Alison Creech

1991 - Michael J. Keen Memorial Award

B. Sc. Honours Thesis

Foraminiferal Assemblages From a Sediment Core From the Inner Scotian Shelf: Implications for Till-Tongue Stratigraphy and Paleoceanography

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The inner Scotian Shelf juxtaposes the Nova Scotia coastline, ending at the 100 m bathymetric contour, and consists of several zones based on seismic sequences and sea-bottom morphology. At the maximum extent of the last glaciation (Wisconsinan), ice covered almost the entire Scotian Shelf. Micropaleontological and lithological analysis of a 534 cm sediment core (Secunda 93 Core 5), collected from the Basin Zone of the inner Scotian Shelf using the Rossfelder® vibracorer, provides information that permits reinterpretation of till-tongue sediment stratigraphy and determination of new paleoceanographic implications for the inner shelf. Core 5 was subsampled, examined for foraminifera, and divided into foraminiferal assemblage zones based on visual interpretation of foraminiferal species composition and abundance. Seismically, Core 5 penetrates through La Have Clay into acoustically incoherent sediments previously interpreted as till-tongues. Lithologically, the acoustically incoherent section of Core 5 exhibits deformation. Foraminiferal assemblages present in the incoherent sediments are identical to the Elphidium excavatum forma clavatum/Cassidulina reniforme assemblage typical of the acoustically coherent glacial marine Emerald Silt, implying that till-tongues, believed to be wedge-shaped lenses of till formed by oscillations of the glacial margin, are instead deformed glacial marine sediments. Tertiary-Cretaceous (T-K) foraminifera occur in the lower part of Core 5, eroded from their original depositional site by glacial activity. Presence of T-K foraminifera suggests that the present contact location between the sedimentary Mesozoic-Cenozoic (M-C) wedge and the Meguma basement is inaccurately mapped and: a) the contact was once inland of Core 5, rather that the present contact location seaward of Core 5; or b) M-C sediments still underlie the late Quaternary sediments; or c) outliers of M-C sediments still underlie the late Quaternary sediments. Unlike previous studies from further offshore, Core 5 foraminiferal assemblages suggest no mid-Holocene warming period, indicating that the warmer Gulf Stream waters did not influence the inner Scotian Shelf as strongly as the outer Scotian Shelf. An Andercotryma glomerata/Spiroplectammina biformis assemblage, indicating a cold water period, occurs from approximately 160 cm to 260 cm in Core 5, but is absent in cores from previous studies. Carbon-14 dates of approximately 8500 years before present occur just above and just below this cold water assemblage, but have no explanation at this time.

Keywords: benthic foraminifera, reworked Tertiary-Cretaceous foraminifera, inner Scotian Shelf, till-tongues, Mesozoic-Cenozoic sedimentary wedge, paleoceanography
Pages: 68
Supervisor: David Scott