AV俱乐部

Analyses of foraminifera and ostracoda are made from Kimmeridgian-Barremian outcrops in the Lusitanian Basin of Portugal, and wells in the Grand Banks of Newfoundland. In the Praia Azul section, a slightly brackish to marine assemblage of ostracoda including Cytherella, Cytherelloidea and Schuleridea characterize outer lagoon or bay paleoenvironments; associated foraminifera include Everticyclammina, Mesoendothyra, Freixialina and Haplophragmoide. The mesohaline to polyhaline ostracoda Pararanotacythere, Galliaecytheridea and Schuleridea characterize brackish lagoons with local oyster patch reefs; associated foraminifera include Choffatella and Rectocyclammina. Landward of the patch reefs, freshwater influence is indicated by the ostracoda Fabanella, Darwinula and Cypridea; Choffatella is the associated foraminifera. The foraminifera Eoguttulina is ubiquitous to these environments. Classification of the foraminifera on test morphology permits recognition of sub-environments, based on habitat. In the Cabo Espichel section, Choffatella and smaller agglutinated foraminifera are associated with rapid basin subsidence in the early Tithonian, whereas Anchispirocyclina is associated with lower rates of basin subsidence and shallow carbonate sedimentation during the latest Jurassic and early Berriasian. Ostracoda indicate fluctuating marine-brackish salinity. Rapid ostracod speciation in the early Berriasian, combined with opportunistic associations of Anchispiricyclina and Discorbis, may be linked to environmental instability during the Berriasian regression. At Ericeira, smaller agglutinated foraminifera are associated with Berriasian regression. At Ericeira, smaller agglutinated foraminifera are associated with Berriasian-Hauterivian marginal to non-marine sedimentation. Ammobaculites assemblages indicate river estuary paleoenvironments, whereas Ammobaculites-Trochammina and trochamminid assemblages indicate marsh paleoenvironments. Two biozonations are proposed for the Lusitanian Basin.

Examination of cored intervals from Grand Banks wells revealed similarity in the microfossil assemblages from marginal marine environments. Presence of a form of Ammobaculites subcretaceus displaying a novel mode of growth may be a biostratigraphic marker for the top of the Berriasian in Portugal and the Grand Banks. A stratigraphic re-evaluation of the Grand Banks wells is made, and suggests the Jurassic-Cretaceous boundary occurs at greater depth than previously thought. A new foraminiferal biozonation is proposed for the Grand Banks. A review of the stratigraphic and subsidence histories of the North Atlantic rift basins indicates most transgressive-regressive events are linked to tectonic, rather than eustatic events.

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Supervisor:聽 F. Gradstein