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Transcript
THE GEOLOGY OF WESTERN CANADA
(Read this while viewing the maps of the Cordillera in the hallway and at the back of
F360 so you can see where the regions, the physiography and structures are found.)
Physiographic Belts:
The Canadian Cordillera of British Columbia, the Yukon, and parts of Alberta and the
Northwest Territories is a region that is both more scenic and more complex than the rest
of the North American Craton. Across the Cordillera there are a series of NNW trending
physiographic belts totaling 1000 to 1350 km wide, that have distinctive: elevation, relief,
bedrock geology and structures. The Cordillera is essentially the youngest and deformed
westernmost edge of the North American continent. To understand the geological
architecture of the Cordillera is to understand how continents grow.
The distinctive physiographic belts (west to east) of the Cordillera are: The Insular Belt,
The Coast Belt, The Intermontane Belt, The Omineca Belt and The Foreland or
Hinterland Belt. East of this is the Great Plains of the North American Craton, which
is underlain by the strata of the Western Canada Sedimentary Basin overlying various
Precambrian basement terranes of the Canadian Shield.
The Geology and Development of the Different Belts from the Coast to the Craton:
The Insular Belt:
The Insular belt includes Vancouver Island, the Gulf Islands, the Straits and western most
slopes of the Coast Mountains, The Queen Charlotte Islands, the island archipelago of the
panhandle of Alaska (including the Wrangell Mountains), the SW corner of the Yukon
and the SE most corner of mainland Alaska including the Yakutat Block. The
physiography of the southern portion of the Insular Belt (South of Dixon Entrance and
Prince Rupert) ranges from 3km tall glaciated mountains through foothills to sea level
lowlands and basins (Dixon Entrance, Hecate Strait, Queen Charlotte Sound, Georgia
Strait) underlain by Quaternary, Tertiary and Cretaceous sediments. The deepest point is
at Captain's Island in lower Jervis Inlet, where where a fjord glacier eroded to more than
700 m below sea level. Further north in Alaska, NW BC and SW Yukon, the Insular Belt
is more mountainous and its physiography resembles the Coast Belt. Because of the
convergence across the Queen Charlotte-Fairweather Transform fault, and the Tertiary
through modern accretion of the Yakutat Block in the corner of Alaska (near 60N), the
highest peak in Canada, Mt. Logan is found in that region (also Mt. St. Elias) as well as
the highest uplift rates on the planet (~2 cm/yr). Because of both ongoing uplift and
Tertiary through recent subsidence and glaciatiation in lowlying basins, this belt is the
most tectonically active. It includes both Canada's most active fault, the Queen Charlotte
(1949, M=8.1) and a variety of small Quaternary basaltic volcanoes from Revillagigedo
Is. (55.5N) just north of the Queen Charlottes to Mt. Edgecumbe, a Quaternary volcano
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on Kruzof Island (57.1N) a little further north in SE Alaska.
The geology of the Insular Belt is characterized by 2 major allochthonous
tectonostratigraphic terranes: Wrangellia and the Alexander Terrane. The oldest rocks are
marine sediments and volcanics, Lower Paleozoic sediments as determined from rare
conodont microfossils on Moresby Is. and on Vancouver Is. Both sedimentary and
volcanic strata are present in the Paleozoic (Devonian through Permian). Apparently this
crust was part of an intra-oceanic island arc system from its marine volcanics and
sediments (Myra and Nitnat Formations on Vancouver Island) in the Devonian and again
in the Pennsylvanian from the marine sediments and sills of the Sicker Group. The
marine arc volcanics of the Myra formation are the host to volcanogenic massive sulfide
deposits involving copper and zinc like at the Myra Falls mine west of Campbell River.
By Permian this twice thickened Island arc crust was somewhere in more tropical climes
from its layers of crinoidal Buttle Lake limestones (Cowichan Valley and Strathcona
Park). In Upper Triassic time there was a massive outpouring of marine flood basalts
making up to 3 km of pillow and pillow breccias of the Karmutsen Formation. This
section and period is akin to many of the larger high standing intra oceanic lava plateaux.
