dundee reservoirs are very similar in overall composition to the Trenton oils.Devonian Traverse oils are considered to be mainly from a Devonian source with some contribution of Ordovician-type oil and are of intermediate maturity. Based on calculations using kinetic parameters of biological marker reactions, considerably deeper subsidence of the Devonian source rock in the past is implied.

Organic matter has been characterized in samples of Pleistocene, Pliocene, and Miocene sediments from seven Deep Sea Drilling Project sites in the subtropical South Atlantic Ocean. Organic carbon concentrations average 0.3% for most samples, and n-alkanoic acid, n-alkanol, and alkane biomarkers indicate extensive microbial reworking of organic matter in these organic-carbon-lean sediments. Samples from the easternmost parts of the South Atlantic contain an average of 4.1% organic carbon and reflect the high productivity associated with the Benguela Current. Lipid biomarkers show less microbial reworking in these sediments. Eolian transport of land-derived hydrocarbons is evident at most of these oceanic locations.

Drilling at ODP Site 641 (on the western margin of Galicia Bank, off northwestern Spain) revealed a thin, but pro nounced, interval of black shale and gray-green claystone. Our high-resolution study combines the sedimentology, micropaleontology (palynomorphs and others), organic and inorganic geochemistry, and isotopic values of this layer to demonstrate the distinct nature of the sediment and prove that the sequence represents the local sedimentary expression of the global Cenomanian/Turonian Oceanic Anoxic Event (OAE) of Schlanger and Jenkyns (1976), Jenkyns (1980), also called the Cenomanian/Turonian Boundary Event (CTBE).The most striking evidence is that the strong positive 5 13 C excursion characterizing the CTBE sequences in shallow areas can be traced into a pronounced deep-sea expression, thus providing a good stratigraphic marker for the CTBE in various paleosettings. The isotopic excursion at Site 641 coincides with an extremely enriched trace metal content, with values that were previously unknown for the Cretaceous Atlantic. Similar to other CTBE occurrences, the organic car bon content is high (up to 11%) and the organic matter is of dominantly marine origin (kerogen type II).The bulk mineralogy of the CTBE sediments does not differ significantly from the general trend of Cretaceous North Atlantic sediments (dominance of smectite and zeolite with minor amounts of illite and scattered palygorskite, kaolinite, and chlorite); thus, no evidence for either increased volcanic activity nor a drastic climatic change in the bor derlands was found.Results from Site 641 are compared with the CTBE section found at Site 398, DSDP Leg 47B (Vigo Seamount at the southern end of the Galicia Bank).