Sedimentary and igneous rocks overlain by marine clays

Hydrogeological Context

Version Metadata
0ea68738-1bd5-4755-9ed6-ce341fd5f47c
Project
Portneuf

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Sedimentary and igneous rocks overlain by marine clays unit description

Update:2014-01-30
The bedrock includes two major geological units: crystalline bedrock and rocks from the sedimentary platform. The crystalline rocks are massive and little fractured. The Paleozoic rocks of the sedimentary platform are highly fractured rocks, very friable in the case of shales. A thick marine silts and clays unit covers the bedrock. Granular formations such as till or glaciofluvial sediments may be between the rock and fine sediments, but current boreholes data do not allow defining clearly these units. The water in these geological formations flows very slowly and little or no involvement in the groundwater flow process is observed in permeable granular surface. It should be noted, however, that below the low permeability granular units may be units with high potential aquifer such as sand and gravel. Considering that the aquifer potential of surficial formations is minimal or zero, the rock aquifers are the main exploited aquifers in these areas. The rock formations have low aquifer potential. The clay cover confines the bedrock aquifers, and consequently helps to protect those aquifers from potential surface contamination.
Physiography
Hydrogeological regionSt. Lawrence Platform
Hydrogeological context
ReliefSedimentary and igneous rocks overlain by marine clays are dispersed sporadically in the area. They are generally found at low altitudes. Some sections located in the western part have medium altitudes. The terrain is relatively flat.
Area376.00 km2
Lithology
DescriptionThe bedrock includes two major geological units: crystalline bedrock and rocks from the sedimentary platform. The crystalline rocks are mainly gneisses, granites and migmatites with the presence of gabbro and crystalline limestones. These rock formations are massive and little fractured. The Paleozoic rocks of the sedimentary platform are mainly composed of limestones of Trenton Group and shales of Utica Group. These are highly fractured rocks, very friable in the case of shales. A significant marine silts and clays unit covers the bedrock. Granular formations such as till or glaciofluvial sediments may be between the rock and fine sediments.
Source
Aquifer
AquifersList of aquifers sedimentary rock aquifer igneous rock aquifer Granular formations that may be between the rock and fine sediments would form other granular aquifers.

Unit properties

Surficial aquifer media
Description Granular formations such as till or glaciofluvial sediments (high aquifer potential) may be between the rock and fine sediments, but current boreholes data do not allow defining clearly these units.
Bedrock aquifer media
Typical value:karst - fractured
Description The crystalline rocks are massive and little fractured. The Paleozoic rocks of the sedimentary platform are highly fractured rocks, very friable in the case of shales, and affected by the dissolution process that led to an early karstification in limestone of St. Casimir and St-Marc-des-Carrieres.
Confinement
Typical value:confined
Description The fractured rock aquifers are covered by a thick silt and clay layer.
Well depth
26 m Range: [1.2 to 182.88] m
Groundwater depth
1 m Range: [0 to 3] m
Surficial unit hydraulic conductivity
5E-8 m/s
Description The mean hydraulic conductivity of the clays is 5.3E-8 m/s
Methode Hazen formula, based on 8 samples
Regional precipitation
1121.5 mm/y Range: [1063.1 to 1168.2] mm/y
Methode Precipitation data are from Environment Canada. The value represents the average of the climate normals (1981-2010) measured on the stations distributed in or near the hydrogeological unit. The stations were selected so the average precipitation is as representative as possible of the whole hydrogeological unit.
Hydrogeological unit potential
Description The rock fractured aquifers covered by clay and silt show low aquifer potentials.
Aquifer vulnerability
0 Range: [0 to 6]