|Title||Surficial geology for ground water studies on the Coastal Lowland from Deep Bay to Nanoose Harbour, Vancouver Island, British Columbia|
|Source||Canadian Quaternary Association - Canadian Geomorphology Research Group, Conference, program and abstracts; by CANQUA-CGRG 2013 Organizing Committee; 2013 p. 57|
|Alt Series||Earth Sciences Sector, Contribution Series 20140217|
|Meeting||CANQUA-CGRG 2013 Biennial Meeting; Edmonton; CA; August 18-22, 2013|
|Area||Deep Bay; Nanoose Harbour; Vancouver Island|
|Lat/Long WENS||-125.0000 -124.0000 49.5000 49.2500|
|Subjects||surficial geology/geomorphology; glacial features; glacial deposits; glaciation; glacial history; glacial landforms; Quaternary|
|Program||Aquifer Assessment & support to mapping, Groundwater Geoscience|
|Abstract||The Nanaimo lowland on eastern Vancouver Island is completely dependent on groundwater for its potable water. The groundwater occurs within a complex mixture of unconsolidated sediments and fractured
bedrock which is poorly understood. The Geological Survey of Canada is assessing this resource by mapping the three dimensional geometry of the unconsolidated sediments and bedrock fractures, and by hydrogeological modelling. This is a short
description of the surficial materials comprising the study area based on new surficial mapping, boreholes, seismic reflection surveys, and existing water well records.|
The overburden is generally over 100 metres thick in places and the
basement contacts can lie below present sea level. Nevertheless, the thickness of the unconsolidated sediment is also variable, as indicated by numerous bedrock outcrops throughout the area. The surficial materials overlying the Nanaimo lowland
record a long interplay of glacial and nonglacial conditions coupled with fluctuating sea levels.
It is well known that this area was overridden by glaciers at least twice, with intervening non-glacial deposits found in places (Dawson, 1890;
Clapp, 1914). Glaciation of east-central Vancouver Island involved two main sources of ice: local valley and piedmont glaciers, emanating from the northwest-trending Island Ranges, and secondly, the Cordilleran Ice Sheet flowing off the mainland
Coast Mountains. The configuration of the penultimate, Dashwood advance is not known, but it probably had a similar character to the final, Vashon advance. During the latter advance, a piedmont lobe of the Cordilleran Ice Sheet flowed southeastward
down the Strait of Georgia and impinged the eastern flank of Vancouver Island where it made contact with the local glaciers. Fossiliferous glaciomarine deposits associated with both glacial advances indicate high sea levels during both the advance
and retreat phases. The Quadra Sand, an extensive proglacial outwash unit deposited during the advance of the Vashon advance down the Georgia depression, is one of the most important aquifers in the Nanaimo lowland. During the maximum of the last
glaciation, the Cordilleran Ice Sheet surmounted most of the Island Ranges and flowed southwest across Vancouver Island. Deglaciation was marked by thinning of the ice and finally separation of the Cordilleran lobe from the Island valley glaciers.
Once separation occurred, ice-contact glaciofluvial terraces and deltas were deposited along the retreating ice margins. Eventually the sea flooded in from the southeast and glaciofluvial deltas were built into a high postglacial sea, reaching 150 m
above sea level. In some valleys, such as the Englishman River valley, the local valley glaciers seem to have persisted and even readvanced after the retreat of the Cordilleran lobe. With complete deglaciation, sea levels fell rapidly and the
glaciofluvial deltas and terraces were left perched on the mountain flanks. Where there was adequate sediment supply, the main rivers built deltas into progressively lower sea levels. In other places, transverse streams crossing the lowland either
terraced pre-existing glacial deposits, or cut narrow canyons in the bedrock. A cobbly surface till covers most the flat-lying interfluves. But, below the marine limit, the till is partially reworked by marine action or covered by marine sands and
silts in poorly drained areas. Modern alluvium is confined to the lower reaches of the larger rivers.
|Summary||(Plain Language Summary, not published)|
A detailed study of the surficial geology of the east coast of Vancouver Island from Deep Bay to Nanoose Harbour, British Columbia was undertaken as part
of the Nanaimo Lowland Ground Water Assessment Project. This poster describes the results of the study and introduces a surficial geology map incorporating the Pleistocene stratigraphy derived from new seismic reflection surveys, boreholes and
existing water well data. The unconsolidated sediment cover is greater than 100 m thick in places with most deposits derived from two previous glaciations that are separated by non-glacial fluvial and marine sediments. Major sedimentation also
occurred in the valleys immediately after the last glaciation, a time of rapidly falling sea level and abundant meltwater. Modern alluvium is confined to the lower reaches of the larger rivers.