Animikie Group

Brief summary of Huronian Supergroup and Marquette Range Supergroup

The Huron and Marquette Range supergroups are similar sedimentary groups to the Animikie Group; all three are in the Great Lakes region. Rifting of continental plates created many sedimentary basins; the largest of these basins in the Great Lakes area are the Animikie Group in Minnesota, the Marquette Range Supergroup in northern Michigan and Wisconsin, and the Huronian Supergroup in eastern Ontario.

This article focuses on the Animikie Group; following is a brief description of the two supergroups.

Huronian Supergroup

The Huronian Supergroup on the north shore of Lake Huron in Ontario overlies an Archean basement. Huronian sedimentary rocks form a 300 km (190 mi) east–west fold belt and reach a thickness of 12 km (7.5 mi) near Lake Huron. Deposition of sediments began 2,450 to 2,219 million years ago and continued until 1,850 to 1,800 million years ago when the rocks were deformed and metamorphosed during the Penokean Orogeny. The Huronian Supergroup's sedimentary layers are subdivided into lower and upper sequences. The lower sequence is subdivided into the Elliot Lake, Hourg Lake and Quirke Lake groups; the upper sequence is the Cobalt Group. The lower sequences were deposited in a continental rift basin and the upper sequence was deposited in a stable passive margin.

Marquette Range Supergroup

The Marquette Range Supergroup also overlies an Archean basement and is 2,000 to 1,900 million years old. This supergroup consists of the Chocolay, Menominee, Baraga and Paint River groups; they are listed in descending order of age. The Chocolay Group is a shallow-marine layer which was deposited on the Archean basement; Hydrothermal xenotime age data shows that deposition in the Chocolay Group began 2,207±5 million years ago and ended 2,115±5 million years ago.

The radiometric age data in this study implies the depositional age of the Chocolay Group is constrained to ~2.3–2.2 Ga, which proves its correlation with part of the Huronian Supergroup in the Lake Huron Region, Ontario, and reveals the unconformity that separates the Chocolay Group from the overlying Menominee Group is up to 325 million years in duration. The source(s) of the ~ 2.3 Ga detrital zircon populations in the Enchantment Lake Formation and Sturgeon Quartzite remains an enigma because no known rock units of this age are known in the Michigan area. It is speculated that once widespread volcano-sedimentary cover sequences in Michigan were removed or concealed prior to Chocolay Group deposition. The hydrothermal xenotime ages probably reflect basinal hydrothermal fluid flow associated with the period of extension, involving rifting and major dyke formation, that affected the North American provinces between 2.2 and 2.1 Ga.

it is equivalent to the upper part of the Huronian Supergroup, and represents the original continental rifting of the Great Lakes Tectonic Zone and the passive-margin phase of the Penokean Orogen. The Menominee Group is a foredeep deposit. The Menominee Group layers were deposited in second-order basins created by oblique subduction of the continental margin, rather than basins formed on a rifting margin. An extensive foredeep in the western Lake Superior region was the site of iron-formation deposition during arc accretion from the south. The upper Baraga Group represents deeper marine basins, dominated by turbidites and lesser volcanic rocks, resulting from increased subsidence and continued collision. The stratigraphic complexity of the Michigan sequence indicates a supergroup rank.

2,200 million year ago, and also continued until 1,850 million yeas ago.

Along with a recently reported age for the Gunflint Formation in Ontario of 1878 ± 2 Ma.

Age, location and size

The Animikie Group is 2,000 to 1,900 million years old.

These strata are the Animikie Group of formations and are found in the present-day areas of the Thunder Bay, Canada –; Grand Portage, Minnesota, U.S., the Mesabi Range and in east-central Minnesota southwest of Duluth, Minnesota.

Regional uplift of the crust formed mountains, which were eroded down to a broad level erosion surface during the next several hundred million years. Several kilometers’ thickness of rock was eroded exposing the granites that were formed deep in the crust. Rocks of this complex – Superior Province of the Canadian Shield – extend for over 1,600 km (990 mi) miles to the northwest, north and northeast into present-day Canada. As the rifting of the Great Lakes Tectonic Zone continued, the newly created southern margin of the Superior Province – which was caused by the collision of the Minnesota River Valley Subprovince with the Superior Province – subsided, which allowed seas to encroach, depositing sediments such as the iron formations, sand, gravels and smaller amounts of dolomite (a sedimentary carbonate rock). The next great geologic episode involved the deposition of sedimentary strata on top of the eroded Archean basement about 2,200 to 1,900 million years ago.

At this time a sea invaded the central and northeastern part of present-day Minnesota, U.S., and extended eastward through northern Wisconsin and the Upper Peninsula of Michigan. Sediments were composed of quartz-rich sand were deposited along the shoreline of this ancient sea; these were succeeded by thick iron-rich layers and eventually tens of thousands of kilometers (thousands of feet) of mud and muddy sand as the Animikie Basin deepened.

The quartz sand became quartzite, the iron-rich deposits became iron-banded formations, and the mud and sand became shale and greywacke (a muddy sandstone). These strata are the Animikie Group of formations and are found in the present-day areas of the Thunder Bay, Canada –; Grand Portage, Minnesota, U.S., the Mesabi Range and in east-central Minnesota southwest of Duluth, Minnesota.

The rocks of the Animikie Basin form a sequence that is up to 10 km (6.2 mi) thick and indicate a complete transition from a stable shelf environment to deep water conditions. Irregularities in the basement influenced the thickness of the sequence. The succession was deformed, metamorphosed and intruded by the plutonic rocks of the 1860±50-million-year-old Penokean Orogeny.

The Animikie Basin, part of the Penokean Orogen, was an intracratonic extensional (rift) basin developed over crystalline basement of the Archaean Superior Province. The basement comprises 2,750- to 2,600-million-year-old Superior Province's granite-greenstone terrane to the north and a 3,600-million-year-old complex of the Minnesota River Valley Subprovince's migmatitic gneisses and amphibolites to the south, separated by the generally ENE-WSW trending Great Lakes Tectonic Zone which passes just to the south of Duluth on the SW tip of Lake Superior.

The 700 km (430 mi) by 400 km (250 mi) Animikie Basin is elongated parallel to and straddles the Great Lakes Tectonic Zone.

