Manitoba Geology
The Province of Manitoba occupies 650 000 km2
and is underlain entirely by rocks of Precambrian age. Two-fifths of
the Precambrian basement are covered by sedimentary rocks deposited
during the Paleozoic, Mesozoic and Cenozoic eras.
Precambrian
The Precambrian Shield in Manitoba consists of Archean rocks of
the Superior Province in the southeast, Paleoproterozoic and
tectonically reworked Archean rocks of the Trans-Hudson Orogen in
the northwest, and a boundary zone on the north margin of the
Superior Province that is characterized by highly deformed gneisses,
a gravity high, and a distinct aeromagnetic signature and trend. The
geophysical features permit extrapolation of the boundary zone
underneath the overlying Phanerozoic rocks, east into Ontario, and
south into South Dakota.
The Superior Province, which is areally dominated by granitic
rocks, is subdivided into eleven major
lithostructural belts or domains (pdf, 300 kb). Each domain is
characterized by a distinctive rock suite, structural configuration,
and metamorphic grade. They range in width from 40-200 km and are
generally separated by a fault or boundary zone with a steep
metamorphic gradient. Locally, rock units can be mapped or
correlated across the boundary.
The domains comprise three major types: 1) high grade
orthogneisses, granitoid rocks and minor supracrustal rocks (Berens
River, Winnipeg River, Molson, Northern Superior); 2)
granite-greenstone belts (Gods Lake, Island Lake, Uchi, Bird River
and Wabigoon); and 3) metasedimentary rocks, their migmatitic and
anatectic derivatives, and various types of younger granitoid
intrusions (English River). These domains vary widely in age from
ca. 3.5 to 2.7 Ga and are currently interpreted to have been
tectonically amalgamated into a structurally coherent craton during
continent-continent collisions at about 2.7 Ga. Granitic domains,
such as Northern Superior, Berens River and Winnipeg River,
typically contain >3.0 Ga components and may represent Mesoarchean
protocontinents about which younger Neoarchean supracrustal domains
were amalgamated. The supracrustal rocks (Gods Lake, Island Lake,
Uchi and Wabigoon) typically consist of narrow, curvilinear,
isoclinally folded, east-trending belts of metavolcanic and
metasedimentary rocks that were intruded by ellipsoidal tonalite to
granodiorite masses. Metamorphism in the granite-greenstone domains
ranges from greenschist to amphibolite grade and is generally
related to the flanks of the subprovinces and to the granitoid
intrusions. Metasedimentary domains (English River) are typically
formed of detritus derived from bounding granite-greenstone and
granitic domains, and are characterized by amphibolite grade
metamorphism. The northeast trending Pikwitonei domain along the
northwest boundary of the Superior craton consists of orthogneisses
and granite-greenstone domains that were metamorphosed to granulite
facies mineral assemblages during or after the 2.7 Ga cratonization
event.
The exposed Superior Boundary Zone consists of three segments: the
north northeast-trending Thompson Nickel Belt, the Assean Lake
Domain, and the east-trending Fox River Belt. The Thompson Nickel
Belt consists of Archean gneisses of the Superior craton and
rift-related Paleoproterozoic cover sequences that were tectonically
reworked during the Trans-Hudson Orogen. The Assean Lake Domain
consists of a southern segment of Meso- to Paleoarchean rocks and a
northern segment of Paleoproterozoic gneisses and plutonic rocks
that were variably contaminated by Archean rocks. The homoclinal Fox
River Belt, which contains the >250 km long and 2 km thick mafic and
ultramafic Fox River Sill, is the largest known continuous section
of the ca. 1.9 Ga rifted margin rocks bounding the northern Superior
craton.