After some more tropical limestone deposition, by middle Jurassic this transitional crustal
fragment was once more over the top of a subduction zone form the calc-alkaline
volcanics (Bonanza, Yakoun) and related plutons (Island Intrusions) present on
Vancouver Island and in the Queen Charlottes. Those intermediate plutons acted like heat
engines in the upper crust and convected enough hot water around to form both
disseminated porphyry copper deposits like that on the north end of Vancouver Island at
Island Copper and copper-iron skarns like at Tasu where the plutons cut the Karmutsen
volcanics and adjacent limestones. Paleomagnetic evidence on these Jurassic rocks (180
to 168 Ma) gives a paleolatitude of about -30 (southern hemisphere). By about 80 Ma in
the Cretaceous, the microfossils indicate more a northerly latitude and "North American"
assemblages. The inference from the sedimentary rocks of the Nanaimo Basin on Eastern
Vancouver Island is that the Insular Terrane had been emplaced against the cordillera by
this time and was now part of North America. While the exact timing for this
emplacement is under discussion, folding of about 95-90 Ma may mark the emplacement
of Wrangellia and the 80-65 Ma rocks of the Nanaimo Group probably mark an overlap
assemblage that covers up the newly accreted real estate. At the onset there is a big
erosional unconformity with the rocks of Wrangellia and some major river valleys that
preserve bolder conglomerates along eastern Vancouver Island. The lowest strata of the
Comox formation are sometimes coal bearing. (This was the economic incentive for
immigrants to settle Vancouver Island.) From the Comox Formation on upsection, the
setting then included a broad shallow continental shelf or fore arc basin somewhat wider
than Georgia Strait today, with large free floating ammonites, benthic flat clams like
inoceramus and big predatory swimming reptiles including: giant turtles, the crocodilian
Mosasaur, and long necked Elasmosaurs like those found near Courtnay. Because of the
repetitive number of massive conglomerate beds, there was probably active faulting and
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tectonic activity to the east in the region of the modern Coast Mountains. During and
since the Upper Cretaceous there has been a succession of subduction zones just offshore
including the Farallon Plate since 95 Ma in the south (Queen Charlottes to Mexico), the
Kula Plate from 56 to 10 Ma (SE Alaska) in the North and the Explorer, Juan de Fuca,
and Gorda Plates as successors to the Farallon beneath Vancouver Island, Washington
and Oregon since Upper Miocene (about 16 Ma). Some of the 56 Ma seafloor basaltic
crust and deepwater sediments have been accreted to the coast in westernmost Oregon,
Washington and southernmost Vancouver Island (south of the Leech River Fault) and as
the Yakutat Block in SE Alaska. In the south this was complete by 40 Ma (Late Eocene)
because of the Sooke Formation overlapping and including rocks fragments from the
Sooke Gabbro and Metchosin and Crescent Basalts together with locally derived pebbles
from the older rocks of Wrangellia. The terrane of marine basalts and deep-sea sediments
that lies from Sooke to Mendocino California and the west coast to the Cascades is called
Siletzia. This piece of real estate was accreted by Upper Eocene and was responsible for
the uplift and high-grade metamorphic rocks on southern Vancouver Island. It is this
accretion and uplift event that formed the lode gold deposits at Gold River and the
Manganese deposits of the Cowichan valley, both typical of forearc environments.
In the north of the Insular Belt, in Southeast Alaska, the Yakutat Block is still being
accreted today, leading to much compression and very large uplifts. In between the 2
subduction zones (Cascadia and Aleutians) from Eocene through Upper Miocene (45-5
Ma) there was an extensional basin along the central coast of Queen Charlotte Sound and
the Queen Charlotte Islands. There was up to 7 km of subsidence and up to 4 km of rift
related volcanics and up to 3 km sediments deposited in a whole series of small grabens.
