Showing papers by "Michael A. Arthur published in 1980"

Carbon Isotope Fluctuations in Cretaceous Pelagic Limestones: Potential Stratigraphic and Petroleum Exploration Tool

Abstract: Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major ^dgr13C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of stan ard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.

Initial tectonic deformation of hemipelagic sediment at the leading edge of the Japan convergent margin.

Abstract: Diatomaceous mudstones at depth under the deep sea terrace and the trench inner slope off Japan have been variably affected by tectonic stress. Veins, healed fractures, and microfaults occur at all sites except the shallow Site 435 on the upper trench inner slope and Site 436 on the Pacific Plate. Veins, fractures, and faults occur in cores from below 620 meters (lower to middle Miocene) in the landward sites (438 and 439) on the deep sea terrace, and are probably related to normal faulting seen in seismic records. The depth to "consolidated" sediment and to the first occurrence of veins and healed fractures shallows progressively toward the trench. The intensity of deformation also appears to increase seaward. However, no sediments younger than upper Pliocene are deformed. Open fractures may exist in situ at Sites 434 and 441 at levels between about 150 and 500 meters sub-bottom. The Japan Transect sediments—in contrast to deposits in the zone of initial deformation at other convergent margins—though highly deformed, are not highly overconsolidated. However, sediment at depth in the trench inner slope sites is overconsolidated relative to that at the same depth in the landward reference site. None of the deformed Japan margin sediments recovered at Legs 56 and 57 sites originated by accretion of oceanic plate material—also in contrast to sediments at some of the margins previously studied. We suggest that tectonic stress related to convergence has been communicated to the slope sediments on the trench inner slope, either continuously or periodically, causing rapid tectonic dewatering and inducing fracturing and faulting. If episodic, the latest of these deformational periods may have occurred during the late Pliocene. The faults and fractures are either rehealed by continued overburden pressure (sediment loading) or may remain open at shallower levels. Fracturing and dewatering of semiconsolidated sediment beneath an unconsolidated but impermeable mud veneer may cause overpressured zones at depths of 200 to 500 meters. These overpressured zones possibly locally reduce shear strength and cause downslope mass movement of sediment, even on low-angle slopes on the trench inner slope.

Lower Cretaceous Lacustrine Source Beds from Early Rifting Phases of South Atlantic: ABSTRACT

Abstract: Significant amounts of oil are produced from Lower Cretaceous pre-evaporite, nonmarine sequences of these west African marginal basins. Gabon, Cabinda, Congo Brazzaville, and Angola. Organic-rich lacustrine source beds attain thicknesses up to 900 m. Their geometry and sediment similarities among several basins indicate a small number of large Early Cretaceous lakes extending along the South Atlantic rift with dimensions and conditions similar to Lake Tanganyika. The organic-rich facies is underlain by a sandy facies deposited during initial rifting. The lacustrine phases deposited green clays and fluviolacustrine-deltaic sands, which were abruptly terminated by marine incursions in the Aptian. Our study of conventional whole cores from the Melania Formation of Gabon provides further evidence that these source beds were deposited in brackish to freshwater environments in a deep lake. Logs show characteristic low bulk densities and high resistivity which allow correlations over 80 km. Large-scale cyclic preservation of organic matter in a stable, low energy environment with anoxic bottom conditions is interpreted. There is little clay in the organic-rich "shales" which typically comprise finely laminated, carbonaceous and dolomite-rich rhythmic couplets. Bulk organic carbon concentrations up to 20% are not uncommon. Palynomorphs, ostracods, and algae also indicate temperate conditions around a low salinity environment. The slope sequences are richest in uniform laminates with /P> End_Page 680------------------------------ some intercalated thin turbidites and enterolithic slump folds, whereas coarse turbidites were funneled into the deep basin plain. These hydrocarbon-saturated turbidite sandstones are derived from fluvial systems draining crystalline hinterland. End_of_Article - Last_Page 681------------

Oxic-Anoxic and Carbonate Cycles in Cretaceous Organic Carbon-Rich Marine Strata: ABSTRACT

Abstract: Sedimentary sequences of Early to middle Cretaceous End_Page 671------------------------------ age that are rich in organic carbon (C-org) are common in the deep seas and in shallow interior and shelf basins of the world. High concentrations of C-org and the presence of fine laminations suggest that these strata accumulated in environments that were anoxic or nearly anoxic at and possibly above the seafloor. Many organic-rich sequences display a marked cyclicity in amount of C-org and, if deposition was near or above the carbonate compensation depth, in CaCO3 as well. These cyclic C-org and CaCO3 variations result in interbedded lighter and darker shale, marlstone, and limestone that have cycle periods ranging from 20,000 to 100,000 years. Variations in degree of bioturbation parallel changes in C-org and CaCO3 because burrowing organisms were m st active at times of deposition of relatively low C-org and high CaCO3 sediments. Such cycles are best developed in sediments deposited during episodes of lower dissolved oxygen concentration (so-called "oceanic anoxic events"), but similar cycles of C-org and CaCO3 variation having periods of tens of thousands of years occur in strata of almost every geologic age. The periodicities are similar to those in pelagic sediments of Pleistocene age that are known to be climatically induced. Studies of geochemistry, stable isotopes, and mineralogy show that older organic cycles are also related to climatic changes that produced fluctuations in the amount of wind- and river-borne clastic sediment and terrigenous organic matter delivered to the oceans, and to coincident variations in surface-water salinity, productivity, and mid- to deep-water oxygen concentratio . Regional paleogeography and paleodepth determined the amount and type of organic matter preserved within the sediments. End_of_Article - Last_Page 672------------