The Trans-Hudson Orogen is a major, >450 km wide Paleoproterozoic
orogen that extends from South Dakota, through western and
northwestern Manitoba, across Hudson Bay and into Northern Quebec,
Labrador, Baffin Island and Greenland. In the northwest it comprises
an ensialic component including variably reworked Archean crustal
segments (Mudadtik, Nejanalini) and tectonically reworked Archean
basement and Paleoproterozoic cover rocks (Wollaston, Seal River,
Great Island). In the south, it comprises a tectonic collage of
juvenile components, the Reindeer Zone, including ocean floor,
oceanic arcs, Andean-style magmatic arcs and related sedimentary
deposits (Chipewyan, Southern Indian, Lynn Lake, Kisseynew, Flin
Flon). The ten major lithostructural domains recognized in the
Trans-Hudson Orogen in Manitoba are characterized by a distinctive
association of supracrustal and intrusive rocks, range in
metamorphic grade and structural style. Strong metamorphic and
deformational events accompanied peak orogenic activity between 1.85
and 1.81 Ga.
Phanerozoic
Exposures of Paleozoic rocks generally are sparse in both the Hudson
Bay Lowlands and the Manitoba Lowlands, except locally along shores
of the major lakes and rivers and in the area north and west of
Grand Rapids. These Paleozoic strata comprise the northeastern flank
of the Williston Basin, a major sedimentary basin centred in
northwestern North Dakota. Within the outcrop belt, Paleozoic strata
dip gently to the southwest at 2 to 4 m/km. To the southwest, in the
subsurface, dips increase progressively towards the centre of the
Williston Basin, to as much as 10 m/km in the extreme southwestern
corner of the Province. Here, the total Paleozoic sequence attains a
thickness of 1200 m and reaches a depth of 2300 m. Paleozoic strata
consist almost entirely of dolomite, dolomitic limestone and
limestone with only minor argillaceous and/or sandy intervals; the
main exception is the basal sandstone-shale sequence of the Winnipeg
Formation. The Paleozoic formations are overlain with marked angular
unconformity by Mesozoic strata which rest on Mississippian beds in
the southwestern part of the Province and progressively overstep
older Paleozoic strata to rest directly on Precambrian basement in
the area southeast of Winnipeg. The apparent degree of truncation of
the Paleozoic sequence averages 3 to 4 m/km. The youngest Paleozoic
strata, of Upper Devonian to Mississippian age, are not exposed in
outcrop.
Several areas of local structural complexity are evident. Devonian
strata show a very irregular (and uncertain) outcrop pattern due to
erratic local structural relief of up to 90 m. This results from
salt solution and collapse, with draping over buried Winnipegosis
reefs. Other local but complex structural features include the Lake
St. Martin crater, the Highrock Lake structure, and the Denby
structure. The Lake St. Martin structure1 is probably of meteorite
impact origin and is approximately Permian in age. The Highrock and
Denby structures are indicated by Precambrian structural highs, and
may also be crater structures.
Industrial minerals products obtained from the Paleozoic formations
include petroleum (Mississippian); high-calcium limestone
(Devonian); dolomitic limestone for building stone (Ordovician);
dolomite (Silurian); and silica sand (Ordovician). In addition,
extensive deposits of salt and potash occur in the subsurface
(Devonian).
Paleozoic strata of the Hudson Bay Lowlands region dip gently to the
northeast, towards the Bay. Although information is sparse, core
hole data indicate that the dip increases progressively from about 2
m/km near the erosional edge to about 7 m/km at the shore of the
Bay. The maximum onshore thickness of Paleozoic strata is 884 m, but
the estimated thickness in the central part of the Bay probably
exceeds 1800 m, indicating that the Hudson Bay area was a major
depositional basin during much of Paleozoic time. Strata consist
mainly of limestone, dolomitic limestone and dolomite, except for an
upper Silurian succession of argillaceous and sandy clastic beds.
The formations of the Hudson Bay Basin are only partially
correlative with the Paleozoic sequence of the Williston Basin of
southwestern Manitoba.
Mesozoic beds dip gently to the southwest, towards the Williston
Basin. Dips increase progressively from 1 to 3 m/km. To the
northeast, however, several outliers or channel deposits appear
almost flat lying. Maximum thickness of Mesozoic beds in the
southwestern corner of the Province is approximately 1070 m. The
eroded Paleozoic surface beneath Mesozoic beds, shows considerable
paleotopographic relief with numerous scarps, erosional valleys, and
probable incipient karst development. As a result, numerous outliers
or channel-fill deposits of Mesozoic sediments occur within the
Paleozoic outcrop belt, the most prominent being the major channel
in the Dominion City.
