Bulletin 37: Geology of the Cowichan Lake Area, Vancouver Island, British Columbia
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Bulletin 37 describes the geology of the Cowichan Lake Area of British Columbia. The area referred to in this report as the Cowichan Lake area includes about 280 square miles of mountainous country on southern Vancouver Island lying mainly north and northwest of Cowichan Lake.
Fault-line scarps and fault-controlled valleys are the most prominent physiographic features of the region. The north side of Cowichan Lake is a fault-line scarp; and Rift Creek, the west fork of Cottonwood Creek, and the upper part of South Nanaimo River flow in fault controlled valleys. The continental ice-sheet covered the entire region. The ice appears to have moved southward.
The oldest rocks in the area belong to the Sicker group. They include thin-bedded, cherty, argillaceous, and feldspathic tufts; limestones; coarse and fine breccias; and minor intercalated amygdaloidal basalt flows. Crinoidal limestones at the top of the Sicker group contain Lower Permian fossils. The Sicker group is conformably or disconformably overlain by the Vancouver group.
The lower part of the Vancouver group is made up of a thick sequence of massive, pillow, and amygdaloidal basalt flows and related sills, dykes, and irregular bodies of diabase. The intrusive masses are equivalent to the Sicker gabbro-diorite-porphyrite described by Clapp. The upper part of the Vancouver group includes the Sutton limestone, which contains Upper Triassic fossils, and an overlying sequence of clastic sediments.
Rocks of the Sicker and Vancouver groups are highly deformed into northwesterly trending folds, many of which are overturned with both limbs dipping southwestward.
Plutons of Saanich granodiorite belonging to the Coast intrusions cut the Sicker and Vancouver groups. They are dominantly quartz diorite and granodiorite, but a roof facies of granite and aplogranite occurs near the top of Mount Buttle. The plutons are mainly steeply dipping, elongate bodies that are less than 2 miles wide and several miles long. In cross-section they cut across structures in the pre-granitic rocks; in plan, long axes are parallel to fold axes in the older rocks.
Close to plutons of granodiorite the pre-granitic basaltic rocks have been metamorphosed, exhibit granoblastic textures, and contain minerals characteristic of the granodiorite. Farther from granodiorite plutons, pre-granitic rocks have undergone a low grade of regional metamorphism that appears to be spatially related to the granodiorite.
Erosional remnants of Upper Cretaceous detrital sediments belonging to the Nanaimo group unconformably overlie the Saanich granodiorite and pre-granitic rocks. The Nanaimo group has been gently folded and displaced by steeply dipping faults. Most of the movement on the faults appears to be parallel to the dip, and the dip slip on several is more than 1,000 feet.
Small irregular masses of gabbro, possibly of Tertiary age, intrude rocks of the Sicker group and t he Saanich granodiorite.
Copper has been shipped from the Blue Grouse property on the south side of Cowichan Lake, and manganese from the Hill 60 deposit 4 miles east of Lake Cowichan. Copper occurs in chalcopyrite-skarn deposits at the Blue Grouse property and on the divide between Widow Creek and Chemainus River.
Quartz veins near bodies of granodiorite commonly contain molybdenite. On the Allies property on Mount Buttle, quartz-molybdenite veins have been prospected by trenches and open pits.
Shear zones in massive volcanics of the Vancouver group on El Capitan and the northeast slopes of Mount Service contain sulphides and gold. They have been explored by underground workings on the Silver Leaf and El Capitan properties.
At many places cherty rocks of the Sicker group contain manganese silicates, mainly rhodonite and manganese garnets. Within a few feet of the surface the silicates have been oxidized, and siliceous manganese oxides have been mined at the Hill 60 deposit.
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