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Albian oceanic anoxic events in northern Tunisia: Biostratigraphic and geochemical insights 总被引：1，自引：0，他引：1
Albian pelagic successions of the Nebeur area in northwestern Tunisia consist of radiolarian-bearing and organic-rich black shale beds, which represent the lower part of the Fahdene Formation. The carbonate content of the organic-rich beds ranges between 40 and 48%. Total organic carbon (TOC) analyses via Rock Eval pyrolysis yielded values ranging between 0.7 and 2.8% and a mixed marine/terrestrial origin. Tmax values vary between 424 and 450 °C, indicative of submature to mature organic matter. High resolution planktic foraminiferal and radiolarian biostratigraphy suggest that the black shales beds span the mid- to late Albian, confined to the middle part of the Ticinella primula zone, upper Biticinella breggiensis zone and lower appeninica + buxtorfi zone. Episodes of organic-rich deposition in the “Tunisian Trough” are interpreted as being the sedimentary record of the global oceanic anoxic events OAE1b, c, and d respectively. Age-diagnostic radiolarian assemblages recovered from late Albian organic-rich black shales lie within the UA13–UA14 boundary biochronozones. The abundance of radiolarian and calcispheres (i.e. pithonella) within the black shales suggests high productivity periods and eutrophic conditions probably triggered by upwelling currents. 相似文献
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Mohsen Layeb Moez Ben Fadhel Yosra Layeb-Tounsi Mohamed Ben Youssef 《Arabian Journal of Geosciences》2014,7(8):3349-3363
From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions. 相似文献
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