A major erosional break also occurred in early Cretaceous time. This
erosional surface shows considerable local and regional relief, and
consequently the basal Cretaceous Swan River beds deposited on this
unconformity surface show marked variations in thickness, and an
irregular outcrop distribution. Cretaceous outliers also occur as
channel-fill deposits within the Paleozoic outcrop belt, such as
north of Arborg.
In contrast to Paleozoic strata, Mesozoic formations consist almost
entirely of shales and sandstones, with some limestone and gypsum
occurring in Jurassic strata. Siliceous, calcareous, and
carbonaceous (bituminous) shales are present, and several beds of
bentonite occur in the Pembina Member. Mineral products include
gypsum (Jurassic); bentonite (Cretaceous); and brick clay and shale
(Jurassic and Cretaceous).
Cenozoic (Paleocene) strata of the Turtle Mountain Formation are
limited to a relatively small isolated outlier capping the
topographic high of Turtle Mountain. These strata, consisting
primarily of fine sandy, silty shales, rest unconformably on the
sandstones of the Upper Cretaceous Boissevain Formation and are flat
lying or dip gently to the south.
Superior Province
Northern Superior
Northern Superior geochronological programs in the Archean
Northwestern Superior Province in Manitoba have provided insight
into the nature of the geological components and timing of events
that formed the northern flank of the Superior Province about 2700
million years ago. The exciting revelations are still interpretive,
and subject to further work, but point to an origin involving
continent-continent collision between 2730 and 2700 Ma.
This new model for the development of the northwestern margin of the
Superior Province is based on Nd isotopic data and U-Pb
geochronology of plutonic and supracrustal rocks from the
northwestern Superior Province. This data suggests that the northern
Superior Province is composed of three fault-bounded crustal
terranes that are, from south to north, the Munro Lake, Oxford
Lake-Stull Lake and Northern Superior:
- The Munro Lake terrane (Molson Domain) comprises mainly plutonic
rocks intruded between 2.84 and 2.72 Ga. The isotopic signature of
these plutonic rocks shows that they have recycled older crust,
probably Mesoarchean granitoid basement and <2.86 Ga platformal
sediments and komatiites of the reworked margin of the North Caribou
terrane (Island Lake and Berens River domains). For this reason, the
Munroe Lake terrane is suggested to be a product of recycling of the
older North Caribou margin and continental growth on its north
margin.
- The Oxford Lake-Stull Lake terrane (Gods Lake Domain) consists of
2.83 Ga submarine, depleted tholeiitic basalts, formed in a
predominantly juvenile oceanic environment, and an isotopically
juvenile, 2.73 Ga continental margin arc. The continental margin arc
is interpreted to have been formed during crustal accretion and
thrusting of the Oxford Lake-Stull Lake terrane over the Munroe Lake
terrane prior to 2.73 Ga.
- The Northern Superior superterrane (Northern Superior and part of
Pikwitonei Domains, also includes the Orr Lake and Split Lake
blocks), on the north side of the northwest-trending North Kenyon
fault, comprises mainly 2.84-2.71 Ga plutonic rocks that have much
older isotopic ages and contain inherited zircons as old as 3.57 Ga.
Docking of this reworked Paleoarchean crust with the Oxford Lake -
Stull Lake terrane resulted in continued 2.73-2.72 Ga arc volcanism.
Eruption of synorogenic <2.71 Ga alkaline and shoshonitic lavas and
subsequent deposition of continental sediments with a vast range of
detrital zircons that mimic the regional ages from all three
terranes (3.6 to 2.71 Ga), reflect amalgamation of the three
terranes during a ca. 2.7 Ga orogenic event.
Descriptions on Mineral Deposit types in the northern Superior
Province.
Southeastern Manitoba
Southeastern Manitoba consists of three granite-greenstone domains
(Western Wabigoon, Bird River, Uchi) separated by the
plutonic-dominated Winnipeg River domain and the metasedimentary-dominated
English River domain. The granite-greenstone domains contain
abundant Neoarchean supracrustal rocks including volcanic rocks of
the Lake of the Woods, Bird River and Rice Lake greenstone belts.