Because of the great sedimentary thickness, high heat flows and good organic source
rocks this is Western Canada's most prospective frontier petroleum basin with many
geological similarities to Cook Inlet SE Alaska. To date there are only natural oil seeps on
land and beneath the sea but so far no productive wells. There has been a defacto
moratorium on exploration since 1969, but recent interest in opening this back up from
both industry and government. Because of ongoing subduction, the western (outermost)
side of the Insular belt is being uplifted, while the inland eastern edge is subsiding. The
westerly uplifts are due to compression and doubling up the curstal thickness. The
easterly subsidence coincides with a steepening in the dip of the subducting slab. This
hinge line runs from the Western side of Puget Sound, along the western edge of Georgia
Strait. To the west the rocks are folded, faulted and uplifted with Mesozoic and older
rocks exposed at the surface. To the east, it is a deep body of water with several hundred
metres of glacial depsosits over about 2 km of Tertiary strata having only minor
deformation.
The Coast Belt:
The Coast Belt is dominated by the Coast Mountains, which run from Vancouver to
Alaska. In Canada this is not strictly the west coast, but rather the western side of the
inner coast eastwards through the mountains that adjoin them. Further South in
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Washington and Oregon the equivalent mountain belt is called the Cascades. The Coast
Mountains and the Cascades are both underlain by Paleozoic and Mesozoic rocks in a
series of eastwards directed thrust sheets with intervening folds. While you might think of
the Cascades Mountains as being our young continental margin volcanic arc, these
subduction related volcanics represent only a small percentage of the total volume of the
Coast Belt mountains (Coast Mountains and Cascades). These belts are quite distinctive
in postion, elevation and geology from the Coast Ranges of Oregon and California which
lie along the west coast further south and are all comprised of Eocene to Oligocene
seafloor and overlying sediments. In the Coast Belt the highest peaks are in the Central
Coast at Mt. Waddington and Silverthrone.
The bedrock geology is dominated by plutonic suites and metamorphic assemblages that
are mainly Mesozoic in age but include Tertiary and younger plutons as well. Much of
this bedrock has been uplifted from the mid to lower crust, although there are a few
panels of low grade rocks and roof pendants in some of the batholiths that are equivalent
to rocks of the Wrangellia allochthon. East of Prince Rupert there is a single Eocene
pluton called the great Tonalite Sill (Diorite) that is 5 km thick. This marks the addition
of a magma body 5 km thick to the lower crust here. This is tantamount to increasing the
crustal thickness by 10-15% in a single intrusive event. The Cascades volcanic Arc in
Oregon and Washington has a series of major strato-volcanoes up to 3,500 m spaced
about every 50 km from Mt Shasta in northern California through Mt. Baker near
Bellingham. While we have Quaternary to Recent volcanics north of the border too
(Garibaldi, Meagher, Cayley, Salal), these are only thin veneers of volcanic deposits over
much older bedrock. This is not due to any great change in arc activity but is mainly due
to the fast uplift and erosion in our mountain belt coupled with intense Quaternary
glaciations that removed all but the feeder dykes for older volcanic piles.
The Coast Belt to the north of Prince Rupert (in SE Alaska) is unique in the whole of the
cordillera, in having some rocks with inherited Precambrian zircons. This is a conundrum
as only rocks adjacent to old continental shields exhibit this. This might be due to an
older age or layering of the crust there or perhaps to its proximity to Ancestral North
America. Otherwise there is no continental crustal affinity for this dominantly Mesozoic
welt of crust. Because of the thickness and high metamorphic grade (Garnet +
Sillimanite) particularly in the Coast Belt, it is thought that this crust was enormously
thick in Cretaceous time and represented an Andean type margin. But all of this former
thickness has been reduced by crustal extension since the end of Farallon Plate
subduction by rifting from there through the Queen Charlotte Basin and profound local
erosion that removed most of the supercrustal strata preceeding the Miocene to modern
uplift of the Coast Belt.
