| Vashon-Capilano |
| The Vashon-Capilano hydrogeological unit overlies the Quadra Sand with thicknesses generally up to 30 m, except at observed locations where depressions left by deeply eroded Quadra Sand were filled by approximately 60 m of sediment (Fyles, 1963). It is composed of glaciofluvial deposits composed of gravel, sand and lenses of till forming the eskers, kame terraces, ice-contact fans, and deltas. The Vashon Drift geological unit is split across two hydrostratigraphic units; Vashon coarse Sediments (of the Vashon-Capilano) and Vashon Till. This unit is considered as an aquifer; however, it generally has poor aquifer potential due to its spatial discontinuity. Deposition is inferred to have been from glaciofluvial streams that initially would have flowed along the margins of the retreating Cordilleran and local glaciers, with some deltas or terraces building into short-lived ice-dammed lakes. As the Cordilleran ice occupying the Georgia Depression thinned, the sea inundated coastal lowland to an elevation of at least 150 m. The geochemical signature of Vashon Drift has a number of complicated trends over the unit thickness related to various element groupings (Knight, 2015). |
| Till de Vashon |
| The Vashon Till overlies the Quadra Sand with thickness generally up to 30 m, except where depressions left by deeply eroded Quadra Sand were filled by approximately 60 m of drift (Fyles, 1963). Vashon Till is composed of gravel, sand, but mostly silt and clay. The unit is considered an aquitard. The till of this unit was deposited during the last deglaciation. Over the lowland, Vashon Till tends to be a sandy diamicton except within some valleys where the matrix is more clayey. The Vashon Drift unit is composed of two hydrostratigraphic units: Vashon coarse Sediments (of the Vashon-Capilano) and the Vashon Till. The geochemical signature of Vashon Drift has a number of complicated trends over the unit thickness (Knight, 2015). |
| Capilano-Salish |
| The Capilano-Salish unit overlies the Vashon Drift (aquitard); however, locally it can unconformably overlie the Quadra Sand. The Capilano-Salish hydrogeological unit is considered to be an aquifer, however its aquifer potential is low, due to the thin unit thickness (median thickness of 1 m) and its spatial discontinuity. The Capilano-Salish unit is composed of coarse Capilano glaciofluvial outwash gravel and sand and includes the modern deltaic and alluvial deposits of Salish unit that extends to present sea level. The Salish sediments along rivers are mostly recent fluvial deposits that are not considered to be aquifers, but could form important water storage units and could play a crucial role in the interaction between surface and groundwater. Geochemically there is a decrease in concentration from the base of the Capilano Sediments upwards for Ba, Ca, K, Rb and Zn and there is an increase in the concentration for Cu, Fe, Mn, and V (Knight et al., 2015). |
| Dashwood-Mapleguard |
| The Dashwood-Mapleguard hydrogeological unit includes a part of the Dashwood Sediments and the Mapleguard geological unit. Dashwood Sediments overlie the Mapleguard unit. They are composed of cobbles and gravel (Armstrong and Clague, 1977; Fyles, 1963; Hicock and Armstrong, 1983). The gravel is a mixture of plutonic rocks derived from the Coast Mountains and volcanic and sedimentary rocks from the east coast of Vancouver Island. The geochemical signature of Dashwood has an elevated concentration of metals compared to Quadra Sand (Knight et al., 2015). Dashwood Drift was deposited during the penultimate glaciation early Wisconsinan, as deduced from the presence of marine shells which date beyond the range of radiocarbon in overlying sediments (Clague, 1980; Hicock, 1980; Hicock and Armstrong, 1983; Ryder and Clague, 1989). Mapleguard Sediments unconformably overlie bedrock and are the oldest Pleistocene deposits present in the Nanaimo Lowland. They are composed of bedded sand, silt, clay and minor gravel up to 10 m thick along the sea cliffs; however Fyles (1963) reported more than 20 m in some boreholes. They are considered to be outwash deposited at the onset of the penultimate glaciation. According to Fyles (1963), Mapleguard proglacial outwash deposits could be coarser than the Quadra Sand and are expected to have good aquifer potential; however, the vertical and spatial extent of this unit is not very well known. |
| Englishman River |
| The hydrogeological context of the Englishman River is completely different from the other contexts as it is predominantly bedrock and where present, the sediment cover is much thinner. The distribution of this context closely matches the watershed limits of the Englishman River, which essentially flows on till and bedrock units. Like other major rivers in the study area, seepage faces occur along slopes incised by river erosion. Capilano deposits along the Englishman River (especially downstream) could form important riverbank storage and regulate exchanges between groundwater and surface water. |
| Lowland (A) |
| The Lowland context covers most of the study area. This hydrogeological context integrates the complete succession of all the hydrogeological units: Capilano-Salish, Capilano, Vashon-Capilano, Vashon till, Quadra, Cowichan-Dashwood, Dashwood-Mapleguard, Lower Nanaimo Group, and Metamorphic and Intrusive Basement. This context includes downstream portions of Nile Creek, Qualicum River, Little Qualicum River and French Creek. Most of the hydrostratigraphic units are present within the Lowland hydrogeological context, except near the main rivers where the sequence has been extensively eroded by fluvial incision. Along river valleys most surficial units, particularly Quadra Sands, are not in direct hydraulic link with the rivers. Groundwater seepage zones are present along the incised slopes. Groundwater in seepage areas can reach rivers by surface runoff or by hypodermic or saturated flow through the permeable Capilano sediments present at the base of incised slopes, commonly as low terraces. These sediments can also play a role in moderating interactions between surface and groundwater due to their water storage capacity even if their spatial distribution is limited. |
| Cameron and Horne Lakes |
