G e o l o g y
of
Sargeant Bay Provincial Park
by John Newell
As everywhere,
the
scenery we enjoy at Sargeant Bay is closely tied to its geology. In the
age of dinosaurs, some 150 million years ago, the west coast of North
America was well to the east of its present position. The oldest rocks
on the Sechelt Peninsula formed in an offshore volcanic island arc,
much like Japan and the islands of the eastern Caribbean. Subsequently,
the endless movements of the Earth's tectonic plates carried these
rocks downward, deep beneath the edge of continental North America.
Under intense heat and pressure they melted and recystallized to create
the granitic rocks that now form the Coast Mountains. Remnants of the
original island arc are widely preserved. Locally, the Caren
Range provides many examples .
A stroll along the beach at Sargeant Bay
illustrates
the many variations in these rocks. There are dark pebbles and light
pebbles, some coarsely crystalline, others fine grained.
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Pebbles on
the
beach
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However, the bedrock exposed within the park, from the prominent bluff
at the east end of the berm, northwards to Triangle Lake, shows
relatively little variation. It is classified as diorite, one of a
number of igneous rock types loosely referred to as “granite”. It is
comprised of medium-grained, interlocking crystals of the black mineral
hornblende and greyish white plagioclase feldspar .
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Most of the
rocks
are Diorite
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Small slivers of the original volcanic and sedimentary rocks can be
found within the diorites on the ridge south of Triangle Lake. They are
dense, fine-grained, dark grey rocks, called hornfels.
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Hornfels
south of
Triangle Lake
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Some show relict sedimentary banding, reflecting their origin
as mud and silt deposited on an ancient seabed.
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Sediments
near
Triangle Lake
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The
present
landscape of Sargeant Bay has been much modified by glaciation during
the Ice Age that ended only 10,000 years ago. There are two
topographically
distinct regions in the park. Northeast of Colvin Creek there are
extensive
outcroppings of diorite, with only a thin, discontinuous veneer of
overburden.
In the area of more subdued topography to the southwest, the granitic
rocks are covered by unconsolidated sediments of Pleistocene age (1.6
million – 10,000 years before present). The most extensive younger
sediments
are Capilano marine and glacial outwash deposits derived from reworking
of earlier glacial tills. They have been eroded from the lower part of
the Colvin Creek valley, exposing the underlying pre-Vashon sediments,
the product of an earlier Pleistocene glaciation. Examples of these
glacial sediments can be seen on the north side of Redrooffs Road,
opposite the park gate. Here they consist of poorly sorted, gravelly
glacial till containing a diversity of pebbles, overlying a bed of
fluvial sand.
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 Glacial
sediments
at Redrooffs Road
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The wetland, arguably the most important feature of the park,
is itself a product of continuing geological processes. Capilano
sediments, forming the bluff west of the bay, are easily eroded. Wave
action carried the coarser material towards the head of the bay, where
it accumulated to form a barrier bar protecting a tidal lagoon. The
buildup of gravel eventually choked off the entrance and silt flushed
down Colvin Creek was trapped behind the berm. Silting up of the lagoon
allowed freshwater marsh vegetation to become established, creating an
ecological system that
is unique on the Sunshine Coast.
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