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by C. Hart The Yukon Territory occupies the northern portion of a
large geologic (and physiographic) province known as the Cordillera. This
province is composed of relatively young mountain belts that range from
Alaska to Mexico. Like most of the Cordillera, Yukon is composed of a
diverse array of rock types that record more than a billion years of geological
history. Most of the rocks have been affected by folding, faulting, metamorphism
and uplift during various deformation events over at least the last 190
million years. This deformation has resulted in a complex arrangement
of rock units and the mountainous terrain we see today. In Yukon, there
are two main geological components which are largely separated by a major,
northwest-trending fault (the Tintina): 1) the northeastern region is
composed of a thick, older sequence of sedimentary rocks which was deposited
upon a stable geological basement; and 2) the southwestern region is composed
of a younger, complex mosaic of varying rock types that amalgamated and
accreted to the stable sedimentary package. This paper briefly describes the geological framework of
Yukon south of 65 degrees N and, with some exceptions, uses the Tectonic
Assemblage Map of the Canadian Cordillera (Wheeler and McFeely 1991) and
the Terrane Map of the Canadian Cordillera (Wheeler et al. 1991)
as a foundation. However, some of the names used on these maps have been
superseded by new terminology and they are included in this paper. Recent
brief syntheses of Yukon physiography and geology are rare (Tempelman-Kluit,
1979; 1981), although geological compilations of Cordilleran geology are
numerous and contain useful information about Yukon geology (Monger et
al., 1982; Monger, 1989; Gabrielse and Yorath, 1992).
The Canadian Cordillera is composed of five northwest-trending
morphogeological belts that are parallel to the continental margin: from
west to east they are the Insular, Coast, Intermontane, Omineca and Foreland
belts (Figure 1). Together these five belts form the Cordilleran continental
crust which varies from less than 3 km thick in the west to 50 km thick
in the east. These belts reflect the sum of geological processes which
interacted over the past billion years to produce the geological framework
of Yukon. Each belt is different due to the different rock types contained
and the different geological history, as well as the varied effects of
climate and glaciation. The close ties between geology and physiography
has led some to call these morphogeological belts. Generally speaking,
younger mountain belts are more topographically extreme. The age of a
mountain belt refers to the timing of its uplift and not the age of the
rocks. The Insular Belt comprises very high,
huge and craggy mountain ranges that are composed of mainly volcanic and
sedimentary rocks of oceanic origin. Much of the Insular Belt has been
tectonically uplifted during the last 15 million years at a rate of approximately
3 cm/year, or 3 km every million years. Rapid uplift combined with several
periods of extensive glaciation, as well as erosion in the past 1.6 million
years have resulted in the extreme topography we see today. The rugged, high relief, steep-sided mountains of the Coast
Belt are mainly composed of granitic rocks. These rocks are rich
in silica which is resistant to weathering, however well-developed fractures,
or joints which are typical of granitic rocks result in steep or vertical
mountain sides. Like the Insular Belt, the Coast Belt has also experienced
dramatic tectonic uplift, but about 50 million years ago. Coast Belt topography
has been modified by numerous glacial events, and more recently by excessive
precipitation which has steepened the mountain sides by intensive gully
erosion. The Intermontane Belt is characterized by
subdued relief and rounded or flat-topped mountains with long, straight
slopes. This character is largely the result of the recessive nature of
the sedimentary rocks composing this belt, and slow and continuous erosion
over most of the past 100 million years. This region has also not experienced
the tectonic uplift that affected the Insular and Coast Belts. The Omineca Belt is the most complex and
varied in Yukon and is composed of variably metamorphosed sedimentary
rocks and granites. Most of this belt contains large mountain ranges with
localized centres of high mountains called massifs. However, the northwestern
part of this belt is characterized by low, rolling, heavily vegetated
hills. This area was not reached by northward advancing glaciers. Consequently,
millions of years of erosion has established a thick cover of soil and
weathered bedrock. Where glaciated, this cover has been scoured away and
exposed craggy bedrock. Massifs in the glaciated portion are usually centred
around granitic intrusions whose heat cooked and hardened the surrounding
sedimentary rocks. The boundary between the Omineca and the Foreland Belts
is defined by the easternmost exposures of granitic rocks. The Foreland Belt contains long, linear
ranges of mountains composed entirely of sedimentary rocks. Unlike the
Intermontane Belt, the Foreland Belt rocks have been affected by a period
of deformation that stacked and thickened the sedimentary rocks along
numerous, generally northwest-trending, folds and thrust faults. The larger
faults constitute zones of weakness in the rock which easily erode and
give rise to long linear valleys (e.g. upper Hess, Stewart, Bonnet Plume,
Wind and Snake Rivers) between the mountain ranges.
