Barriers, barrier platforms, and spillover deposits in St. George's Bay, Newfoundland: Paraglacial sedimentation on the flanks of a deep coastal basin

Abstract

St. George's Bay and adjacent coastal areas of southwest Newfoundland contain thick sequences of late Quaternary glaciogenic sediments and have been subject to relative sea-level fluctuations over a rang of 70 m during the past 13.5 ka. Glaciogenic sediments have been mobilized by littoral processes to form two types of deposits. These are: (1) the prograded barrier beach and spit at Stephenville and Flat Island, respectively, which are associated with large subaqueous sandy platforms; and (2) a wedge of well-stratified sand and gravel on the landward flank of the shallow sill which extends across St. George's bay at an average depth of about 25 m. The subaerial barriers and spits are composed of gravel beach ridges, overlain locally by thin aeolian deposits, with freshwater and salt-marsh peats in swales. These barriers developed during the last several thousand years, as relative sea level approached its present position from an early postglacial minimum of approximately − 25 m. The prerequisite for beach-ridge formation was the deposition of large, sandy subaqueous platforms which appear on shallow seismic reflection records as clinoform wedges prograded into the deep basin of inner St. George's Bay. The barrier platform at Stephenville is constructed across a deep submarine valley. The Flat Island barrier platform sits on the flank of a glacially-overdeepened trough, partly filled by glacial and postglacial sediments, and envelops a delta formed during the early postglacial low stand of relative sea level. The platform and barrier together constitute a thick coastal sequence coarsening up from sandy mud at the base to gravel at the top. The wedge of well-stratified, coarse sediment, up to 50 m thick, situated on the landward flank of the shallow sill overlies older, glaciogenic sediments and passes laterally into postglacial basin muds. Steep foreset reflections dipping 10°–20° are evident on shallow seismic reflection records, suggesting that the wedge was formed by spillover of sediment, entrained by wave and current action, into the relatively deep basins of the inner bay. Active gravel ripples testify to the continued mobility of coarse sediment on the sill. The formation of the spillover deposits, barriers and platforms demonstrates the potential for glacially -overdeepened basins to host thick, heterogeneous, coarse-grained coastal and inner-shelf deposits.