The Uchi domain is bounded to the north by the Berens River Domain
which, along with the Island Lake Domain, comprises the North
Caribou terrane, a potential Mesoarchean, ca 3.0Ga protocontinent
that is approximately 400 by 850 km in size.
The Wabigoon granite-greenstone domain, which is over 900 km long
and 150 km wide, is mainly exposed in Ontario, with the Manitoba
portion largely covered by Quaternary glaciogenic deposits and, to
the west, flat-lying platformal Phanerozoic strata. The Western
Wabigoon region is characterized by a series of interconnected
greenstone belts surrounding large elliptical granitoid batholiths
(e.g., Lake of the Woods area).
The Winnipeg River plutonic domain, which is up to 70 km wide and
400 km in length, is mainly exposed in Ontario with its western
extension covered by Phanerozoic cover rocks. This diverse suite of
granitic rocks and volumetrically minor supracrustal rock includes
both Mesoarchean and Neoarchean components.
The Bird River granite-greenstone domain includes the Bird River
greenstone belt in Manitoba and the Separation Lake greenstone belt
in Ontario. The Bird River greenstone belt is a diverse mafic to
felsic Neoarchean volcanic sequence that contains a distinctive
chromite- and PGE- enriched layered mafic-ultramafic intrusion, the
Bird River Sill. Geochemically evolved pegmatitic granites and
related rare earth element enriched pegmatites in the Bird River
granite-greenstone belt are host to world class Ta deposits as well
as reserves of Cs, Li, other metals and industrial minerals.
The English River metasedimentary domain is over 800 km long by 50
km wide with the western 90 km exposed in Manitoba. It comprises
highly metamorphosed and migmatized clastic sedimentary rocks
intruded by a diverse suite of intermediate to felsic plutons. In
Manitoba, weakly metamorphosed equivalents of the metasedimentary
rocks overlie Neoarchean volcanic rocks of the adjacent Uchi
granite-greenstone domain. These subaerially and subaqueously
deposited sedimentary rocks were derived, at least in part, from the
underlying granite-greenstone domain. Detrital zircons in
metasedimentary rocks and their less metamorphosed equivalents
indicate deposition between 2.70 to 2.71 Ga.
The Uchi granite-greenstone domain is 550 km long by 50 km wide but
only the western 120 km is exposed in Manitoba. The Uchi domain
includes several strands of Mesoarchean and Neoarchean volcanic
rocks that comprise the Rice Lake greenstone belt in Manitoba and
the Red Lake, Bee Lake and Pickle Lake greenstone belts in Ontario.
Mesoarchean volcanic rocks in the Rice Lake belt include mainly
komatiites and komatiitic basalts whereas the Neoarchean volcanic
rocks are more lithologically diverse and include both tholeiitic
and calc alkaline volcanic members. The Rice Lake greenstone belt
hosts a number of gold deposits including both past and present gold
producers. The Red Lake greenstone belt is host to world class Au
mineralization.
The Berens River domain, which is dominated by granitic plutonic
rocks, includes an older ca. 3.0 Mesoarchean suite and a younger ca.
2.7 Ga Neoarchean suite. The Mesoarchean rocks comprise a ca. >3.0
Ga granitoid basement complex unconformably overlain by a >2.97 Ga
platform/rift succession of quartzite, carbonate, iron formation and
komatiite. In Manitoba the platform/rift succession has been
documented along the southern flank of the Berens River Domain at
Wallace Lake, Wanipigow Lake and on the eastern shore of Lake
Winnipeg. Neoarchean supracrustal rocks of the Rice Lake greenstone
belt were structurally juxtaposed against the Mesoarchean component
of the Berens River Domain at ca. 2.71 Ga with local deposition of
arkose and conglomerate in strike slip basins during convergence of
the two domains. Sedimentary rocks in these strike slip basins are
similar in age to metasedimentary rocks of the English River domain
to the south. Neoarchean plutons in the Berens River Domain are
similar in age to the ca. 2.73-2.72 Ga volcanic rocks in the Uchi
Domain suggesting that both may have been products of the same
magmatic event.