In their Great Lakes occurrence across the Minnesota-Ontario border, Animikie sediments occur in outcrop (as about Thunder Bay, Ontario) as undeformed platform sediments on a profound nonconformity cut across basement rocks of the North American craton. In their southward extent, Animikie sediments thicken greatly toward where they occupy several sedimentary basins in a mobile belt on the northwest side of the Grenville Boundary fault zone. The culminating orogeny of the mobile belt that dates from 1,850 through 1,600 million yers ago is called the Penokian in the U.S. and the Huronian in Canada. Synfolded in the Animikie sedimentary strata are 1.3-1.8 Gy intrusive pegmatites.

The Animikie Series forms a belt which extends from the Mississippi River to the extreme northeast part of Minnesota, at Pigeon Point, and into Canada at Thunder Bay. It is over 320 km (200 mi) long and almost always less than 16 km (10 mi) should be 10 miles wide. The usual width is 4 km (2.5 mi). The rocks are generally arranged in a gentle monocline which dips south. It often has a decided east-of-south tendency; the angle averages 10 –15°.

Formation of Animikie Basin

The formation of the Animikie Basin and the accumulation of sediments in it appear to have been related to a great thrusting movement that shoved part of the midcontinent to the north, toward the basin. This crustal deformation is known as the Penokean Orogeny; it has been dated to about 1,850 million years ago. The intense, northward-directed pressure thickened the crust and folded the shale and greywacke of the southernmost unit – the Thomson Formation – and induced the development of rock cleavage as the shale was metamorphosed into harder slate. The Animikie strata on the Mesabi Range and in the Grand Portage area were far enough away so they escaped this deformation and metamorphism. The Mesabi Range and the Grand Portage area contain some of the oldest unmetamorphosed sedimentary deposits in the world.

A long period of crustal uplift and resulting erosion followed, exposing Thomson Formation rocks that had been folded several miles beneath the surface. About 1,100 million years ago the third dramatic event occurred in the Lake Superior region. A slowly rising plume of hot, slightly plastic rock in the earth’s mantle beneath present-day Lake Superior, the crust and the underlying rigid uppermost part of the mantle (the lithosphere) began to stretch and break apart. This great zone of crustal thinning and fracturing is the Midcontinent Rift System. It extends as a great loop for over 2,200 km (1,400 mi) from northeastern Kansas northward through Iowa, under the Twin Cities of Minnesota, beneath Lake Superior, and then south through the eastern Upper Peninsula of Michigan and beneath the central Lower Peninsula of Michigan. Rocks of this age associated with the Midcontinent Rift are known as Keweenawan, after the Keweenaw Peninsula in Michigan’s Upper Peninsula. The rising mantle rock began to melt as it approached the surface –; the higher pressures at a deeper depth kept it solid –; and huge volumes of magma were produced; one recent estimate is 1,300,000 cubic kilometres (310,000 cu mi). This is roughly a times the volume of Lake Superior.

A large proportion of the magma was produced beneath present-day Lake Superior, where the hotspot was located. This magma was of basaltic composition. Basaltic lavas solidify to rocks which are relatively rich in iron, magnesium and calcium; and relatively weak in silicon and potassium, compared to other common igneous rocks. The high iron content gives these rocks a dark color &ndsh; generally grey, brown or black. This is the type of lava now being erupted in Hawaii; it is the most common type of lava. As the crust was being pulled apart, many fractures developed, generally parallel to the rift axis, allowing great volumes of the basaltic magma to rise up and be erupted at the surface. These were mostly fissure eruptions, with lava pouring out along long cracks in the earth’s crust. They produced flood basalts, not volcanoes, which are large lava flows that spread out on the flat landscape over hundreds of square kilometers. Fissures in which magma has cooled and solidified are called dikes. Several hundred of these basalt flows accumulated on top of each other along the rift, and are visible around the Lake Superior basin. Those along the Minnesota coast are the North Shore Volcanic Group.

Preston Cloud, former geology professor at the University of Minnesota, has described the conditions during rifting as:

The eruptive scene is unimaginable! A mini-ocean of eerily fuming basaltic lava. A region the size of Maine engulfed in the acrid fumes of volcanic gases. The searing heat of a blast furnace across the whole of it. Literally, hell on Earth.

— quote

As the crust was being stretch thin and more material flowed out onto the surface from below, the axis of the rift subsided continuously. This caused the flows toward the edges of the rift system to tilt inward toward the center. These are the rocks that now form the North Shore of Lake Superior and their tilting or dipping is visible in lakeshore and river gorge exposures. The biggest flow of all – the Greenstone Flow – can be traced for {convert|80|km|mi|abbr=on}} along the Keweenaw Peninsula in Michigan’s Upper Peninsula; it continues under Lake Superior to form the backbone of Isle Royale, just off the northeastern tip of Minnesota, where it dips in the opposite direction. A typical cross-section of moderate-sized basalt flows of the North Shore Volcanic Group has a concentration of amygdules – filled gas bubbles – toward the top, the characteristically smooth surface of the more abundant olivine tholeiite variety showing that it was very fluid, and the columnar joints that commonly develop as the lava flow cools and shrinks.

Many of these features can be seen the Gooseberry Falls and Temperance River state parks of Minnesota. The rifting, stretching and volcanism stopped after a few million years; one reason could be that the Greenville Orogeny stopped the rift process when that collision occurred. Usually, when a continent splits apart the rift widens to become a new ocean basin; that didn’t happen in this case. Subsidence continued for several million years after the volcanism had ceased; immense volumes of sediments – sand, gravel and mud – were washed off the barren landscape into the still sinking basin along the rift axis. As much as 8 km (5.0 mi) of sedimentary rocks accumulated in the center before the sinking stopped and the region stabilized. Some of these post-volcanic sandstones can be seen at Fond du Lac southwest of Duluth, and along the Kettle River near Sandstone, Minnesota, Hinckley, Minnesota, and Minnesota’s Banning State Park. During this period of sedimentation, compressive forces squeezed up parts of the central zone of the rift system along big thrust faults.