Just the same as magnetic minerals having blocking temperatures below which they retain
their magnetization, so also do datable minerals have specific blocking temperatures for
the retention of their daughter isotopes. Each mineral has a different blocking temperature
eg. Hornblende (450) > Biotite (235) > Apatite (~ 100) especially for the retention of
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fission tracks due to alpha particles from U or Th decay. By collecting a suite of minerals
form different elevations through a granitic batholith it is possible to date the uplift age by
this method rather than the primary cooling age one would get from proportions of
daughter isotopes. It is also possible to estimate past geothermal gradients during the time
of uplift. This has been done by Randy Parrish and Marcos Zentilli and their students for
a variety of localities. The surprise is that the Coast Mountains are relatively young and
seem to have been formed only in the last 10-12 Ma. This is a factor of 5 or more younger
than the Rockies or 100 or more times younger than the Appalachians. As a consequence
we now understand many of the coastal rivers to have been lowered through the Coast
Mountains during their uplift but that the rivers themselves (Stikine, Iskut, Skeena, Fraser
etc.) are appreciably older as physiographic features. Not only do they cut declivitous
canyons (enlarged to fjords) through the Coast Mountains, but these old rivers carried
distinctive lithic sediments into coastal basins like the Queen Charlottes. As a final
indication of the youth of the uplift in the Coast Belt, there are Miocene volcanic strata on
the Eastern side of the Coast Mountains that are tilted down to the East by the young
uplift in the last 10 Ma.
The Intermontane Belt:
The Intermontane belt is a losenge shaped region of sediment and volcanic filled
lowlands and plateaux that makes up about 40% of the interior of BC. It extends from the
Stikine in the north to the Okanagan in the south and from the Eastern slopes of the Coast
Mountains to the Rocky Mountain Trench and the Western slopes and foothills of the
Kootenays and Ominecas. This belt includes the flat lying or low relief forested and ranch
country of the Nechako, The Chilcotin, the Cariboo and the Okanagan regions. The
basement architecture is a series of oceanic tectonostratigraphic terranes, which from west
to east are called: Stikinia, Cache Creek Terrane and Quesnellia. The bedrock geology
is dominantly formed of Upper Paleozoic marine sediments: limestones, cherts, turbidites
and shales, Upper Triassic basaltic volcanics (Nicola, Skolai etc.) and Lower Jurassic arc
volcanics (Hazelton Group). The Lower Jurassic marine volcanics are particularly
important as they are the host to all of the major volcanogenic massive sulfide (VMS)
deposits in the cordillera (Kutcho, Eskay Creek etc.). This welt of 3 terranes was accreted
to BC in Middle Jurassic time as one large block or allochthon that is called Superterrane
I. In north central BC between Smithers and the Stikine, the large Jurassic Bowser basin
formed, collecting clastic sediments from the first uplift of the Rockies a few hundred
Km to the east (Omineca Mountains). Since the accretion of Superterrane I, there have
been a series of arc related Middle Jurassic and Cretaceous plutons and Cretaceous
volcanics that mark the time of Farallon Plate Subduction, causing crustal thickening and
the transformation of this oceanic material into more Continental style crust. These large
batholiths formed the many porphyry copper and molybdenite deposits of B.C.'s interior.
The Cretaceous volcanics (Tip Top formation) in particular indicate deep mantle sources
with Garnet and Amphibole, implying a very thick, Andean style crust (~80 km or more).
This is to be expected from the incredibly fast Farallon plate subduction with rates up to
28 cm a year (this is not a typo)! The birth of a new spreading ridge and the Kula Plate
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offshore transformed the Farallon subduction margin into a Kula plate subduction and
transform margin by Lower Eocene. This greatly reduced convergent stresses across the
northern half of the cordilleran margin and the crust relaxed and "flowed out" westwards
causing many extensional basins and major right lateral strike slip faulting along the
Cordillera. The subduction (or over riding) of the easterly directed Kula Farallon Ridge
by the Cordillera, created a series of "no-slab" windows where hot asthenosphere came
right up to the base of the crust. This eroded the crust from below by partial melting and
warmed the crust so that it extended and thinned down to less than 30 km thickness and
exposed the Mesozoic volcanic greenstones and greenschist facies Paleozoic sediments
over widespread areas. The formation of many elongate grabens, half grabens,
rhombochasms and strike slip basins permitted structural lows to collect and preserve
mixed extensional continental volcanic suites and continental sediments including some
economic coal deposits along the length of the belt, and stream gravel hosted "roll front
style" Uranium deposits in the Okanagan Kettle River region. This warmed crust and
oblique shear continued crustal thinning in the Intermontane Belt throughout the Miocene
to Recent and gave rise to large regions of extensional flood basalts of the Chilcotin,
Nechako and Stikine regions and isolated centres of long lived (15 Ma - 1 Ma) alkaline
within-plate shield volcano complexes (Itchas, Ilgachuz, Rainbow Range in the Cariboo
and Hoodoo Mtn., Edziza, Level Mountain and Heart Peaks in the Iskut and Stikine). To
give you an idea of the size of some of these features, Level Mountain is 100 km across,
comparable the hotspot volcanoes like the big island of Hawaii. There are also many
mantle derived small basaltic lava flows and cinder cone fields sprinkled the length of the
belt with more than 178 eruptive centers between the Okanagan and the Stikine many of
these erupted in the last 10,000 years since the end of the last ice age.