Superior Boundary Zone
Thompson Nickel Belt
The northwestern foreland margin of the Superior craton is underlain
by various blocks of Archean basement gneiss, one block containing
rocks >3.5 Ga, among the oldest in North America. Paleoproterozoic
sedimentary and volcanic rocks are currently interpreted as
rift-related marine basin fill deposited unconformably on the
drowned Archean platform margin. Mafic dykes that fed some of the
volcanic flows, and prominent ultramafic to mafic sills that
intruded the cover have been dated as 2.1 to 1.86 Ga.
In the Thompson Nickel Belt (TNB), the cover rocks (Ospwagan Group)
comprise quartzite to pelite, carbonate and ferruginous to silicic
chemical deposits overlain by mafic volcanic rocks and intruded by
ultramafic sills that host world-class nickel deposits. These rocks
extend over 400 km along strike but are complexly deformed and
highly metamorphosed. Moreover, they are poorly exposed in the north
and covered by Paleozoic carbonates in the south. A
government-industry collaboration between MGS and Inco, Falconbridge
and Hudson Bay Exploration and Development Ltd has produced new 1:50
000 scale maps of the entire TNB.
The Fox River Belt, to the northeast, and the buried Winnipegosis
Komatiite Belt, to the southeast, feature weakly metamorphosed,
nearly undeformed komatiitic to basaltic flows and sills as well as
mudstone and ironstone. Whereas rocks of the TNB were strongly
deformed during continental collision tectonics with the adjoining
Trans-Hudson Orogen, those in the other extensional
volcano-sedimentary belts are remarkably well preserved, probably
because of their more inboard location on the Superior craton
margin.
All these rocks form challenging exploration targets for nickel, as
has been proved in the TNB, and for other platinum group elements (PGE)
as is currently suggested in the Fox River Belt.
Northwest Superior Boundary
An MGS- and NSERC-supported, integrated mapping, geochemistry and
isotopic study of the Western Superior craton margin northeast of
Thompson (PDF, 220 k) has been partnered with researchers from the
University of Alberta. Over the past three years, this work has
indicated that a re-interpretation of the location and nature of the
boundary zone between the Archean Superior Province and the
Paleoproterozoic Trans-Hudson Orogen is required.
In 1997, the first hints of ancient crust were discovered at Assean
Lake. This discovery has led to extensive research in the Assean
Lake area and the adjacent crustal domains of the Superior Boundary
Zone. The Assean Lake ancient crust comprises a collage of Archean
crustal segments, trending approximately 090–110°, that are
overprinted by Neoarchean and Paleoproterozoic metamorphism and
deformation, the latter forming structures trending 060°. The Assean
Lake crustal complex is subdivided into a southern Meso- to
Paleoarchean segment and a northern Paleoproterozoic segment. The
southern segment comprises migmatitic quartz arenite, arkose, and
metagreywacke gneiss, with local silicate-facies iron-formation,
metabasalt and ultramafic rocks. The northern segment comprises
Paleoproterozoic plutonic rocks and subordinate ortho- and
paragneiss which display varying degrees of Archean contamination.
Combined Sm-Nd isotopic and U-Pb geochronological results, obtained
using thermal-ionization mass spectrometry (TIMS) and sensitive
high-resolution ion microprobe (SHRIMP), indicate that Assean
ancient crust underwent a complex and prolonged history spanning
more than two billion years.
Fox River Belt
The Fox River Belt (FRB) is an approximately 300 km long and 10 to
30 km wide, Paleoproterozoic supracrustal sequence intruded by
coeval ultramafic and mafic sills and dykes. It is a homoclinal,
north-dipping and north-facing sequence that developed in a marginal
rift at ca. 1.9 Ga. The belt is relatively undeformed and features
low metamorphic grades (subgreenschist to lower greenschist facies).