As immense amounts of hot, basaltic magma were rising through the crust toward the surface, some of it became trapped within the crust at various levels, and cooled and crystallized slowly in place to form intrusive rocks. The slow cooling allowed crystals to grow larger – become coarser – in intrusive rock masses as compared with volcanic rocks. The slower cooling also allowed the intrusive rock bodies to adjust more easily than the volcanic rocks to the contraction stresses that occur when a rock cools. This resulted in fewer, more widely spaced fractures in the intrusive bodies compared to the more intensely fractured volcanic bodies.

When later exposed to stream and glacial erosion the intrusive rocks were more resistant, and form the present-day ridges, hills and knobs. The largest of these intrusive bodies is the Duluth Complex. Underlying much of the territory south and east of the Iron Range and Ely, it also holds up the higher, southwest part of the City of Duluth from the Point of Rocks and Mesaba Avenue down to Spirit Mountain and Bardon Peak. The Duluth Complex is comprised of gabbro, a variety of rock types. Gabbro has the same general chemical and mineral composition as basalt. Other smaller intrusions cut the volcanic rocks along the shore. When basaltic magma is intruded at relatively shallow depths in the crust, it cools at intermediate rates and forms diabase.

A large concentration of diabase intrusions is in the Beaver Bay – Silver Bay – Finland area in Lake County, Minnesota; it is the Beaver Bay Complex. As the magmas were working their way up from the upper mantle, they broke off many blocks of an unusual rock, anorthosite; and carried them up to the level where the diabase crystallized and froze those inclusions in place. Anorthosite is comprised nearly entirely of the mineral plagioclase, is light colored, in contract with the darker diabase; it is particularly resistant to erosion. The diabase intrusions of the Beaver Bay Complex, with their blocks of anorthosite, give the rugged topographic character to Split Rock Lighthouse, Tettagouche and Crosby Manitou state parks ad Carlton Peak in Temperance River State Park. Another important effect of the basalt magmas working their way up from the mantle was to partially melt some of the Archean basement rocks in the lower crust.

This produced a new magma of a different composition, granite, which is relatively rich in silica and potassium and poor in iron, magnesium and calcium compared to basalt, gabbro and diabase. When these granitic magmas became trapped in the crust they formed masses of reddish granite, but much was able to rise to the surface and erupt to form the volcanic rocks rhyolite. Because rhyolites have less iron, they are lighter colored than basalt and diabase; they do contain enough iron to color many of them red or pink. The rhyolite flows were erupted intermittently as basaltic volcanism was continuing; they are interlayered with basalt flows. Their total volume was about one-tenth to one-quarter of the total accumulation of volcanic rocks. Some of them were erupted as very hot and fluid lava flows; they can be followed for many miles across the area. Other rhyolitic units were erupted more explosively &ndash the magma was literally blown to bits as it came out, then settled to form a widespread layer of pumice fragments stuck together with ash. Magma always contains a small amount of volatile constituents dissolved in it, such as water and carbon dioxide. These would be in the gaseous state because of the magma’s high temperatures, but the pressure at the depths the magma exists keeps these volatile gases dissolved in the magma (like pressure keeps beverages carbonated).

The pressure is released as the magma comes to the surface, the volatiles form bubbles. This has two effects: the expanding bubbles help push the magma out the volcanic vent or fissure, maybe even producing an explosive eruption, and some the bubbles get trapped as the lave solidifies. This forms bubble-shaped cavities called vesicles. Many of the bubbles were able to rise in the still-fluid lava, only to become trapped near the top of the flow, leaving the lower and middle parts without vesicles. As the hundreds of lava flows accumulated along the rift, the earlier ones became buried, some to depths of several kilometers. Heated ground water slowly percolated through the fractures in the rocks. Gradually, over thousands of years, the water dissolved material from the rocks, and deposited minerals in the cracks and vesicles. The hot-water solutions are hydrothermal solutions and the filled vesicles are amygdules. Most of the lava flows throughout the North Shore Volcanic Group are amygdaloidal, especially near the top of each flow. The temperature at which this process occurred was 150 to 300 °C (302 to 572 °F), in contrast to the temperature of the erupting lava (1,000 to 1,200 °C (1,830 to 2,190 °F)). These hydrothermal solutions have deposited many different minerals. Many of them are light colored and contrast with the dark basalt. They are called secondary minerals because they formed after the rock had solidified. Most of these minerals are soft and easily eroded. Of particular interest in the North Shore area of Lake Superior are two semi-precious gems – agate and thomsonite. The Lake Superior agates are extremely fine-grained quartz, of the chalcedony variety. They were deposited in layers, starting at the outside surface of the cavity and gradually filled in toward the center. In many cases the last interior portion is lined with coarser quartz crystals. Most agates are found in glacial deposits. Because Chalcedony is very hard and tough, agate may be the only part of a weathered basalt to survive the destructive process of glacial erosion and long-distance transport. At various times during the early Paleozoic Era [Cambrian, Ordovician and Devonian periods], shallow seas encroached the interior of North America and deposited relatively thin layers of sandstone, shale, limestone and dolomite on the eroded Precambrian basement in southeastern and far northwestern Minnesota. There is no evidence that these seas reached northeastern Minnesota; this area probably remained above sea level, slowly weathering and eroding. Slow long-continued chemical weathering tends to produce residual soils from the disintegration of the underlying rock and has a layer of transition where the bedrock is gradually being broken down, altered and converted to the surface soil. Unweathered fragments would resemble the underlying rock. Such deep residual soils are common in most tropical and subtropical areas of the world. The Great Ice Age is known as the Quaternary Period. Nearly all of it is contained in the Pleistocene Epoch; the last 10,000 years or so is the Holocene Epoch, the time since the last ice sheet started to melt back. As the successive ice sheets spread southward from Canada, they found the sandstones and other sedimentary rocks that had been deposited in the sinking Midcontinent Rift to be much softer than the more resistant igneous and metamorphic rocks on either side. Over hundreds of thousands of years this strong erosive action gouged out the basin of present-day Lake Superior.< ref name=nsstateparks/> Along the North Shore, the Superior Lobe deposited major moraine along its northwest margin, the Highland Moraine. Bedrock striations and ice-molded elongate hills of till (drumlins) show that the ice was moving upslope out of the basin to the west, or even northwest as it carried debris to the Highland Moraine. The weight of the great continental ice sheet, several thousand kilometers thick, had depressed the earth’s crust several hundred meters by slow outward plastic flow of mantle rock beneath the crust. The retreat of the ice had been so rapid that the crust had not rebounded much by the time the ice had cleared the Sault Ste. Marie outlet about 9,500 years ago. The crust gradually rebounded a few meters, then centimeters per century so the water outlet rose. Abandoned shorelines on Minnesota’s North Shore of Lake Superior have been warped upward to the northeast because of the gradual rebounding of the crust after the ice sheet melted away. The gradual rise of the crust was not uniform across the area. Very little rebounding occurred at Duluth, with progressively more to the northeast, where the ice was thicker and lasted longer. Lake Superior has been at approximately 200 m (660 ft) should be 602 feet above sea level for several thousand years, longer than any of its post-glacial ancestors.

