Seismic stratigraphy and late Quaternary sedimentary history of Bonavista Bay, northeastern Newfoundland

Cumming, Ewan Hugh (1990) Seismic stratigraphy and late Quaternary sedimentary history of Bonavista Bay, northeastern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Late Quaternary glacial and sedimentary history of Bonavista Bay, on the NE coast of Newfoundland, is interpreted from 545 line km of high-resolution seismic reflection data and three piston cores. Seismic data from the open, outer region of Bonavista Bay display three seismic units (1-3) overlying acoustic basement within two broad, shallow basins. The surveyed inner region of Bonavista Bay contains narrow, deep basins which commonly host four seismic units (A-D) above acoustic basement. -- Glacial sediments are represented by seismic units 1 and A, both of which are acoustically incoherent. Unit 1 is generally <15 ms thick and has a variable morphology. It is correlated with a stiff, sandy, gravelly mud (diamicton). Unit A is up to 110 ms thick and ponded. Units 2, B and C are proglacial sequences. Unit 2 is acoustically stratified, <20 ms thick and has a draping basin-fill morphology. It is correlated with an interlaminated sequence of distal turbidites and hemipelagic/ice-rafted sediment. Unit B is acoustically transparent, ponded and up to 30 ms thick. It corresponds with a proglacial homogeneous silty mud, rapidly deposited under open water conditions until 12,790 y BP (in Chandler Reach). Unit C, overlying B, is acoustically stratified, up to 90 ms thick and has a draping basin-fill morphology. It is correlated with a sandy, pebbly and shelly mud, deposited by hemipelagic, terrestrial and ice-rafted sedimentation from 12,790 y BP to 10,170 y BP. Seismic units 3 and D are acoustically transparent post-glacial sequences with an average thickness of 20 ms. They display a draping basin-fill morphology which has been altered by erosion and are correlated with a sandy, pebbly and shelly hemipelagic mud; initially deposited in the outer bay by 13,500 y BP and in the inner bay by 10,170 y BP (Chandler Reach). -- Bonavista Bay was host to a grounded Late Wisconsinan ice sheet. Deglaciation of the bay was rapid and occurred at a time of rising sea level. Initially a thin basal till was deposited beneath grounded ice. With lift-off, an ice shelf formed over the basins of the outer bay. Retreat of the grounding line paused briefly on the basin margins, where discontinuous moraines were deposited. The ice sheet also became buoyant within the northeasternmost basin of the inner bay during this period. Diamictons (seismic units 1 and A) were subaqueously deposited within basins beneath the floating ice. Following ablation of the ice shelf, ice margins persisted near the SW and western regions of the outer bay, providing sediment for proglacial turbidity currents within the basins (unit 2). Geostrophic currents largely limited deposition of hemipelagic sediments to within the basins of the outer bay. -- The basins of the inner bay were gradually deglaciated while unit 2 was deposited. Fine-grained outwash sediment (unit B), transported by interflows, was rapidly deposited as the ice margin retreated to a terrestrial position by at least 12,790 y BP. The region of the inner bay remained under the influence of one or more ice margins until 10,170 y BP, with ice at the position of an inland end moraine and probably on the Bonavista and Gander Peninsulas. Normal marine conditions were present in the outer bay by 13,500 y BP and throughout the entire region by 10,170 y BP. The Labrador Current has eroded Holocene and perhaps Wisconsinan sediments. -- The deglacial sequence of events within Bonavista Bay and the sediments present, suggest that ice was channelized in the bay during the Late Wisconsinan, either at glacial maximum or during a deglacial surge which was driven by a collapsing ice cap from the SW.

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