Geoscience

Manitoba Geology

 

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General Geology:
 Precambrian
 Phanerozoic 
Geology of Manitoba Map PDF format
Superior Province
Southeastern Manitoba
Superior Boundary Zone
Trans-Hudson Orogen
Phanerozoic
Quaternary

 

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. 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 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. 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.

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Phanerozoic

Exposures of Paleozoic rocks generally are sparse in both the Hudson Bay Lowland 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 Lowland 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 throughout the Cretaceous section. 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.

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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:

  1. 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.
     
  2. 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.
     
  3. 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.

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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.

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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 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.

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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 162 million tonnes of sulphide have already been mined or are in development within these deposits. 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.

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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.

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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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