Subsiduary formations

Thomson Formation

The 1,880-million-year-old Thomson Formation contains folded and metamorphosed felspathic greywacke, siltstone, mudstone and slate.

The Thomson Formation was formed in the early Early Protozoic Eon about 1,900 million years ago. Originally deposited in a sea as horizontal beds of mud and sand, it was soon afterward involved in a major Orogeny that subjected the rocks to intense compression from the south. This folded the layers into east-west trending anticlines and synclines, and caused the development of the rock cleavage in the formerly muddy beds, turning them into slate, a metamorphic rock. This slately cleavage is approximately vertical and in most places it is at a considerable angle to the layering. The more massive, resistant beds of greywacke don’t generally show cleavage. Over a hundred years ago there were several small quarries in the Thomson area extracting slate for roofing tiles. Several basaltic dikes, from the lava of the Midcontinent Rift period, can be seen cutting across the Thomson Formation slates and greywackes. Most of these dikes trend in a northeasterly direction and express the tensional forces that were trying to rift the earth’s crust apart. They represent magma that was rising in fissures in the crust. It is probable that these fissures once fed great flood-basalt eruptions, but those lava flows were all eroded away before the present glaciated bedrock surface was formed. The present western margin of the lowest and therefore earliest flood basalts is now located in the Nopeming – Ely’s Peak area; two to three miles northeast of Minnesota’s Jay Cooke State Park. Some fine examples of dikes can be seen in the rock bed of the river between the Thomson Dam and Highway 210, just west of Thomson. Another easily accessible example is just east of the Swinging Bridge in the park. Both the basalt dikes and the slates are black and may be hard to distinguish, but the dike has formed crude columnar joints that are horizontal – perpendicular to the dike walls – and has no slatey cleavage. The eastern margin of this dike has been crushed and broken. A different and younger rock formation – the Fond du Lac Formation – underlies the southeastern part of the park, but only its very basal few feet are exposed in the park. Good exposures of the main part of this red, sandy sedimentary unit can be seen in the low areas around the village of Fond du Lac, where Highway 210 meets Highway 23; quarries here once shipped tons of this “brownstone” down the river to help build the burgeoning city of Duluth. It is typical of sediments that have been deposited in great rift basins on continents throughout the world. The basal layers of the Fond du Lac Formation are visible in the beds of several of the tributaries on the northwest shore of the St. Louis River, east of Oldenburg Point. Whereas Thomson Formation slate is exposed at the trail crossing and for 33 m (108 ft) upstream, the Fond du Lac outcrops are a few meters (yards) downstream and form a streambed pavement. This basal Fond du Lac material is a quartz-rich conglomerate, with bedding that dips gently to the southeast. It was deposited probably by streams, on the eroded-off surface of the much older, folded Thomson Formation. The deposition of the Fond du Lac took place in response to the continued sinking of the central part of the Midcontinent Rift System, well after all the volcanic activity had ceased. This disconformity between the steeply folded Thomson Formation and the nearly flat Fond du Lac Formation represents a gap of 800 million years.

Nopeming Formation

The Nopeming is a basal sedimentary unit which is overlain by Ely's Peak basalts. The Nopeming Formation has 10 m (33 ft) of interbedded conglomerate and quartz arenite, with minor siltstone beds in the uppermost part of the unit. Much of the sandstone is medium- to coarse-grainded, well sorted and rounded quartz arenite. The uppermost sulty parts of the Nopeming contain load casts and other structures indicating soft-sediment deformation. The overlying Ely's Peak basalts were deposited subaqueously.

Puckwunge Formation

The Puckwunge Formation is located near Grand Portage, 240 km (150 mi) should be 150 miles to the northeast, and has nearly identical stratigraphic sequence as the Nopeming. The Michigan Bessemer Quartzite also has a similar stratigraphic position.

Recent work has shown that there are earlier sequences – the Mille Lacs and North Range – that underlie the Animike; the Animikie extends further to the north onto the Archean craton. These formations are difficult to interpret because of their the poor exposure and the tectonic rearrangement that occurred during later Penokean events.

Generally during rifting, two generations of sediments collect in an opening basin. The first deposits occur during the initial stages of extension in the continental crust, they tend to be normal faulting basin fill of coarse breccias conglomerates and sands. As the crust expands it reaches the limit of continental block extension, and basaltic oceanic crust is extruded along the center rise. Sedimentation generally ceases during this transitional period because of the elevation of the area is above sea level, and because the normal faulting has slowed or stopped, allowing the basins to fill in completely. When the rift widens enough to allow the area to subside, seas come in, and the second batch of sediments begin to be deposited unconformably on the basin fill. As time progresses, many thousands of feet of sands and silts are deposited in the sea off its coast of the unstreched portion of the continent. In these later stages, the spreading center that broke the continent apart is adding oceanic crust which is heavier than continental crust, so the relative elevation of the center ridge is below sea level allowing a continuous sea between the two rifted halves. A modern analogue of the last stage is the relationship between the eastern seaboard of the United States and Europe, with the mid-oceanic ridge in the center of the Atlantic ocean.

The oceanic sediments associated with the last stage of rifting contain the banded iron formations. These sediments were laid down on the calm passive margin over two hundred million years or so and extend intermittently along roughly the same trend as the Great Lakes Tectonic Zone, from Minnesota through upper Wisconsin and Michigan into eastern Canada where they terminate against a later formed tectonic terrane called the Grenville province.

