Cyclic Sedimentation along the Continental Margin of Northwest Africa

Thin but discrete pelagic limestone beds intercalated among the black mudstones near the top of the extensive Mesozoic black shale sequence of the Falkland Plateau are reminiscent of similar occurrences in the central and North Atlantic and may be cyclic in nature. They have been studied via carbonate, organic carbon, stable isotope, nannofloral, and ultrastructural analysis in an attempt to determine their mode of origin. Nannofossil diversity and preservation suggest that selective dissolution or diagenesis did not produce the interbedded coccolith-rich and coccolith-poor layers, nor did blooms of opportunistic species play a role. Stable isotope measurements of carbonate do not adequately constrain the origin of the cyclicity; however, the δ 13 C data suggest that the more nannofossil-rich intervals may be due to higher nutrient supply and overturn of deeper waters at the site rather than influxes of well-oxygenated waters into an otherwise anoxic environment. Such an explanation is in accord with the nannofloral evidence.

Section: Introductionmentioning

confidence: 79%

Section: Previous Workmentioning

confidence: 99%

Section: Previous Workmentioning

confidence: 99%

See 1 more Smart Citation

Carbonate and Organic Carbon Cycles in AptianAlbian Black Shales at Deep Sea Drilling Project Site 511, Falkland Plateau

Parker¹,

Arthur

Wise³

et al. 1983

“…A similar compilation was made for the upper Berriasian Barron et al (1985), Arthur et al (1984), and Pratt (1982), Upper Cret., w. interior U.S. Barron et al (1985), unspecified ocean basins Arthur (1979), Upper Cret., Italy (Gubbio) Dean et al (1978), lower Tertiary, DSDP Site 366 Arthur et al (1984), unspecified ocean basins de Boer (1983), de Boer and Wonders (1984), mid-Cret., Italy Roth (1983a), mid-Cret., Pacific and Atlantic Einsele (1982), unspecified ocean basins Cotillon and Rio (1984), Lower Cret., Gulf of Mexico (DSDP Leg 77) Robertson and Bliefnick (1983) through Aptian sediments in Hole 534A, located in the Blake-Bahama Basin (Fig. 1).…”

Section: Pelagic Facies Subunits and Regional Correlationsmentioning

confidence: 99%

Lower Cretaceous Pelagic Sediments of Deep Sea Drilling Project Site 603, Western North Atlantic: A Synthesis

Ogg¹,

Haggerty²,

Sarti³

et al. 1987

Pelagic sedimentation during the Early Cretaceous at Site 603 produced alternations of laminated marly limestone and bioturbated limestone-a facies typical of the "Blake-Bahama Formation" of the western Atlantic. This limestone is a nannofossil micrite, rich in calcified radiolarians, with variable amounts of organic matter, pyritized radiolarian tests, fish debris, and micaceous silt. The laminated marly limestone layers are enriched in organic matter when compared with the intervals of bioturbated limestone. The organic carbon is predominantly terrestrial plant debris; where the organic-carbon content is in excess of 1%, there is also a significant marine-derived component. Laminations can result either from bands of alternately enriched and depleted opaque material and clay, or from bands of elongate lenses (microflasers) of micrite, which could be plastically deformed pellets or diagenetic features. The alternating intervals of laminated and bioturbated structures may have resulted from combined changes in surface productivity, in the influx of terrigenous organic matter, and in the intensity of bottom circulation, which led to episodic oxygen depletion in the bottom water and sediments.Variations in the relative proportions of laminated clay-rich and bioturbated lime-rich limestone and in the development of cycles between these structures make it possible to subdivide the Lower Cretaceous pelagic facies into several subunits which appear to be regional in extent. Bioturbated limestone is dominant in the Berriasian, laminated marly limestone in the Valanginian and Barremian-lower Aptian, and well-developed alternations between these end members in the Hauterivian. The Hauterivian to lower Aptian sediments contain abundant terrigenous clastic turbidites associated with a submarine fan complex. These changes in the general characteristics of the pelagic sediment component of the Blake-Bahama Formation at Site 603 are synchronous with those in the Blake-Bahama Basin (Sites 534 and 391) to the south. Carbonate sedimentation ended in the early Aptian, probably because of a regional shoaling of the carbonate compensation depth.

“…Dean et al (1978) and Gardner et al (1978) suggested that variations in rate of supply of organic detritus, much of which may be terrestrial (Habib, 1979;Simoneit, 1978a;Tissot et al, 1979), caused the cyclic interbeds of more-reduced and less-reduced strata ranging in age from Late Jurassic to Eocene along the continental margin of northwest Africa.…”

Section: Origin Of Organic-carbon-rich Stratamentioning

confidence: 99%

Origin of Organic-Carbon-Rich Mid-Cretaceous Limestones, Mid-Pacific Mountains and Southern Hess Rise

Dean¹,

Claypool²,

Thiede³

1981

Self Cite

Limestones of mid-Cretaceous age containing as much as 8.6 percent organic carbon were cored at one site (463) in the Mid-Pacific Mountains and at two sites (465, 466) on southern Hess Rise, central North Pacific Ocean, during Leg 62 of the Deep Sea Drilling Project. In the Mid-Pacific Mountains, three layers containing more than 2% organic carbon are associated with volcanic ash in silicified limestone of early Aptian age (-112-113 m.y. old), a time when the site was south of the equator and considerably shallower than at present (2525 m). The beds of laminated, organiccarbon-rich limestone, implying deposition in an oxygen-deficient environment, are interbedded with normal pelagic limestones. This sequence lies on interbedded pelagic limestones and clastic limestones containing locally derived shallow-water carbonate debris, and it is overlain by interbedded green, gray, and pink limestones. The organic-carbonrich strata on southern Hess Rise are dark-olive, laminated limestones with rare interbeds of altered volcanic ash of the late Aptian to early Cenomanian ( -98 to 103 m.y. old), a time when the site was passing under the equatorial highproductivity zone and subsiding from shallow to intermediate water depths. The organic-carbon-rich limestone sequence overlies volcanic basement (trachyte) in this part of Hess Rise. The association of volcanogenic sediments with organic-carbon-rich strata on Hess Rise is not as striking as in the Mid-Pacific Mountains, but the occurrences do suggest a coincidence of mid-plate volcanic activity and accumulation of organic matter at intermediate water depths in the tropical North Pacific Ocean during the mid-Cretaceous.Pyrolysis assays indicate that most of the organic matter in the limestone on Hess Rise is composed of lipid-rich kerogen derived from marine organisms. Limestones from the Mid-Pacific Mountains contain lower concentrations of organic matter. Pyrolysis assays and extractable hydrocarbons indicate that the organic matter in samples from Hole 463 is also predominantly of marine origin, and that contributions of terrigenous organic matter were probably minor. Analyses of stable carbon isotopes indicate that the organic carbon in all analyzed samples is isotopically light (δ 13 C -24 to -29‰) relative to most modern marine organic carbon (δ 13 C -9 to -30%), and that the lightest carbon (δ 13 C -28 to -29‰) is also the most lipid-rich.The organic-carbon-rich mid-Cretaceous limestones on Hess Rise, the Mid-Pacific Mountains, and other elevated plateaus and seamounts in the Pacific Ocean are approximately equivalent in age (mid-Cretaceous, -85-120 m.y.) to organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean and parts of the Indian Ocean. However, strata of equivalent age deposited in the Pacific Ocean at greater depths do not contain any carbonrich horizons. This observation, together with the evidence that the plateau sites were considerably shallower and close to the equator during the mid-Cretaceous, suggests t...