It is likely the best-preserved Paleoproterozoic rift basin sequence
on earth, contrasting with the complexly deformed rocks present in
the Thompson Nickel Belt. Past and current exploration interest in
the FRB is for Ni-Cu-PGE deposits similar to those within the Raglan
Camp in northern Quebec and the Thompson Nickel Belt in central
Manitoba. The belt consists of two sedimentary rocks formations
(Lower and Upper sedimentary formations) that enclose an igneous
domain comprising the Lower and Upper Volcanic formations and
laterally continuous, <50 m to 2.5 km thick ultramafic-mafic
intrusions. The largest intrusion in the FRB is the Fox River sill.
Smaller ultramafic to mafic intrusions (Lower Intrusions) occur in
the northern part of the Lower Sedimentary formation.
The stimulus for the recent, increased levels of geoscience
programming in the FRB was provided by the minerals industry, in
particular Falconbridge Limited and Mr. B. Dunlop, who began
regional exploration of the FRB for Ni, Cu and platinum-group
elements (PGE) in 1998. In 1999, the Manitoba Geological Survey and
the University of Manitoba initiated field and geochemical
investigations in the FRB with the support of Falconbridge Limited.
Trans-Hudson Orogen
Flin Flon Belt
The Flin Flon Belt (FFB) is in the juvenile internal zone of the
Trans-Hudson Orogen and consists of Paleoproterozoic volcanic,
plutonic and minor sedimentary rocks. The exposed portion of the
belt is 250 km long by 75 km wide. Although it has an apparent
easterly trend, this is an artifact of the belt’s tectonic contact
with gneissic metasedimentary, metavolcanic and plutonic rocks to
the north (Kisseynew Domain) and the east-trending trace of
Phanerozoic platformal cover rocks to the south. In reality the Flin
Flon greenstone belt extends hundred of kilometres to the
south-southwest beneath a thin, geophysically transparent
Phanerozoic cover. To the north the FFB is tectonically overthrust
by younger metasedimentary rocks of the Kisseynew domain and by
nappes of metavolcanic rocks that are the same age as those in the
FFB.
The FFB is composed of structurally juxtaposed volcanic and
sedimentary assemblages that were emplaced in a variety of tectonic
environments. The major 1.92-1.88 Ga components include areally
significant juvenile arc and juvenile ocean-floor rocks, and minor
ocean plateau/ocean island basalt. The juvenile arc assemblage
comprises tholeiitic, calc-alkaline and lesser shoshonitic and
boninitic rocks similar in major and trace element geochemistry to
modern intraoceanic arcs. Ocean-floor basalt sequences are
exclusively tholeiitic, and are geochemically similar to modern N-
and E-type MORBs erupted in back-arc basins. Evolved arc assemblages
and Archean crustal slices are present within the FFB as minor
components.
Collectively, these tectonostratigraphic assemblages were
juxtaposed in an accretionary complex at ca. 1.88-1.87 Ga,
presumably as a result of arc-arc collisions. The collage was
basement to 1.87-1.83 Ga post-accretion arc magmatism, expressed as
voluminous calc-alkaline plutons and rarely preserved calc-alkaline
to alkaline volcanic rocks. Unroofing of the accretionary collage
and deposition of continental alluvial-fluvial sedimentary rocks (Missi
Group) and marine turbidites (Burntwood Group) occurred ca.
1.85-1.84 Ga, coeval with the waning stages of post-accretion arc
magmatism. The sedimentary suites were imbricated with volcanic
assemblages in the eastern FFB during 1.85-1.82 Ga juxtaposition of
the supracrustal rocks along pre-peak metamorphic structures. Post
ca. 1.83 Ga structures formed the present southwest-verging fold
style at the northeastern end of the FFB.
The tectonostratigraphic architecture of the FFB is of essential
economic significance. The belt is one of the largest Proterozoic
volcanic-hosted massive sulphide (VMS) districts in the world,
containing 27 Cu-Zn- (Au) deposits from which more than 183 million
tonnes of sulphide have already been mined. Most of mined VMS
deposits in the Flin Flon belt are associated with the juvenile arc
volcanic rocks, providing a powerful focus for exploration in the
belt. New geochronological work indicates, however, that at least
one of the mined deposits is hosted by rhyolites dated at 1869 Ma
and thus is associated with the post-accretion arc magmatism.