Yukon's geology divides into two essential components that
are, for the most part, separated by the Tintina Trench. Rocks northeast
of the Tintina Trench are old (>1000 to 300 million years), mainly
sedimentary and represent the Ancient North American margin. Rocks
southwest of the Tintina Trench are mostly young (350 to 20 million years
old), mainly igneous and metamorphic, and represent numerous crustal fragments
called accreted terranes whose place of origin is uncertain. During
most of the Yukon's geological history, the terranes were not attached
to North America, but were accreted to the western margin of Ancient North
American between 190 and 120 million years ago. Rocks in the zone between
the accreted terranes and Ancient North America have been extensively
deformed and form a belt known as the Teslin Suture Zone. This
belt has subsequently been cut by the Tintina Fault which has caused some
complexity in this region. Ancient North America Prior to 190 million years ago, the western edge of the
Ancient North American continental craton extended far out into the ancient
Pacific Ocean. This submerged continental shelf is composed of crystalline
basement rocks (akin to the Canadian Shield) that are at least 1.7 billion
years old. These rocks provided a stable continental platform upon
which sediments, dominantly limestone and sandstone accumulated for over
a billion years. Shale, sandstone and chert accumulated in regions of
deeper water known as basins. These two different depositional
environments (platforms and basins) gave rise to differing packages of
rocks characterized mainly by limestone and shale, respectively. Today,
millions of years later, these limestone and shale packages are largely
in fault contact with each other. Sediments deposited on the platform formed a thick succession
of rocks that are now exposed in the Mackenzie and Cassiar Mountain Ranges.
The Mackenzie and Cassiar Platforms (Figure 2) accumulated between 5 and
25 kilometres of mainly limestone and sandstone over a one billion year
period. The limestone accumulated during quiescent periods in warm, shallow
and clear water. The sandstone accumulated from detritus that eroded from
exposed rocks of the Canadian Shield craton which were carried west by
ocean currents. Each platform is composed of rocks of the Wernecke Supergroup
and the Mackenzie Mountains Supergroup. These two thick stratigraphic
packages include the Mackenzie, Purcell, Wernecke, Windemere, Rapitan,
Pinguicula and Gog assemblages. Although parts of the Cassiar Platform
are west of the Tintina Trench, the rocks there are so similar to those
on Ancient North America margin that it is certain they are North American
in origin but have been displaced along the Tintina Fault. On some maps
the displaced fragments of North American continental margin are called
the Cassiar and Dorsey Terranes (Figure 3). The Selwyn Basin and Richardson Trough were two major basins
that formed within the platforms. Because the basins create regions of
much deeper water, limestone growth is impossible and the currents which
move the sands that form sandstone are not as strong. Instead these basins
slowly accumulated muds and biogenic silica that later formed successions
of black shale and chert. These shale basins existed from about 800 to
320 million years ago. Rock units in the basins are dominated by: the
sandstone, maroon and green shales and rare marble of the Hyland Group;
the chert and black shales of the Road River/Rocky Mountain Group and
the black shale and chert-pebble conglomerate of the Earn and Imperial
Groups. The black shales host numerous deposits of zinc-lead-silver and
barite such as those at Faro and Macmillan Pass. Accreted Terranes Southwest of the Tintina Trench there exists a mosaic of
rock packages that are different from each other and are separated by
faults. Individual rock packages or crustal fragments are known as terranes.