Banded-iron formations

The banded-iron formations of the Lake Superior District have provided one of the world’s greatest sources of iron ore since mining began here in the late 19th century. The iron-banded formations contain laminated beds laid down by ancient algae and – in Ontario, Canada along the Lake Superior shore – a variety of microscopic fossil life forms.

The major banded-iron formation units, represented by the Animikie Group, were deposited either directly on Archaean basement or on eroded remnants of the Mille Lacs Group. The major iron formations in different parts of the basin represent either virtually contemporaneous near-strandline shelf sedimentation on either side of the main basin, or deposits formed simultaneously in isolated sub-basins of the main basin. The deposition of iron formation was terminated by the onset of the overlying deep water carbonaceous mudstones, greywacke, siltstone and mafic to felsic volcanogenic rocks that accompanied minor deformation and uplift to form the upper parts of the Animikie Group. Locally, deep water turbiditic deposition continued, to form the Paint River Group. Deposition was terminated by the Penokean orogeny.

Banded iron formations have been recognised over a number of intervals (or ranges) around the margins of this basin, five of which (including the Cuyuna Range) contained sufficient concentrations of iron mineralisation to be economically exploited. The stratigraphic successions have been correlated between each of these ranges, although physical continuity between the individual districts has not been demonstrated.

The Cuyuna Range iron and manganese deposits are located some 100 km (62 mi) southwest of the Mesabi Range in east-central Minnesota, U.S. defining a district that is approximately conver|110|km|mi|abbr=on}} by 32 of tightly folded iron formation. Mining commenced in the district in 1911 and finished in 1985. The iron formations of the Cuyuna Range are hosted by rocks of the Palaeoproterozoic Animikie Group which are best known in the Mesabi Range further to the north-east.

The major iron formations of the Great Lakes region of North America are hosted within the Paleoproterozoic 2,200- to 1,750-million-year-old Animikie Group, which was deposited within the Animikie Basin.

The Mesabi Iron Range is 240 km (150 mi) long. Natural ore is oxidized hematite- or geothite-rich and leached iron formation.

The ore deposits are localized along a set of fault zones that presumably provided the plumbing system for fluids that first oxidized the formation and produced permeability, and leached silica from the porous zones. Natural ores contain up to 50% iron and less than 10% silica. Some taconite contains about 30% iron and 50% silica. Taconite is used to refer to unoxidized iron formations containing sufficient iron to be mined profitably using today's technology.

These are large, thick sedimentary packages contain millions of tons of iron and other minor ores that have been mined in the Great Lakes region since before the turn of the century. The sedimentation on this calm coastal passive margin came to a close when the Penokean Orogeny began 1.85 million years ago. These sediments took on a different character shortly before they were crumpled up during this orogeny. The pattern of sedimentation from this rifting environment continues into the Penokean orogeny which is the next major tectonic event in the Great Lakes region. The sediments are part of the banded iron formations, even though professionally these sediments are now considered part of the Penokean orogeny due to its effect on them.

The beginning of the Penokean orogeny is recorded by the banded iron formations which are sedimentary strata of late Archean and early Proterozoic time.

The iron-bearing rocks extend as far east as to Gunflint Lake on the U.S.-Canadian border. From here the Animikie Series continues east to Pigeon Point. It is best known around Thunder Bay.

These sediments are also thought to record the introduction of abundant free oxygen into our atmosphere. This change was important, first because life could have only begun in the low free oxygen reducing environment of the Archean, and second for life to progress from simple prokaryotic cells to the more complex eukaryotic cells, required a change to a oxygen-rich environment.

Life was not only produced in the early atmosphere, it also played a pivotal role in the change of atmospheric conditions by releasing free oxygen as a by product of photosynthesis. This free oxygen was taken up by the elements with strong affinities for it like hydrogen, carbon, and iron. One of the big producers of this free oxygen were colonies of cyanobacteria like algal stromatolites. Over a billion years or so, it's thought that organisms like cyanobacteria were able to gradually change the atmosphere enough to fill all the oxidizing needs in the environment, producing a state of excess oxygen that continues to this day.

The evidence is in the sediments of the earlier Archean that are characteristically are dark brown and black caused by unoxidized carbon, iron sulfide and other elements and compounds that were present. As the atmosphere and oceans changed, so did the sediments. In the late Archean, sediments went through a transitional stage with the banded iron formations, after this transition they show an oxygen-rich environment, evidenced for instance by iron stained sandstones called red beds that persists to this day.

Banded iron formations are alternating sedimentary strata of iron and chert. They can be found on every continent in sediments that were deposited in the late Archean and early Proterozoic, when oxygen levels were fluctuating around a value sufficient to produce chert at that time. Chert is a type of silica oxide or quartz that can be precipitated chemically in oceans, and indicates an abundance of oxygen in the seas at that time of deposition.

The pH of the water also plays an interesting and complicated, although minor, part in these formations. On one hand iron stays in solution when in acidic aqueous conditions, and precipitates in alkaline conditions. Chert, or silica behaves in the reverse. The complication is in the fact that rains at this time were most likely acidic, making the oceans acidic as well, prohibiting the formation of the iron bands. Another wrench in the works is the difficulty in changing the pH of an entire ocean in a short period of time. This rules out pH as a major variable, but undoughtely was a factor. There may have been a difference in the behavior of iron during this period of Earth¹s history that can account for this quandary.

A better explanation for concise changes in banding seen in the rock that requires an almost spontaneous changes in ocean chemistry, is the fluctuation in concentration of free oxygen in solution during this period. However, almost spontaneous changes in oxygen levels is unlikely, unless you have highly localized sources of oxygen in the oceans, for instance algal stromatolites. This is generally how the idea that life changed the nature of the atmosphere and oceans came to be accepted.

Sudbury impact on oxygen levels.

Sedimentation styles of the passive margin changed as they came to a close. The sedimentary environment we see recorded near the end in these rocks changed from deep water shales derived from Archean rocks, to coarser clastic rocks derived from a younger Proterozoic source. This change is interpreted to be from the Penbine-Wausau ? island arc as it closed in on the passive margin from the south just before its collision with the passive margin. Sediments that were shedding off of the island arc piled on top of the previously deposited passive margin sequences in a style called forestepping, where the younger sediments are deposited farther and farther inland on the stable craton as sediments are still being shed from the craton.