Gold mineralization in the FFB is less thoroughly studied but at
Flin Flon has been shown to be intimately associated with late
brittle-ductile shear zones that follow peak tectonic and
metamorphic activity within the Trans-Hudson Orogen. At Snow Lake,
however, preliminary investigations suggest a long history of gold
mineralization with at least some gold introduced prior to
metamorphism.
Lynn Lake Belt
The Paleoproterozoic Lynn Lake
and Rusty Lake greenstone belts occur in the interior (Reindeer)
zone of the Trans-Hudson Orogen, that is, in the zone formed as new
ocean floor, or in oceanic to continental volcanic arcs and related
marine and continental sedimentary basins. Metavolcanic rocks in the
Lynn Lake – Rusty Lake belts are 1.89-1.88 Ga, the same age as
metavolcanic rocks in the Flin Flon Belt. They are flanked by
metasedimentary gneisses to the south (Kisseynew Domain) and to the
north (Southern Indian Domain).
Regional mapping by the MGS and follow-up geochemistry in the 1970s
and 1980s provided a gross stratigraphy of basalt to andesite and
rhyolite from the Saskatchewan border to Barrington Lake, and a
different volcanic stratigraphy of basalt and thick rhyolite in the
Rusty Lake Belt, near the town of Leaf Rapids. U-Pb zircon dating
has shown that the latter are among the youngest volcanic rock in
the area, only slightly older than the large granodiorite plutons in
the Lynn Lake Belt. Recent geochemistry has shown that the Lynn Lake
Belt had a complex tectonic evolution in a variety of volcanic
environments. Similar volcanic environments in the Flin Flon Belt,
to the south, contain highly productive copper-zinc and gold mines.
Although base metal mining will cease with closure of the Ruttan
Mine near Leaf Rapids in May, 2002, the Lynn Lake nickel-copper
deposits and the various gold properties are undergoing renewed
exploration interest. The origin of the volcanic massive sulphide
(VMS) deposits and shear-hosted gold deposits and the nature and
structure of their host rocks is under active investigation.
Phanerozoic
In Manitoba, Paleozoic, Mesozoic, and Cenozoic sedimentary rocks
accumulated in sedimentary basins of the Western Canada Sedimentary
Basin. Two major basins influencing sedimentation in Manitoba are
the Hudson Bay Basin, centred in Hudson Bay, and the Williston/Elk
Point basins, centred in northwestern North Dakota. The Paleozoic
strata comprise limestone, dolomite, shale, sandstone and evaporite
deposits. The Paleozoic rocks contribute to Manitoba’s
industrial
minerals through products such as building stone and crushed stone
derived from dolomitic limestone and dolomite, high-calcium lime
from limestone and salt brine from subsurface evaporites. Petroleum
is also extracted from some Paleozoic units.
Mesozoic sediments include red siltstones, sandstones, shales,
gypsum and bentonite. Gypsum and petroleum are important commodities
extracted from Mesozoic rocks. Cenozoic strata are limited to the
Turtle Mountain area and consist primarily of fine sandy and silty
shales.
Investigations designed to locate Prairie-type and Mississippi
Valley-type mineralization have and are presently being undertaken
in Paleozoic and Mesozoic rock formations by the Manitoba Geological
Survey and private industry. Surface and subsurface mapping of all
rock types continues by the Manitoba Geological Survey, augmented by
corehole drilling with the Survey drill. The Manitoba Stratigraphic
Database (a listing of all subsurface well data) is instrumental for
subsurface mapping.
Quaternary
Manitoba’s glaciated terrain is characterized by a freshly eroded
rock surface that has not been chemically weathered. Vast areas of
streamlined glacial sediments, for example in the lowlands of south
central Manitoba, clearly indicate a complex ice-flow pattern.
However, glacial ice generally advanced towards the southwest.