Most terranes are different from rocks of the Ancient North American margin
and their place of origin is uncertain. These are called suspect
terranes. Some terranes are similar to Ancient North American rocks but
cannot be absolutely correlated -- these are called pericratonic
terranes. Other terranes have features that indicate that they formed
in an environment totally unlike that of Ancient North America -- these
are called exotic terranes. The one thing that all of these terranes
have in common is that they were accreted to the ancient western margin
of ancient North America - consequently they are all called accreted
terranes. The Yukon is composed of ten of these terranes (Figures 2 and
3). Geological evidence further suggests that several of these terranes
may have amalgamated with each other prior to their accretion to Ancient
North America. These groups of terranes constitute composite and
superterranes -- three of these are found in Yukon. Yukon-Tanana Composite Terrane The Yukon-Tanana Terrane is a name that was not included
on recent compilation maps but is preserved here because of its common
and continued usage in Yukon. The Yukon-Tanana Terrane is the largest
of Yukon's terranes, covering a large portion of the Omineca Belt, and
extending into adjacent Alaska and British Columbia (Figure 3). The Yukon-Tanana
Terrane is composed of several metamorphic rock assemblages -- from oldest
to youngest they are the Nisling assemblage (or Terrane), the Nasina assemblage,
the Pelly Gneiss and the Nisutlin assemblage. Each of these components
appear to have been deposited upon one another during this Terrane's 500
million year long history. The Nisling assemblage is a metasedimentary package composed
of quartzite, quartz-mica schist and marble that is at least 400 million
years old but may be as old as a billion years old. The 400-320 million
year old Nasina assemblage rocks are also dominated by quartzite and schist,
but contain large amounts of carbon that make these rocks black, or graphitic.
The Pelly Gneiss and Nisutlin assemblage are composed of 350 to 250 million
year old granitic and volcanic rocks respectively, that have been subjected
to heat and pressure which has deformed and metamorphosed them. The Pelly
Gneiss still retains its granitic composition but is strongly foliated
and locally displays mineral banding. The metamorphism has turned the
Nisutlin assemblage into a light green quartz-mica schist package that
underlies the Klondike goldfields and is known as the Klondike schist.
The complexity of the Yukon-Tanana Terrane largely results from the diversity
of rock types and the numerous metamorphic events it has undergone throughout
its long history. The metamorphism is locally of extremely high temperature
(650 degrees C) and high pressures that correspond to crustal depths of
approximately 25 kilometres. Most of the metamorphic rocks that comprise Yukon-Tanana
Terrane were originally sedimentary rocks. The stratigraphy, or the order
in which the different sediments were deposited, is similar to that of
rocks on Ancient North America. This has resulted in the Yukon-Tanana
Terrane's assignment as a pericratonic terrane. However, the Yukon-Tanana
Terrane encloses, and is amalgamated with the terranes that comprise the
Intermontane Superterrane. This has led many geologists to include the
Yukon-Tanana Terrane as part of the Intermontane Superterrane. Intermontane Superterrane The Intermontane Superterrane is composed of five dissimilar
terranes that were amalgamated approximately 180 million years ago, including
Stikinia, Quesnellia, Slide Mountain, Cache Creek and Windy-McKinley. Stikinia is the largest terrane in the Cordillera but in
Yukon is restricted to the area of the Intermontane Belt. Stikinia is
composed of a linear belt of 220 million year old volcanic rocks of the
Lewes River Group. The volcanoes formed in an oceanic setting called an
island arc that is similar to present-day Japan. A seven kilometre thick
sequence called the Whitehorse Trough consisting of slightly younger (210-160
million years) sedimentary rocks was deposited in a marine basin adjacent
to the Lewes River arc. These rocks are mainly sandstone, conglomerate
and limestone of the Laberge and Lewes River Groups. These rocks are exposed
between Whitehorse and Carmacks, and include the limestone of Grey Mountain
and the conglomerates near Braeburn. The limestone unit hosts the copper
deposits of the Whitehorse Copper Belt. Quesnellia in Yukon is composed of volcanic rocks known
as Nikolai Group that are the same age and similar to those in Stikinia.