The Iron Formations are distributed throughout Northern Wisconsin and the Upper Peninsula as well as parts of Minnesota. There are differences in the composition of the formation which may be evidence of the presence of several sub-basins in the area. The Iron Formation is found only in rocks dated as 2 billion years old or older. This is considered to be evidence of a change in the atmosphere at around this time, as organisms are putting oxygen into the air, and in turn causing the iron to precipitate out of the water. This increase in oxygen levels not only causes the iron present to precipitate out of solution but prevents iron from being dissolved in the waters at the previous quantities.

The unconformity after deposition of the Iron Formation represents a missing 300 million years. During this time, there was a change from passive to convergent tectonics in the area. The top of the succession is the Michigamee greywacke/ slate which is found in most places in the Lake Superior region but is called by several different names. The Michigamee is dated at about 1860 million years old

These are complex and include lavas erupted under an ancient sea, sedimentary rocks comprised of the erosional debris of the volcanic rocks, iron formations formed by chemical precipitation in the sea and granitic intrusions.

Cuyuna Iron Range

The Cuyuna Iron Range of east-central Minnesota was discovered in 1904 and became a major mining district of Fe and Fe-Mn ore. The range is divided into three structural entities: Emilv District, North Range and South Range, each with its own characteristic stratigraphic and structural attributes. The rocks of the North and South ranges are separated by a major structural discontinuity inferred to be a north-verging thrust fault, and both are overlain unconformably by strata of the Emily District. The rocks of the South Range have been assigned to the Mille Lacs Group, those of the North Range, to the North Range Group, and those of the Emily District, to the Animikie Group. This stratigraphic arrangement is considerably different from that in the older literature, which assumed that the various parts of the Cuyuna range could be correlated via a single interval of deposition of iron-rich sediments.

The North Range Group has been divided into three formations, the Mahnomen, Trommald and Rabbit Lake. The lowermost Mahnomen Formation can be divided into a lower member, which lacks ferruginous components, and an upper member dominated by beds of femrginous argillite and lean iron-formation interlayered with nonfemrginous argillite, siltstone, and quartzose sandstone.. The Trommald Formation, the main iron formation of the North Range, is a chemically precipitated unit that includes units of oxide, carbonate and silicate iron formation. The uppermost Rabbit Lake Formation can be divided into a lower member of black mudstone intercalated with beds of iron formation and units of volcanogenic origin, and an upper member of slate, carbonaceous mudstone, greywacke and thin units of iron-rich strata. The North Range of the Cuyuna range was regionally metamorphosed to the upper greenschist facies during the Penokean Orogeny, which peaked between 1870 and 1850 million years ago. We estimated an equilibrium metamorphic temperature from coexisting carbonates within the upper part of the Trommald Formation. Most of the carbonates fall compositionally along or very close to the calcite-rhodochrosite binary join, which was experimentally investigated by Goldsmith and Graf.

On the basis of their determination of the position of a solvus between kutnahorite and rhodochrosite, the equilibrium metamorphic temperature for the Trommald Formation near the Merritt mine was estimated at between 480°C and 490°C. A temperature of 450°C was estimated for the cross-cutting carbonate veins. A regional metamorphic pressureh as not yet been determined for the Penokean Orogeny in this area. Seventy-five km to the east, the pressure of the Penokean metamorphism was estimated to range between 4.5 and 8 kbars.

Mesabi Iron Range

The oldest rocks in the region are north of the Mesabi Range, in the Eveleth–Ely–Boundary Waters area.

Layers

The three different formations exposed along the Mesabi Iron Range were deposited along the leading edge of a foredeep basin – the Animikie Basin &ndasg; that transgressed north over the Archean craton during the Penekoan Orogeny. Deposition of the basal Pokegama quartzite, the medial Biwabik and Gunfline Iron chemical and the upper Virginia and Rove turbiditic sediments represents a transgression of near-shore, shelf and slope environments, respectively. These three layers were formed during the Paleoproterozoic time period of 2,500 to 1,600 million years ago.

From 2.6 to 1.7 Ga igneous activity subsided, seas withdrew and this was followed by a long, quiet period of erosion which is considered to mark the beginning of the Middle Precambrian Period.

Pokegama Quartzite

The Pokegama Quartzite is the lowest of this sequence of three differemnt formations that constitute the Animikie Group. The lower member of the Pokegama Quartzite contains shale, siltstone and minor sandstone, which Ojakangas has interpreted as having been deposited in a low-energy upper tidal flat environment in a sea that peneplaned the Archean surface. It is younger than 2,500 million years old.

Biwabik/Gunflint Iron formations

These formations are about 1,880 million years old.

Virginia/Thomson/Rove formations

These formations are between 1,800 and 1,600 million years old.

The seas returned two times and laid down great deposits of sediments which were converted into thick layers of rock. The upper deposits were laid down about 1.9 Ga to form the bedrock that is exposed as the shales, slates and mudstones of the Rove, Virginia and Thomson formations.

During the Penokean Orogen the Minnesota River Valley Subprovince overrode the Superior Province.

The Thomson Formation has steeply dipping beds of greywacke, siltstone and slate. The Thomson is the upper stratigraphic unit of the Paleoproterozoic Animikie Group, presumable equivalent with the Virginia Formation. Geochronologic data indicate that the Thomson Formation was deposited 1,880 to 1,870 million years ago, and was deformed by the Penokean Orogeny 1,850 million yeas ago. It is unconformable overlain by redbeds of the Mesoproterozoic Fond du Lac Formation. The Thomson is assigned as the uppermost unit of the Paleoproterozoic Animikie Group, along with the Vriginia and Rove formations.

