Geology of British Columbia and Vancouver Island

The Geology and Geological History of

Vancouver Island

Steven Earle, PhD

Department of GeologyMalaspina University-College

Nanaimo, British Columbia, Canada

Vancouver Island is part of the Insular Superterrane of western B.C.

Most of Vancouver Island is made up of the Wrangellia Terrane (part of the Insular Super Terrane) shown here in shades of green. The Wrangellia Terrane arrived on the west coast of North America some time around 100 m.y. ago. The Nanaimo Gp. Rocks were deposited on top of the Wrangellia rocks (from 85 to 65 .y. ago), and the rocks of the Pacific Rim and Crescent Terranes arrived here later(at 55 and 42 m.y. respectively).

The oldest rocks of Vancouver Island are Devonian in age (ca. 370 m.y.) and include sea-floor and terrestrial volcanic rocks as well as some limestone of the Sicker and Buttle Lake Groups.

Myra Falls Mine

Devonian (Sicker Group) Rocks

Dyke in pillow basalt

Chert overlain by Cretaceous conglomerate

The Karmutsen and Quatsino Formations are part of the Vancouver Group. The Karmutsen is dominated

by sea-floor basalt , while the Quatsino is primarily limestone. The Karmutsen

is the most common rock type exposed on Vancouver Island

Karmutsen Formation

(Triassic) sea-floor pillowed basalt

Quatsino Formation

limestone (with a mafic dyke).

The Bonanza Fm. includes volcanic rocks that erupted on land, for the most part.

The Westcoast Crystalline complex (WCC) includes granitic rocks that are the intrusive equivalents of the Bonanza Fm.

The WCC also includes metamorphic rocks in the Victoria region.

Jurassic aged Island

Intrusion granite with a

quartz-feldspar vein

The rocks shown in colour on this map

comprise the Wrangellia Terrane, which collided

with North America at around 100 m.y. ago.

Nanaimo Group (65-

85 m.y.)

The Nanaimo Group sedimentary rocks were deposited into the basin between Wrangellia and North America.

The total thickness is roughly 5000 m. The oldest rocks may be as old as 92 m.y. but most basal units are no more than 86 m.y. The youngest rocks are probably more than 65 m.y. but could be younger.

Most, but by no means all, of the Nanaimo Gp. sediments were deposited under marine conditions, largely as submarine fans offshore from coastal shelf deposits.

Nanaimo Group (Cretaceous) Sandstone,

mudstone, comglomerate

The City of Nanaimo exists because of coal mining, which took place from around 1850 to 1950. It could be argued that this is also why

British Columbia is part of Canada.

The only coal mining at present is from the Quinsam mine near to Campbell River.

Exploration for coal bed methane is currently underway in the area north of Nanaimo

The Pacific Rim and Crescent Terranes were accreted onto

Vancouver Island as a result of plate movements.

Pacific Rim at 55 m.y. and Crescent at

42 m.y.

Leech River Complex

(Pacific Rim Terrane)

phyllite at Niagara Falls

Crescent Terrane Tertiary,

Metchosin Igneous Complex, Sea-floor pillowed

basalt

These collisions led to compression of western North America and the up-thrusting of Nanaimo Group and

Wrangellia rocks onto Vancouver Island

Sedimentary rocks along the outermost west coast (the Carmanah Fm.) were deposited during the Paleogene (up until around 24 m.y. ago.)

Cross-section from the edge of the North American plate at the subduction front, across Vancouver Island to the Strait of Georgia. This section shows the Pacific Rim Terrane in brown

and the Crescent Terrane in red. Cretaceous and Tertiary sedimentary rocks are shown in yellow.