Although they cover a large area in British Columbia, Quesnellia in the
Yukon is represented by only a few small fragments east of the Teslin
River. Slide Mountain, Cache Creek and Windy-McKinley Terranes
are composed of volcanic rocks that formed on the oceanic sea floor, as
well as overlying successions of chert, limestone and shale. These terranes
are similar in age and range from 320 to 190 million years old. Both Slide
Mountain and Cache Creek Terranes are thought to represent ancient oceans
that existed between other terranes. Slide Mountain Terrane represents
the up-thrusted remains of the ocean floor that once separated Quesnellia
from North America, whereas the Cache Creek Terrane represents the ocean
floor that existed between Stikinia and Quesnellia/Yukon-Tanana. Locally,
Slide Mountain, Cache Creek and Windy-McKinley Terranes have outcrops
of ultramafic rocks. These are rocks that are rich in iron and magnesium
and originally formed the base of the oceanic crust. Ultramafic rocks
that are faulted up and exposed on the land surface contain the Clinton
Creek and Cassiar asbestos deposits as well as forming the Midnight Dome
at Dawson City. Sedimentary rocks of the Cache Creek Terrane contain a
particular assemblage of fossils that are found in Asia and not in Ancient
North American rocks. This suggests that the Cache Creek Terrane likely
originated far from North America and may have existed on the other side
of the Pacific Ocean. It is therefore considered exotic. These
Cache Creek limestones form the large white mountain across from Jakes
Corner and Bove Island. Insular Superterrane The Insular Superterrane is mainly composed of two older
terranes that were amalgamated by 320 million years ago -- they are Wrangellia
and Alexander Terrane. Both of these terranes are composed of island arc
and ocean floor volcanic rocks with thick assemblages of overlying oceanic
sedimentary rocks that range in age from 400 to 220 million years old.
Wrangellia in particular, has a several kilometre thick package of platform-type
limestones. The Insular Superterrane hosts a 230 million year old package
of volcanic rocks (the Nicolai Group) that contains the Windy Craggy copper-cobalt-gold
deposit in northernmost British Columbia and the Wellgreen nickel-copper-platinum
deposit near Burwash. The Chugach and Yakutat Terranes are not part of
the Insular Superterrane, but are within the Insular Belt. These two terranes
are composed of young (20-90 million year old) sedimentary rocks that
were originally deposited on the floor of the Pacific Ocean. These sedimentary
rocks were subsequently scraped off of the ocean floor by tectonic processes
and accreted onto the western margin of North America. Overlapping Assemblages Numerous rock packages were deposited for the large part
after all the terranes were amalgamated and accreted to North America.
These overlapping assemblages of rocks are not specific to one
terrane but are found on or in two or more terranes, or on Ancient North
America. These post-accretionary assemblages could be felsic plutonic
(igneous) rocks that intrude two or more terranes, or volcanic or sedimentary
rocks that are deposited on, or across two or more terranes. Felsic Plutonic Rocks Many felsic plutons intrude into Yukon's basement rocks
(the assembled terranes and Ancient North America) in all morphogeological
belts, except for the Foreland Belt. These rocks vary in composition from
granite to granodiorite, to quartz monzonite to diorite to syenite but
in this summary are coined as "granites". The largest concentration
of plutons occurs in the Coast and Omineca Belts. The Coast Plutonic Complex takes up most of the Coast Belt.
It is not a terrane, but rather a linear belt composed almost entirely
of felsic plutons. The plutons of this vast granitic region are bounded
to the east by western margins of Stikinia and the Yukon-Tanana Terrane,
which they intrude, and are truncated on their western margin by the Denali
Fault. The Coast Plutonic Complex ranges in age from 185 to 55 million
years with most of the older rocks along its eastern margin. The western
margin of the Coast Plutonic Complex experienced a tremendous amount uplift
about 50 million years ago that has exposed rocks that were previously
20 km deep in the crust. These rocks are exposed near Haines Junction
and Skagway, Alaska. The remaining felsic plutonic rocks are grouped into suites
according to their age and composition. Granitic plutons of the Klotassin
suite are approximately 210-180 million years old and intrude the Yukon-Tanana
Terrane and western Stikinia. Granitic plutons of the St. Elias suite
are approximately 130 million years old and underlie parts of the St.