The Rove Formation sediments in Minnesota’s Grand Portage State Park are very similar in origin to the Thomson Formation in Jay Cooke State Park, and were deposited in seawater in the same broad Animikie Basin. Here on the northern part of the Animikie Basin these rocks escaped the crustal deformation and thickening that characterizes the equivalent strata of the Thomson Formation and in northern Wisconsin and Michigan’s Upper Peninsula. This left the Rove Formation unmetamorphosed and essentially flat lying and without slaty cleavage. At about 1.9=2.0 billion years old, these are some of the oldest undeformed and unmetamorphosed sedimentary rocks in North America. The dikes and sills of the Rove Formation intruded during the Midcontinent Rift. The diabase bodies are known as the Logan Intrusions, named for the Sir William Logan, first Director of the Geological Survey of Canada. There are two sets of diabase intrusions in the area. A slightly older set consists of sills, which are layers that were squeezed while liquid between the subhorizontal beds of the Rove shales. Large examples of the Logan sills are prominent in the great mesa of Mount McKay in the Thunder Bay, Ontario, Canada area; the many east-west ridges between the Boundary Waters lakes north of the Gunflint Trail, northwest of Grand Marais, Minnesota. The second, slightly younger set of diabase intrusions forms large dikes. These solidified from basaltic-composition magma that was passing upward through the crust along great fissures. Most of them trend in an east-northeast direction in the Grand Portage area, but a few, such as the one that forms Hat Point and Mt. Josephine on the north side of Grand Portage Bay, trend northwesterly. These are very large dikes. The high, steep ridge alongside the highway on the north side of Wausaugoning Bay is principally one of these Logan dikes (also known as Pigeon River intrusions). Some of the Rove Formation sedimentary rocks, baked to various degrees by the great heat of the adjacent dike, are preserved along the lower slopes of the cliff. This dike extends east-northeast to and across Pigeon River where it forms the Pigeon, or High, Falls of the Pigeon River. The river, eroding its way down through the bedrock, has been able to cut deeply through the Rove Formation to form a deep canyon just below the falls, but this big diabase dike has been more resistant. The dike prevents the river from eroding below its level on the upstream side where the Rove sediments again underlie the riverbed. Farther upstream, in the southeast quarter of Section 24, the river has cut another deep gorge, with falls about 20 to 28 m (66 to 92 ft) should be 60 to 80 feet high. The rocks here are a combination of a northeast-trending dike, Rove shale, and one or two Logan sills, all are cut by a fracture zone that has helped to establish the straight course of the river gorge. Large veins of white calcite have been deposited along this fracture zone. Middle Falls, at the northwest corner of the park, is also associated with diabase intrusions. A combination of faults and another diabase dike have determined its position. The river first cascades over a 3 m (9.8 ft) should be 10 ft drop, then soon over a 7 m (23 ft) should be 20 ft freefall into a large pool.

Except for the rocky ridge, the park is covered with glacial till overlain by red clays deposited in the post-glacial lake. Much of the southeastern part of the park, below the High Falls, is covered by sand and gravel deposited as a river terrace or delta during a receding lake stand.

The Pigeon River has a low gradient for a considerable distance from its entrance into Lake Superior, similar to the Gooseberry River.

The Animikie Series has three regions: the western Mesabi, from the Mississippi River to east of Embarrass lakes; the eastern Mesabi, from the Embarrass lakes to southwest of Gunflint Lake; and the international boundary region extending from Gunflint Lake into Canada.

Also about this time the rocks of the McGrath Gneiss Formation, east of Lake Milles Lac.

The Penokean orogeny was a major early Proterozoic (1875-1825 Ma) tectonic event in the Great Lakes region. In east-central Minnesota, it is marked by multiply deformed and highly metamorphosed supracrustal rocks of the early Proterozoic Denham and Thomson Formations. Structural features similar to those in the supracrustal rocks also exist in the basement Archean (2700 Ma) McGrath Gneiss. Such features are here explained in a tectonic model consisting of southward-directed oblique subduction along the Great Lakes tectonic zone. Intense deformation occurred in the footwall of the major thrust, which marked the boundary between downgoing and overriding plates during A-type (continental) subduction. Sedimentary rocks (Thomson Formation) deposited on the footwall during loading caused by thrusting eventually became incorporated into the deformation zone. Early-formed structures related to footwall deformation are a dominantly well-developed foliation in the gneiss and isoclinal, recumbent folds with a bedding-subparallel foliation in the Denham and lower Thomson formations. Progressive metamorphism during subduction reached the garnet zone of the amphibolite facies. Various deformation inversions show that this early phase of deformation involved extreme flattening (with Z vertical) and large amounts of extension in both the north-south and east-west directions.

Footwall deformation was followed by imbrication and accretion onto the hanging wall during uplift associated with continued compression and isostatic rebound. Later-formed structures associated with imbrication and deformation within the hanging wall consist of folding of the foliation and development of shear zones in the McGrath Gneiss and open to close, upright-to-overturned folds in the Denham and Thomson Formations. The peak metamorphic event (represented by staurolite) occurred after the later deformation at temperatures of about 470-530 °C and a minimum pressure of 3.4 kbar (minimum depth of 12.4 km). Increasing temperature associated with decreasing pressure (uplift) is explained by conductive relaxation caused by crustal thickening and erosion. This tectonic model may have more widespread implications for explaining similar structural features found in many Precambrian terranes worldwide.

The major Early Proterozoic tectonic event is the 'Penokean Orogeny', which occurred about 1850-1900 Ma ago and included deformation, high-grade regional metamorphism, and extrusive and intrusive igneous activity.

The succession within the Animikie Basin, which unconformably overlies the Archaean basement, is characterised by three Groups: i). the basal Mille Lacs Group on the north-western side of the basin, and the Chocolay Group on the south-eastern rim, ii). the ~1878 to 1777 Ma Animikie Group on the north-western margin of the basin, and the lower Menominee and overlying Baraga Group on the south-eastern rim - these units contains the economic BIF units, and iii). the upper most Paint River Group.

The Mille Lacs Group is absent in the iron districts on the north-western margin of the basin and sections of the eastern rim, where the Animikie or Menominee Group sits directly on the Archaean basement. Similarly, the Paint River Group is only locally represented, with the unconformably overlying late Mesoproterozoic (1.10 ±0.01 Ma) Keweenawan basaltic lava flows of the Midcontinent Rift resting directly on the Animikie or Baraga Group.

The three subdivisions listed above, each represents a grossly fining upwards depositional cycle. The Mille Lacs and Chocolay Groups commence with predominantly quartz rich conglomerates and arenites/quartzites. These are overlain by platformal stromatolitic dolomites and shales on the margins of the basin, grading to mafic and intermediate subaqueous volcanogenic rocks, black (carbonaceous) shales and minor chert BIFs towards the axis of the basin.

