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The hydrogeological characteristics of the Upper Cretaceous De Courcy Formation (Nanaimo Group), from a subsurface core, groundwater observation well, Cedar, British Columbia

A 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.

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