Elias and Icefield Ranges in the extreme southwestern part of the territory.
Granitic rocks of approximately 120-65 million years of age comprise several
plutonic suites that range from the Kluane Mountains across all the terranes
of the Yukon into Ancient North American rocks as far east as the Northwest
Territories. This most widespread and voluminous age of granite pluton
formation includes the Kluane, Whitehorse, Cassiar, Surprise Lake Tombstone
and Selwyn plutonic suites. These plutons are important since they are
responsible for the formation of numerous deposits of copper, gold, molybdenum,
tungsten and tin including Mactung, Logtung and Cantung tungsten deposits,
Red Mountain molybdenum deposit, Casino Copper, Brewery Creek and Dublin
Gulch gold deposits as well as the deposits of the Whitehorse Copper Belt.
Plutons of approximately 55 million years in age are common in the Coast
Plutonic Complex and western Yukon-Tanana Terrane and locally host copper-molybdenum
mineralization but are rare in eastern Yukon. Volcanic Rocks There are four main packages of post-accretionary volcanic
rocks. Mount Nansen Group rocks are about 100 million years old and are
sporadically located across Stikinia and the western Yukon-Tanana Terrane.
Rocks of the similar aged South Fork volcanics form huge caldera complexes
on Ancient North America north of Ross River. The younger Carmacks Group
(75 million years old) forms numerous thick successions of volcanic rocks
along the contact between Stikinia and Yukon-Tanana Terrane and through
the Dawson Range northwest of Carmacks. This volcanic event is responsible
for much of the mineralization in the Dawson Range including the Laforma
gold veins and the huge Casino copper-molybdenum-gold deposit. The 55
million year old Skukum Group forms discreet volcanic calderas that occur
in a linear array from the south end of Atlin Lake (in British Columbia)
to Bennett Lake then to Aishihik Lake. This group of rocks hosts the Mount
Skukum gold deposit southwest of Whitehorse. The Yukon's youngest volcanic rocks are the less than 10
million year old Fort Selkirk, Miles Canyon and Tuya basaltic lavas that
occur near Fort Selkirk, Whitehorse and Watson Lake, respectively. One
of the youngest volcanic events represented in the Yukon is so young that
it is not even a rock yet. The thin strip of white ash that is common
near the top of road-cuts in western Yukon is the White River Ash. This
ash resulted from a volcanic explosion in the St. Elias Range near the
Yukon-Alaska border about 1250 years ago. Lavas near Fort Selkirk were
formed even more recently, during the early 19th century. Sedimentary Rocks There are few occurrences of young (<150 million years
old) sedimentary rocks in the southern Yukon. The Dezadeash Formation
is composed of a thick succession of muddy sandstone called greywacke
that was deposited in a huge submarine fan about 150-100 million years
ago. The Front Ranges, as seen from Haines Junction, are composed of these
rocks. These rocks stretch from north of Haines, Alaska, northerly to
Dezadeash Lake and Haines Junction and were deposited on the Insular Superterrane
and the Yukon-Tanana Terrane. The Tantalus Formation occurs in small isolated
exposures that range from south of Whitehorse to Carmacks and to just
south of Dawson. These exposures are mainly of quartz-rich sandstone and
conglomerate and host the Whitehorse, Division Mountain, Tantalus Butte
and Haystack Mountain coal deposits. Tantalus Formation rocks range in
age from 140 to 60 million years old and are deposited on Stikinia, Quesnellia
and Yukon-Tanana Terrane. Unnamed, localized deposits of conglomerate
were deposited within the Tintina Trench about 55 million years ago. These
deposits also host coal deposits most notably near Dawson and Ross River.