Unlike in the Mesabi Range, where the Animikie Group rests directly on the Archaean crystalline basement, the intervening Mille Lacs Group is represented by the following sequence from the base: Denham Formation - Quartz arenite and siltstone, oxide iron-formation, marble, mafic hypabyssal intrusions and fragmental volcanic rocks metamorphosed to the staurolite grade of the amphibolite facies. Glen Township Formation - Commencing with conglomerates and quartz sandstones, grading upwards into thin bedded fine grained metasediments with thin dolomite beds, locally developed silicate­carbonate-oxide facies iron formation, carbonaceous shale with sulphides and basalts. Mafic volcanics of the formation have tholeiitic to calc-alkaline affinities and have been dated at 2197 ±39 Ma. Metamorphosed to greenschist to lower amphibolite facies. Little Falls Formation - Quartz-rich slate, argillite and schist in the northwest segment of the unit and coarse-grained megacrystic garnet-staurolite-schist in the southeast. Trout Lake Formation - Quartz arenite, siltstone, and chert-rich dolostone.

The overlying Animike Group which hosts the ore deposits, comprises, from the base: Mahnomen Formation - Claystone, shale, siltstone, and graywacke metamorphosed to the greenschist facies. Trommald Formation - a 14 to 150 m thick unit composed of carbonate-silicate iron formation and associated manganese oxide deposits. The iron formations have been oxidised and leached near surface to hematite iron formation. Also contains substantial quantities of volcanic and hypabyssal rocks of generally mafic composition. Metamorphosed to the greenschist facies. Rabbit Lake Formation - Mudstone, slate, greywacke, with lean, siliceous, argillaceous iron formation and associated mafic metavolcanic rocks, all of which have been metamorphosed to greenschist facies. Includes thin beds of carbonate-silicate iron formation.

The sequence in the Cuyuna Range has been deformed into a series of broad, open, eastward-plunging folds with near-vertical axial planes, in contrast to the monoclinal structure of the Mesabi Range. In addition, the stratigraphic position of the Biwabik Formation in the Mesabi Range is occupied by the Trommald Formation represented by several lenticular units of iron-formation intercalated the underlying Mahnomen Formation and the basal section of the overlying Rabbit Lake Formation. Stratigraphic relationships imply that the basal contact of the Animikie Group unconformably overlies older folded rocks of the Mille Lacs Group in the Cuyuna North Range. That unconformity marks the boundary between rocks of the Penokean fold and thrust belt which have undergone two pulses of deformation, while the Animikie basin sequence has only been subjected to one period of deformation.

The main BIF of the Trommald Formation, Unit A, was deposited during two transgressive-regressive cycles in a basin interpreted to have been deposited between a strandline to the west and deeper water to the north and east. It has been divided into seven lithotopes, as follows, from the base: i). Laminated to very thin bedded, to thick bedded to structurless (upward gradation) epiclastic sandstone, siltstone and shale, >30 m thick, with a matrix of sericite-chamosite-silica as well as calcite and siderite; ii). Thick bedded to upper structureless mixed epiclastic (quartz arenite) to jaspery chert, 5 to 10 m thick, with a matrix of mainly goethite amd manganese oxides; iii). Thick bedded to structureless oolitic and pisolitic grainstone iron formation, 18 to 20 m thick, with a matrix of psilomelane, cryptomelane and pyrolusite with lesser goethite and chert; iv). Thick bedded granular iron formation, with a matrix of manganese oxide and recrystallised grey to white chert, 11 to 30 m thick, including 5 to 7 m containing parallel and irregularly banded oxide rich layers comprising mosaically intergrown layers of hematite or goethite, with or without chert, and intercalated bands of manganite or cryptomelane; v). Interlaminated thick to thin granular and oxide rich to non-granular banded iron formation, with individual beds of chert, minnesotaite, stilpomelane and siderite with traces of magnetite and chamosite, 20 to 25 m thick; vi). Thin bedded to laminated, non-granular iron formation with individual beds and laminae similar to the underlying lithotype, predominantly minnesotaite; vii). Thinly laminated, non-granular ferruginous chert with layers of chert and hematite with lesser siderite and minnesotaite, 7 to 40 m thick.

While Unit A is mineralogically, texturally and chemically similar to other iron formations of the Animikie Group, it locally contains 10 to 100 times greater manganese oxides than the norm of 0.6 to 0.8% in the Biwabik Iron-formation elsewhere in the basin. Manganese oxides are principally concentrated in the coarser grained parts of Unit A as disseminated grains, thin pods or lenses, and layers as thick as 1.5 m that typically contain about 10% Mn, and locally as much as 20 to 30% Mn. Two laterally persistent zones about 15 to 18 m thick carry 10 to 50% Mn and coincide in general with stratigraphic positions occupied by oolitic-pisolitic iron-formation. These zones contain various proportions of psilomelane and cryptomelane, as well as hematite and quartz. Goethite and manganite are locally abundant representing secondary (supergene) mineralisation formed during a period of intense chemical weathering that modified these rocks in Late Jurassic or Early Cretaceous time.

Between 1904 and 1984, some 106 Mt of iron ore was mined from the Cuyuna Range, much of which contained more than 10% Mn, although no Mn was sepatately recovered due to the metallurgical difficulties involved. The ores from this production averaged 43.3% Fe with 6.5% Mn. Reserves of similar enriched ore contained 13% SiO2, 0.06% P2O5 (Klemic, 1970).

Potential resources of un-enriched iron formation have been estimated as 4400 Mt @ 28% Fe (Klemic, 1970). Other sources suggest resources of 1480 Mt @ 32% Fe. Sections of the iron formation that are manganiferous account for a resource of 20 Mt @ 10.5% Fe.

Black cherts of the Gunflint Formation are exposed along the North Shore of Lake Superior at Schreiber. They have yielded a large group of prokaryotic organisms. Included in the Gunflint biota are cellular filaments that are the remains of blue-green algae, bacteria and some microorganisms of complex structure but doubtful affinity. The Gunflint microfossils are about 1.9 By.