Title: Stratigraphy and sedimentation in the anoxic Bannock Basin (Eastern Mediterranean)
Abstract: Ten gravity and/or piston cores, raised from the floor and the eastern flank of the Bannock Basin and from nearby plateaus, were investigated lithologically and biostratigraphically. The Bannock Basin is a large, rimmed depression with a vertical relief of over 600 m containing deep-seated brines. The brines are derived from submarine dissolution of Messinian evaporites in a physiographic setting (deep, rimmed basin) that does not permit dilution and removal by bottom currents. Plateau cores range in age from Holocene (Emiliania huxleyi Acme-zone) to Middle Pleistocene (Pseudoemiliania lacunosa Zone); they consist of low-sedimentation rate pelagic sediments, with sapropels and tephras as minor isochronous lithologies. Two cores on the flanks recovered Pliocene pelagic sediments. One of these cores displays an unconformable contact between Holocene anoxic sediments and early Pliocene (Reticulofenestra pseudoumbilica Zone) marls. The other core is entirely Pliocene, consisting of a debris flow with a matrix that is referred to the Discoaster tamalis Subzone. Cores from the deeper part of the basin consist of laminated, non-bioturbated, poorly oxygenated dark sediments rich in H2S and containing large gypsum crystals. Organic carbon content is higher than in normal Eastern Mediterranean pelagic sediments, but lower than in true sapropels; the kerogen is dominated by amorphous organic matter. Although the oxygen content of the bottom waters was not measured, sedimentary evidence suggests that poorly oxygenated but not entirely anoxic conditions exist in the basin at the sediment/water interface. In particular, benthic foraminifers would be much less abundant than they actually are and org. C content would be higher, if bottom waters were anoxic. Poor oxygenation started to develop in the Bannock Basin some 180 ka B.P., after the deposition of sapropel S-6 as documented in Core 84-02. This long-lasting period of "anoxia" strongly differs from the short anoxic episodes in the Eastern Mediterranean that were controlled by glacial-interglacial changes. The Bannock Basin anoxia is a result of brine formation causing strong density stratification, with a density contrast (20%) that is much higher than the density contrast derived from changes in thermohaline circulation during glacial-interglacial times. During these Eastern Mediterranean basin-wide euxinic episodes the boundary between oxygenated and anoxic waters was much shallower than the −3200 m interface recorded in the Bannock Basin. Reworked and displaced foraminiferal faunas were found in the Pliocene sediments recovered in the flank cores. In particular the occurrence of abraded tests of the shallow-water form Elphidium spp., of Bulimina costata and of other displaced taxa does suggest that the area was subject to deposition by turbidity currents originating on the African margin, and that in Pliocene times no rimmed basin was in existence.
Publication Year: 1987
Publication Date: 1987-04-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 44
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