The youngest package of sedimentary rocks in the Yukon are the Amphitheater
Formation conglomerates and sandstones that occur in three main areas:
near the upper White River, Burwash and Dalton Post. These 25 million
year old rocks also contain deposits of coal but because of their youthful
age the coal is low grade lignite. Faults There are two major faults which extend across the Yukon
(Figures 2, 3). Tintina Fault The long, linear depression that extends northwesterly
across the Yukon from Watson Lake along to Ross River, Faro and Dawson,
and then into Alaska is the Tintina Trench. It is the northern continuation
of the Northern Rocky Mountain Trench in British Columbia. The Tintina
Trench is the physiographic expression of the Tintina Fault. Tectonic
forces caused the block of rocks southwest of the fault to grind up against
the stable North American block and, during a history of innumerable earthquakes,
moved the southwestern block northwest towards Alaska. The grinding along
the fault caused the rock to break up and become less resistant which,
with erosion led to the formation of the trench. Most geological evidence
suggests that there was at least 450 km of right-lateral displacement
(area southwest of the fault moved northwest) along the Tintina Fault
although there may have been as much as 1200 km offset. Volcanic rocks
were deposited in the trench about 55 million years ago - probably at
the same time as some of the motion along the Tintina Fault. These volcanic
rocks host the Grew Creek gold deposit. Denali Fault The Denali Fault originates (on land) at Haines, Alaska and continues north into the Yukon to the south end of Kluane Lake, and further northwest into eastern Alaska. This fault, and the associated Duke River Fault, are still active and cause a steady stream of small earthquakes. The Denali Fault separates the very high mountains of the Insular Belt from the lower mountains east of the fault. The tectonic forces that are causing uplift in the Insular Belt are also responsible for displacement along the Denali Fault. This transfer of force along the fault prevents the region east of the fault from being affected by the tectonic forces that form the high mountains. There has been at least 350 km of right-lateral offset along the Denali fault.
Gabrielse, H. and Yorath, C.J., 1992. The Cordilleran Orogen
in Canada, Decade of North American Geology #4. Geoscience Canada, v.
16, p. 67-83. Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J.,
1982. Tectonic accretion and the origin of the two major metamorphic and
plutonic welts in the Canadian Cordillera. Geology, v. 10, p. 70-75. Monger, J.W.H., 1989. Overview of Cordilleran Geology;
Chapter 2: In: B.D. Ricketts, (ed.). Western Canadian Sedimentary
Basin; Canadian Society of Petroleum Geologists, p. 9-32. Tempelman-Kluit, D.J., 1979. Evolution of physiography
and drainage in southern Yukon. Canadian Journal of Earth Sciences, Vol.
17, p. 1189-1203. Tempelman-Kluit, D.J., 1981. Geology and mineral deposits
of southern Yukon. In: Yukon Geology and Exploration 1979-80,
Exploration and Geological Services Division, Yukon, Indian and Northern
Affairs Canada, p. 7-31. Wheeler, J.O., and McFeely, P., 1991. Tectonic Assemblage
Map of the Canadian Cordillera. Geological Survey of Canada Map 1712A,
1:2 000 000 scale with legend. Wheeler, J.O., Brookfield, A.J., Gabrielse, H., Monger,
J.W.H., Tipper, H.W. and Woodsworth, G.J., 1991. Terrane Map of the Canadian
Cordillera. Geological Survey of Canada Map 1713, 1:2 000 000 scale with
legend.
Figure 1. Location of the five physiographic or morphogeological belts of the Yukon. The belts underlie regions that have similar geology or have undergone similar geological histories. The Yukon north of 65 degrees is underlain by the Foreland Belt. The lines of latitude and longitude define the 1:250 000 NTS grid.
Figure 2. The Yukon's major tectonic elements indicate that the territory is underlain by two dominant rock packages. Northeast of the Tintina Fault are a thick assemblage of sedimentary rocks that belong to the Ancient North American continental margin. They are platformal (mainly limestones) and basinal (mainly shale) in origin. Southwest of the Tintina Fault are numerous dissimilar crustal fragments called Terranes. The terranes were amalgamated into the Insular and Intermontane Superterranes prior to their accretion to the Ancient North American margin. The zone of deformation between the accreted terranes and Ancient North America is represented by the Teslin Suture Zone.
Figure 3. The numerous terranes that comprise the accreted terrane portion of the Yukon include fragments of oceanic floor, volcanic island arcs, oceanic basins and old metamorphosed crust. Since their accretion to North America over 120 million years ago, their positions have been complicated by displacements along numerous faults.
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