Omineca Belt:
The Omineca Belt exposes metamorphic rocks from greenschist to upper amphibolite
grade in a series of gneiss domes like at Three Valley Gap, Frenchman's Cap etc. The
tectonostratigraphic terranes include the Kootenay Terrane in the south, the Slide
Mountain and the Yukon-Tanana Terrane in the north. The original stratigraphy ranges
from Proterozoic (Late Precambrian) through Mesozoic but much of this has been
metamorphosed and uplifted during the Eocene. Inhereted detrital zircons show clear
affinitiies with the adjacent Precambrian basement rocks of the North American Craton.
Because of this provenance tie to North American basement, this belt is considered to be
peri-cratonic, i.e. next door to the craton. The inference is that while these rocks may be
structurally disrupted and moved tens or even a few hundred kilometers, that they
originated on or close to the continental margin. There are extensive Proterozoic marine
sedimentary rocks like those than that make up the bulk of the Windermere and Purcell
mountains. Three formations in particular, a red argyllite (Apekunny), a green argyllite
(Grinelle) and a dark limestone (Siyeh) extend for hundreds of kilometers along the NW
trending strike. Particular turbidite horizons are the host to massive sulfide deposits (Cu,
Pb, Zn) like at Sullivan near Trail. This was B.C.'s longest operating and most productive
mine and the site of the only working smelter. A particularly pronounced valley is called
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the Rocky Mountain Trench in the south and the Tintina Trench in the north but its origin
is still being debated as to whether it is merely erosional or partly tectonic. Ore deposits
in this belt incluse metamorphic types ranging from Uranium in metavolcanics, tungsten
skarns, Columbite-Tantalite in alkaline intrusions in the Wolverine, Beryl (emerald) and
sedimentary exhalative barite in the Yukon.
Foreland Belt:
The Foreland Belt is the fold and thrust belt of the Rocky Mountains. There is a
dominantly westwards dipping stack of thrusts that involves Proterozoic Rocks in the
Main Ranges and the west side and Paleozoic through Mesozoic rocks on the east side of
the mountains. The physiography is classic for fold and thrust belts with elongate
mountain ridges held up by tough, erosion resistant rocks like limestones and sandstones
separated by valleys underlain by softer more erodable shales. The streams all run either
NW or NE, either along or across strike in a classic trellis drainage pattern. Often the
stack of thrusts superposed 3 sequences of the same stratigraphy. This was accomplished
by E-W convergence during the Farallon Plate subduction. The mountain building event
certainly spanned the Cretaceous and Lower Eocene. The structural style is more thin
skinned with listric thrusts in the south and more thick skinned and blocky often
involving basement blocks in the north. Corresponding to the bend in the coastline of
Alaska and the position of the thickened Omineca Belt in northern BC and the Yukon, the
Foreland belt extends across the Mackenzie Mountains into the Northwest Territories
nearly to Great Bear Lake. this is more than 1330 km from the present coastline, with
nearly all of the additional thickness to the Cordilleran Orogen being taken up by the
Foreland Belt. There are abundant structural plays for oil and gas deposits hosted in
Paleozoic and Mesozoic strata. These include both conventional anticline plays, roll overs
in thrusts, pinch outs against faults and ramp style duplexes. Many of these potentially
productive structures lie in Provincial and National Parks. There are also Cretaceous coal
deposits below massive thrusts and in folded sections from Fernie and the Crowsnest Pass
in the South to Tumbler Ridge in the north.
