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TitleThe hydrogeological characteristics of the Upper Cretaceous De Courcy Formation (Nanaimo Group), from a subsurface core, groundwater observation well, Cedar, British Columbia
AuthorHamblin, A P; McCartneny, T
SourceGeological Survey of Canada, Open File 7628, 2014, 30 pages,
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS92B/13; 92F/01; 92F/08; 92G/04
AreaNanaimo Lowlands; Nanaimo; Parksville; Mill Bay; Deep Bay; Ladysmith
Lat/Long WENS-124.6667 -123.5000 49.5000 48.7500
Subjectshydrogeology; sedimentology; Upper Cretaceous; groundwater; groundwater resources; groundwater regimes; bedrock aquifers; aquifers; depositional environment; depositional history; tectonic setting; sedimentary rocks; sandstones; conglomerates; De Courcy Formation; Nanaimo Group; Comox Formation; Haslam Formation; Extension Formation; Pender Formation; Protection Formation; Cedar District Formation; Northumberland Formation; Geoffrey Formation; Spray Formation; Gabriola Formation; Mesozoic; Cretaceous
Illustrationslocation maps; cross-sections; stratigraphic columns; photographs
Natural Resources Canada Library - Ottawa (Earth Sciences)
ProgramAquifer Assessment & support to mapping, Groundwater Geoscience
Released2014 07 15
AbstractA new inquiry into the groundwater potential of the Nanaimo Lowlands was jointly undertaken by concerned municipal, provincial and federal agencies because rapid population growth and expanding industrial development are, and will continue to, put pressure on the limited groundwater resources. The bedrock component of the project focused on the characterization of the aquifer potential of the Upper Cretaceous Nanaimo Group, as a likely target of importance. This unit is a thick succession of 11 intertonguing sandstone-dominated and shale-dominated formations, of which only the lower 8 are present in the defined study area. As one step in the analysis, a 112.5 m core was obtained as part of the drilling of a Groundwater Observation Well in the Cedar area of Vancouver Island, about 10 km SE of Nanaimo City centre. The entire length of the core comprises the mid-Nanaimo Group, sandstone-dominated (potential aquifer zone) De Courcy Formation, the uppermost coarse grained formation in the study area. The De Courcy Formation present in the studied core is characterized by stacked, thick bedded medium to coarse grained arkosic sandstone separated by units of bioturbated sandy siltstone with thin finer grained sandstone. It includes two main facies: 1) thick bedded, grey medium to coarse grained sandstone interpreted as high-energy density current and turbidity flow deposits emplaced in a moderately deep marine setting on the surface of, and in channels on, a northwestward-sloping submarine fan system, with minor thin beds of bioturbated siltstone, and 2) thinly interbedded dark grey bioturbated siltstone to sandy siltstone interpreted as lower-energy, background sedimentation on the surface of the submarine fan systems, and very fine to medium grained sandstone, interpreted to represent slower, more distal, higher-energy density current turbidity flow events which occasionally punctuated that quiet background sedimentation. The thick bedded sandstone facies represents about 65% of the strata in the core, and has porosity ranging 2.0 to 10.2 %, averaging 6.8 %, and permeability ranging 2.8 to 105.0 mD, averaging 24.4 mD. In thin section, these sandstones are predominantly feldspathic litharenites and lithic arkoses with abundant plagioclase, volcanic rock fragments, quartz and chert, in a clay matrix. Multiple, laterally-extensive units of thick, porous and permeable sandstone, up to 6 m thick, likely represent significant aquifer horizons within the De Courcy Formation. The interbedded siltstone and thin sandstone facies occupies about 35% of the core strata and has porosity ranging 4.0 to 9.7 %, averaging 7.4 %, and overall permeability ranging 1.8 to 40.0 mD, averaging 12.7 mD. However, within this facies, the thin sandstone beds have an average permeability of 15.4 mD, whereas the bioturbated siltstones have average permeability of only 5.4 mD. Multiple, laterally-extensive units of interbedded siltstone and thin sandstone, up to 6 m thick, may represent significant aquitard horizons within the De Courcy Formation. These results, although derived from the De Courcy Formation only, may display comparative analogies to the characteristics of the other (uncored) potential aquifer zones present lower in the Nanaimo Group; the Comox, Extension and Protection formations.
Summary(Plain Language Summary, not published)
As part of the investigation of the groundwater potential of the Nanaimo Group bedrock units, a drill core 112.5 m long was obtained from an Observation Well near the village of Cedar on Vancouver Island. The entire length of the core comprises the sandstone-dominated (potential aquifer zone) De Courcy Formation, and is characterized by thick bedded, medium to coarse grained sandstone (65% of core) with interbedded units of sandy siltstone. Sandstones have decent porosity and permeability and may form subsurface aquifers, whereas siltstones have lesser porosity and permeability and may form intervening barriers to groundwater flow. More importantly, the characteristics displayed in this core provide analogies for deeper and more widespread potential aquifer zones within the Nanaimo Group (Comox, Extension, Protection formations), for which we have no subsurface data.