Stratigraphy and chronology of deglacial events at Highlands, southern St. George's Bay, Southwest Newfoundland

Sheppard, Kevin Rick (2000) Stratigraphy and chronology of deglacial events at Highlands, southern St. George's Bay, Southwest Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes and interprets the sedimentology, geomorphology, and chronology of Quaternary deposits at Highlands, southern St. George's Bay, southwest Newfoundland. These deposits are critical in understanding the glacial and deglacial history of the region, and whether retreat was interrupted by a climatic reversal causing a readvance at ~12.6 ka BP across the lowlands around St. George's Bay. -- The physiography of the Highlands area consists of a low-relief coastal plain backed to the east and south by the Long Range and Anguille mountains, respectively. The coastal plain is dominated by gravel near the coast, with till outcropping farther inland. The uplands are dominated by bedrock interspersed with till veneer. Late Wisconsinan ice, originating on the southern Long Range Mountains, covered the entire area and extended to a terminal position offshore in St. George' Bay. Striations and clast fabrics indicate that ice flow was generally west-northwestward and unconfined by topography. Deglacial ice flow was affected by topographic highs becoming diverted southwestward down Codroy Valley and south of Bald Mountain. -- Retreat of ice across the lowlands occurred in a tidewater environment. Sediments exposed in the coastal sections relate to this retreat and were mostly deposited near the grounding-line on a subaqueous fan, as sediment and meltwater entered the sea via a subglacial jet. Two distinct sedimentary sequences relating to the surface topography occur along the coast. The coast is dominated by planar surfaces at 18 to 20 m asl and 24 to 26 m asl, interrupted by the Highlands ridge ranging from 34 m asl at the coast, to >60 m asl inland. Exposures through the planar surfaces consist generally of bedrock, overlain successively by diamicton, mud, sand, gravel, and sand and silt. These represent a deglacial sequence from subglacial and proglacial deposition of the diamicton; to glaciomarine sedimentation of the mud and sand; and glaciofluvial/fluvial deposition of the gravel on outwash terraces. Sand and silt capping the coastal sections are interpreted as aeolian deposits. -- Exposures through where Highlands ridge intersects the coast (Highlands section) consist of mud, overlain sequentially by sand and gravel; diamicton; gravel and gravelly sand; and sand and silt. This section represents the more proximal part of the grounding-line fan. Diamicton forms a continuous unit along the top of the Highlands section, grading laterally from structureless to stratified, and has characteristics of subglacial, proglacial, sediment gravity flow, and rainout depositional processes. At one site, a fossiliferous diamicton is interpreted to be deposited by a combination of sediment gravity flows, suspension settling from meltwater plumes and ice rafting, and traction currents. A radiocarbon date of 13 680 ± 90 BP (Beta-120124) on paired shells from this diamicton is interpreted to represent the date of its emplacement. This date lies within the range of all other dates (~13.1 to 14 ka BP) on marine organisms from sediments along the coast of southern St. George's Bay, and suggests that deposition of the diamicton was contemporaneous with sedimentation in all areas along the coast. Evidence to support a climatically induced readvance at ~12.6 ka BP was not found.

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