Water Stewardship Information Sources

Citation Hamblin, A.P. and McCartney, T., 2014. The hydrogeological characteristics of the Upper Cretaceous De Courcy
Formation (Nanaimo Group), from a subsurface core, groundwater observation well, Cedar, British Columbia;
Geological Survey of Canada, Open File 7628, 30 p. doi:10.4095/294859
Organization NRCAN
URL http://publications.gc.ca/collections/collection_2015/rncan-nrcan/M183-2-7628-eng.pdf
Abstract/Description or Keywords 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.
Information Type report
Regional Watershed Vancouver Island South, South Gulf Islands, North Gulf Islands
Sub-watershed if known
Aquifer #
Comments
Project status
Contact Name
Contact Email