Thick, stratified glacial sediment, widespread in the northeast
(Hudson Bay Lowland) and the southwest, complicate mineral
exploration. Sand and gravel resources are sporadically abundant
throughout the province. Expansive wetland areas east and north of
Winnipeg comprise thick accumulations of peat, making access
difficult. Discontinuous permafrost occurs north of the 54th
parallel, while continuous permafrost only occurs in the extreme
northeast of the province, near Hudson Bay.
Following the compilation of the first version of the southern
Manitoba Digital Elevation Model (DEM) in 1999, many aspects of the
Quaternary geology of Manitoba have been re-evaluated. As such, the
DEM is a good tool to help describe the Quaternary geology of
Manitoba. For example, ice directional indicators can clearly be
seen on much of the DEM, a testament to the erosive power of
continental glaciation. Above the Manitoba Escarpment, different
landscapes delineate contrasting glacial ice dynamics (e.g., Riding
Mountain hummocky moraine versus the glacially streamlined terrain
to the west).
Although Manitoba has been repeatedly glaciated, the rock substrate
has a profound effect on the glacial sediments in any particular
area. On the Precambrian Shield, glacial sediments are sand-rich and
discontinuous; rock structure can easily be observed (e.g., Ross
River Pluton). Thick accumulations of sediment, including preserved
older sediments, are common in bedrock depressions. Clast lithology
tends to be extremely high in Precambrian rock types. In contrast,
to the west, below the Manitoba Escarpment, glacial sediments are
much finer textured and typically silt-rich with abundant carbonate
lithologies. Above the Manitoba Escarpment the sediments are
relatively clay-rich with abundant shale lithologies, again
mimicking the lithology and texture of the underlying bedrock. In
portions of the province underlain by Phanerozoic bedrock, the
glacial sediments are much more continuous and rock structure is not
visible. Glacial sediments tend to be thick along ice marginal
positions (e.g., Sandilands Moraine) and older sediments are
preserved locally in bedrock depressions.
During glacial retreat, meltwater flow above the Manitoba escarpment
carved large spillways and deposited sand and gravel, in the form of
underflow fans, into Lake Agassiz. Such features are particularly
evident along the escarpment (e.g., Pembina Spillway and Assiniboine
Delta). Following the retreating glacial ice in the Manitoba
Lowlands, Lake Agassiz became the dominant force in modifying the
Manitoba landscape. Lake Agassiz once covered everything east of the
Manitoba Escarpment, other than the higher area of northwestern
Manitoba. Campbell Beach, one of the higher levels of Lake Agassiz,
can clearly be seen at the base of the escarpment, a testament to
the dramatic effect of wave erosion on the landscape. In the deep
basins of Lake Agassiz, thick (in excess of 50 m) accumulations of
clay were deposited (e.g., south of Winnipeg, in Lake Winnipeg and
several areas to the north).
The effects of glaciation are still modifying the landscape of
Manitoba. The surface of the earth is rebounding due to isostasy.
Present-day north-draining lakes, such as Lakes Winnipeg and
Manitoba, are slowly expanding southward, while lakes that drain
southward are diminishing in size. This process is most evident
along the drowned south shore of Lake Winnipeg (e.g., Netley Marsh).
Elsewhere along the south shore, wave erosion is a serious problem
for cottage and recreational development. Along the Manitoba
Escarpment, fine-grained sediments, shale bedrock, and high local
relief have led to the development of one of the largest landslide
areas in North America, on the east side of the Porcupine Mountains.
The clay plain in the Winnipeg area and to the south, which
originally floored Lake Agassiz, is an area prone to flooding and
challenging foundation conditions. Groundwater contamination, in
areas with surface or near surface aquifers, has implications for
livestock management and grain production. One such aquifer, the
Assiniboine Delta, is the largest in southern Manitoba.
Digital
Elevation Model of southern Manitoba.
1 Bannatyne, B.B., McCabe,
H.R. 1984: Manitoba crater revealed; Energy, Mines and Resources Canada, Geos, v.
13, no. 3, p.10-13.
Reproduced with the permission of the Minister of Public Works and Government
Services Canada, 2006 and Courtesy of Natural Resoruces Canada, Geological
Survey of Canada.
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