The Craton andWestern Canada Sedimentary Basin:
In the basement below the sedimentary cover rocks of the Western Canada Sedimentary
Basin, there are a series of NE striking 200-400 km wide Precambrian basement terranes
that include many Archean domains under Alberta (Rae, Slave, Hearne) and Proterozoic
Domains (Hottah) further North beneath NE BC and the Northwest Territories.
Apparently the North American Continent grew by layers (of accreted terranes) towards
the NW in the Precambrian. By Late Proterozoic the continent was stable, drifting and
apparently far from any disrupting tectonic influences such that by about 880 Ma it had
eroded flat forming a continent wide peneplain. In the far west (now about the Alberta B.C. border) and in the far north against the Yukon, deep water passive margin sequences
were deposited. The continent rode below sea level for much of Paleozoic time and
formed massive limestone and shale basins. The Devonian limestones are the main reef
7
formers and oil host rocks below the Alberta and Williston Basins. (Things have never
been the same in Alberta since the discovery well at Leduc #1.) These older limestones
are also hosts to Pine Point and Missisippi style Pb-Zn deposits.
Things really picked up by Middle Jurassic with compressional shortening, the
beginnings of the formation of the Rockies and terrane accretion that added most of B.C..
This caused uplift in the Rockies, shedding sediments into the interior of the Continent
from Jurassic through Eocene. Many of these strata are continental or deposited in a
shallow interior seaway that extended from the Gulf of Mexico to the Arctic on top of
North American Precambrian basement rocks. There are coal basins, sinuous riverine
sands that localize shallow gas deposits and also migrated and biodegraded heavy oils
that make up the Tar Sands of Northern Alberta. While the Cretaceous Volcanic Arc
reached as far east as SW Montana, the Crowsnest Pass and the Southern Yukon,
volcanic ash deposits occur widespread across much of the western Canada Sedimentary
Basin. This is the easterly evidence of the West coast active tectonics. By contrast,
despite looking for distal equivalents to the Paleozoic and Triassic volcanism of BC, none
are found. This corroborates the terrane accretion story for the Cordillera.
The Craton has thick cold Precambrian lithosphere, like a keel well down into the
mantle. In Eocene while there is abundant volcanism in the Cordillera, there are only
isolated tiny kimberlite pipes in Alberta, Saskatchewan and the Northwest Territories.
While these igneous bodies are both rare and tiny (< 1km across). they come from great
depths, circa 160 km and carry up diamonds as accidental inclusions in the peridotites
carried up from the garnet and diamond stability field deep in the Upper Mantle.
Apparently the source for all the productive diamonds is in the subducted roots or former
Archean oceanic plates that build the lithosphere below the North American Continent.
Tectonostratigraphic Terranes and Terrane Accretion:
While the rest of the geological research world studied the ocean basins or other regions
of continental geology, a revolution in thought was happening in western Canada. The
geologists who worked in the Cordillera were forced to understand and assemble maps
where one side of the map had totally different stratigraphy and structural sequences than
the other. It made for complicated correlations or no correlations at all. None of the
familiar stratigraphy of North America was present but new oceanic successions of
volcanics and sediments were described. Large regions of batholiths like the Andes or
Indonesia were found and studied. Soon paleontologists started turning up asian and
tethyan faunas instead of North American ones. The paleomagnetists like Ted Irving,
boldly started talking in terms of displacements of thousands of kilometers. Jim Monger
systematically went through everybody's stratigraphy and started redefining it in terms of
correlatable and non-correlatable tectonostratigraphic terranes. The US geologists in
Alaska and California and workers in western Mexico were coming to grips with many of
the same problems. A new paradigm was born that explained much of the disparate
geologic history and sequences in terms of accreted terranes on active tectonic margins.
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This new theory seemed to fit so well that Precambrian geologists working in shield areas
reexamined their own geology in terms of different terranes. This is now the new
paradigm for how continents grow by continentalizing added pieces of oceanic crust,
island arcs, oceanic plateau and rifted microcontinental fragments. This same concept has
been applied to the mountain building in the Appalachians and Caledonides of the
Atlantic margins.
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