| The Cameron-Horne context is similar to the Lowlands context, except for the surficial unit which is composed of fluvioglacial sand and gravel deposits (the more permeable component of the Vashon). All hydrogeological units are therefore present, with the exception of the Capilano-Salish and Capilano units. This context is located at the outlet of the Cameron and Horne lakes, in the lowland area. The coarser sediments of the Vashon could be an important aquifer and is expected to be either perched over impermeable Vashon till or in direct contact with the Quadra unit. The hydraulic connection between the coarse sediments of the Vashon and the underlying aquifers is not well understood as no stratigraphic drilling is available. The Mapleguard sediments may not be present at this elevation. |
| Lower Nanaimo Group |
| The Lower Nanaimo Group is a terrestrial to marine succession formed within a basin during the orogenic development of the Canadian Cordillera in Jurassic-Cretaceous time. It is up to 4 km thick and consists of 11 interfingering sandstone-dominated and shale-dominated formations. In the study area, only the lower 8 formations are present. The dominant bedrock units are the Comox and Haslam formations, which are regarded as aquifer of moderate and low capacity. The Comox Formation sandstone/conglomerate is commonly between 100-150 m thick, and is overlain by Haslam Formation shale that is 100-150 m thick. |
| Quadra Sand |
| Quadra Sand overlies the Cowichan Head Formation and is generally found above current sea level and below 100 m. It can exceed 75 m in thickness. The Quadra hydrostratigraphic unit aquifer is present across the study area except along the main rivers where the Quadra Sand has been eroded by rivers and where locally eroded and infilled by Vashon Till. It consists of horizontally and cross-stratified, well-sorted sand with minor silt and gravel with wood and peat lenses in its lower portions. Geochemically the Quadra is distinguished from other units by the Sr concentrations throughout the unit (including the upper Cowichan Head Formation; Knight et al., 2015). Quadra is interpreted to be outwash deposited during the transition from non-glacial to glacial conditions at the onset of the Fraser Glaciation (Armstrong and Clague, 1977; Clague, 1977, 1976). In general the Quadra Sands can be regarded as proglacial outwash formed subaerially on outwash plains extending across and along the margins of the present-day Strait of Georgia. The Quadra Sand is the most important water-bearing unit in the study area. The upper part of the Quadra Sand has a fluvial erosional contact. Large incised and subsequently sediment-filled channels can be observed at the top of the unit. |
| Mountains |
| The context is located on the flanks of the mountainous area to the west of the Lowlands. This context is mostly composed of thin till and colluvium over metamorphic and intrusive bedrock. |
| Capilano (aquitard) |
| The Capilano Sediments hydrogeological unit overlies the Vashon Drift, except locally where it unconformably overly Quadra Sand. The thickness of the sediments is highly variable ranging < 1 m up to 12 m and locally reaching 25 m in thickness. These marine and glaciomarine deposits, of considerable thickness, are found in coastal areas that had undergone significant glacio-isostatic submergence prior to uplift. They are mostly composed of silt and clay of marine origin. These deposits are considered to be postglacial, but still affected by rapid emergence and influxes of glacial meltwater during early deglaciation. The marine deposits are relatively impermeable due to their fine grain-size. Geochemically there is a decrease in concentration from the base of the Capilano Sediments upwards for Ba, Ca, K, Rb and Zn and there is an increase in the concentration for Cu, Fe, Mn, and V (Knight et al., 2015). |
| Cowichan-Dashwood |
| The Cowichan Head Formation and Dashwood Till (part of the Dashwood Drift geological unit) form a single aquitard. Cowichan Head Formation unconformably overlies the Dashwood Drift and is up to 21 m thick (Hicock and Armstrong, 1983). The formation has been divided into a lower member of clayey silt and sand with marine shells, and an upper member of sandy silt and gravel, commonly with reddish oxidized hues rich in fossil plant remains (Armstrong and Clague, 1977). There is at least 2.4 m of compact peat, probably deposited in a boggy floodplain (Alley, 1979). Radiocarbon dates range from 25.8 to 40.5 ka 14C BP (Armstrong and Clague, 1977). Fossil pollen and beetle assemblages suggest that the climate fluctuated between conditions similar to present and cooler than present (Armstrong and Clague, 1977). Gravel provenance is a mix of volcanic and sedimentary rocks (Armstrong and Clague, 1977; Clague, 1976). Geochemically the Cowichan has metal concentration similar to Dashwood. The lower Member is interpreted as glaciomarine, whereas the upper Member is attributed to estuarine and fluvial environments (Armstrong and Clague, 1977). Dashwood Drift conformably overlies the Mapleguard Sediments and is commonly less than 10 m thick (Armstrong and Clague, 1977). The section of Dashwood Drift, which is comprised in the Cowichan-Dashwood hydrogeological unit, is composed of muddy diamicton and glaciomarine sediments (Armstrong and Clague, 1977; Fyles, 1963; Hicock and Armstrong, 1983). The geochemical signature of Dashwood has an elevated concentration of metals compared to Quadra Sand (Knight et al., 2015). Dashwood Drift was deposited during the penultimate glaciation early Wisconsinian, as deduced from the presence in overlying sediments of marine shells which date beyond the range of radiocarbon dating (Clague, 1980). |
| Metamorphic and Intrusive Basement |
| Wrangellia rocks are more common in the southern part of the study area and in the mountains. The regional unconformity surface of the basement is commonly fractured and heavily-weathered making the upper meter or so of basement more permeable. The basement topography likely influenced the distribution and thickness of the unconformably-overlying Lower Nanaimo Group units (Johnstone et al., 2006; Ward and Stanley, 1982). |
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