Late Wisconsinan deglaciation of Emerald Basin, Scotian Shelf

Gipp, Micheal Robert (1989) Late Wisconsinan deglaciation of Emerald Basin, Scotian Shelf. Masters thesis, Memorial University of Newfoundland.

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Abstract

Emerald Basin is a glacially overdeepened basin located on the central Scotian Shelf, where Quaternary sediments form a thin veneer over Tertiary bedrock. Nineteen piston cores and 1200 line km of seismic data were obtained from the basin, from which seven acoustic facies were recognized: (i) Cambro-Ordivician metasediments (ii) Tertiary sediments (iii) Scotian Shelf Drift (till) (iv) acoustically stratified Emerald Silt (v) acoustically unstratified Emerald Silt (vi) LaHave Clay (vii) diffused gas. Six depositional sequences were recognized in the Emerald Silt and LaHave Clay facics, and were correlated with sediments recovered in piston cores. Depositional sequence 0 is a strongly stratified package which drapes over the Scotian Shelf Drift. It is correlated with stiff, unbioturbated, slightly gravelly sandy muds, with no in situ molluscs, and is characterised by an ice marginal foraminiferal assemblage. Depositional sequence 1 drapes over depositional sequence 0 and shows onlap on topographical highs. It is correlated with slightly gravelly muds, which are characterised by alternating bioturbated and unbioturbated hands, and monospecific molluscan and foraminiferal assemblages. Depositional sequence 2 asymmetrically drapes over and onlaps onto depositional sequence 1, and is correlated with bioturbated, slighty gravelly muds, with monospecific molluscan and foraminiferal assemblages. Depositional sequence 3 infills the basin deeps, erosionally truncating reflections of depositional sequence 2. It is correlated with bioturbated muds which are characterised by a monospecific foraminiferal assemblage and a slightly diverse molluscan assemblage. Depositional sequence 4 drapes over the erosional upper surface of depositional sequence 3, and is correlated with bioturbated muds which are characterised by a diverse molluscan assemblage. Depositional sequence 5 onlaps onto depositional sequence 4, infilling the basin deeps, and is correlated with soft, bioturbated sandy silts, which are characterised by diverse molluscan and foraminiferal assemblages. -- The present day seafloor in Emerald Basin is marked by small gas escape structures intepreted as pockmarks. Buried pockmards are observed in depositional sequences 4 and 5. Buried iceberg scours are observed on the upper surface of depositional sequence 0, and within depositional sequences 1 and 2. Small ridges, interpreted as "lift-off* moraines, are formed on the upper surface of the Scotian Shelf Drift, and are oriented transverse to ice flow directions during glacial retreat. Wedge-shaped, acoustically unstratified features, interpeted as till tongues, are rooted in moraines on the basin flanks, and were deposited simultaneously with depositional sequences 0, 1, and 2. -- Detailed interpretation of the seismic and core data suggest that glacial ice advanced across Emerald Basin, reaching the shelf edge at approximately 24 ka. Melting of the ice sheet resulted in deglaciation of Emerald Basin at 17.5 ka, by lift-off of the grounded ice sheet to form a floating ice shelf of 10-100 yr duration. Sediment beneath the ice shelf was primarily supplied by subglacial meltwater streams. Prior to lift-off, till was squeezed into basal crevasses in the ice to form features known as "lift-off" moraines. The ice shelf calved at approximately 17.4 ka, causing intense scouring of the seafloor by in situ icebergs, and isolating ice rises on the banks surrounding Emerald Basin. These ice rises disintegrated rapidly, except for the rise on Emerald Bank, which contributed sediment to Emerald Basin until 12 ka. Sediment was primarily supplied by subglacial meltwater, and deposited out of suspension, except near the ice margin, where ice rafting and gravity flow mechanisms were important. Iceberg scouring occurred until about 14 ka. The main ice margin remained on the northern flank of the basin until 15.4 ka, at which time it retreated landward. A widespread erosive event, caused by storm-driven currents, occurred at 13 ka. Rain-out from suspended sediment plumes was the dominant depositional mechanism until 12 ka. LaHave Clay is primarily derived from inner shelf sediments reworked by rising sea level. Storm-generated currents have created an erosional surface at the present day seafloor around the flanks of the basin.

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