Thursday, December 12, 2013

A paper I wrote last year on the Biwabik Iron Formation

The Biwabik Iron Formation
Jeff Harrison
                                                                     Image courtesy Humboldt State University

            The Biwabik Iron Formation is a large formation which accounts for much of the iron that has been historically mined in Northern Minnesota.  This makes it an obvious choice as a subject for a short paper on rock history as the Biwabik Iron Formation is far from being short on history in itself.  From its origins during the oxidation of the world’s oceans and subsequent reduction of ferric materials dissolved in the waters through the following 1.9 billion years, the Biwabik Iron Formation has been through a lot.

            The Biwabik Iron Formation is a thick (100-250m) that extends along the 250km length of the Mesabi Iron Range (Jirsa et al, 2007).  This formation exists as a layering of 4 layers and dips towards the south east at approximately 7°-15° (Severson, et al.). 

      Figure (1) The Biwabik Iron Formation is associated with the Mesabi Iron range shown in black on this image.   (Jirsa et al. 2007)
            As you can see in figure 1 the Biwabik Iron Formation is layered in-between the archean granites that lie underneath and to the north of the formation, and the Virginia Formation that lies over and to the south.  It is also partially buried under the later deposited Duluth Complex which lies generally to the east.
            In the figure 2 you can see that the general orientation of the formation is flat and the relative pinning between the granites to the north and the Virginia Formation to the south is related to the later tilting that the region experienced, much of which should be attributable to the collision of the Penokean Orogeny forming the Animikie Basin and the deposition of the Duluth Complex (Wikipedia, Animikie Group).

Figure (2)  Cross section of the Biwabik Iron Formation showing relative positions of the granite Giants Ridge Batholith, the overlying sedimentary Virginia Formation, and the overlying Duluth Complex (Jirsa et al. 2007)

Age of the Biwabik Formation

            The age of the Biwabik Iron Formation is constrained by the ages of the under lying Granite formations and by the terminal event of the Sudbury Impact Event as there is ample evidence of deformation within the formation by the cataclysmic results of the Sudbury event, along with a strong correspondence between the timing of the Sudbury event and the end of the deposition of banded iron formations world wide at this time (Cannon, 2008).
            The under lying Superior Providence dates between 2.6 and 2.75 Gya (Wikipedia, Animikie Group), where as the Sudbury Impact Event has been dated at 1.85 Gya (±3Mya) (PSSCA, Earth Impact Database, 2013).  This gives us a very loose constraint on the age of the formation, but a dated Zircon crystal “in the nearby Gunflint Iron Formation yielded an age of 1,878.3 ±1.3 million years” (Fralick et al., 2002 as cited in Jirsa et al., 2007) which would be right in line with the deposition of the Biwabik Iron Formation being a process that continued for the several tens of millions of years up until the time of the Sudbury Impact.  By this logic to give an age of 1.9 Gya would be reasonable for the formation of the Biwabik Iron Formation.  Given the thickness of the deposited layers it also would have taken a considerable amount of time for the deposition to occur.  Perhaps 1.90GYA ±50 Mya should be the most accurate constraint for the development of the formation.

Rock Types

            The most basic description of the rock types with in the Biwabik Iron Formation is that it consists of a upper slaty, upper cherty, lower slaty, and lower cherty layers as shown in Figure 3.  These descriptions should not be confused with the petrological definitions, but is a stratigraphic scheme designed early in the exploration of the region, slaty being terminology used by miners, (Severson, et al.) which was much improved later (Jirsa et al. 2007).  The layers are not clearly delineated as slaty and cherty with a few exceptions, but the nomenclature persists as during a recent mine tour that is exactly how the layers were referenced.
            Later efforts to better define and determine the amounts of recoverable magnetite divided the formation up into 22 different layers, each with their own mineralogy and characteristics (Jirsa et al. 2008).

Figure (3)  Map of the southern end of the Biwabik Iron Formation showing locations of mining and a general scale of the general layers present along the extent of the formation. (Severson, et al.)  The black sections correspond to mined intervals within the deposit.

                The mineralogy of the region (with the exception of metamorphosed regions in close proximity to the Duluth Complex) contains a mixture of silicates, oxides and carbonates.  For silicates the most common members are chert, chalcedony, and quartz, along with the sheet silicates stilpnomene, minnesotaite, greenalite, chamosite, and talc.  Also common are carbonate minerals such as siderite, ankerite, calcite and dolomite.  Most importantly to the economy of the region are the oxide materials that made the region famous including magnetite, hematite, and goethite (Severson, et al.).  There are many other minerals present including amphiboles, pyroxenes, olivines and garnets but brevity will preclude listing them all (Jirsa et al.,2007)
            The formation is divided into bedded sedimentary layers of sandstone, shale, chert, and siltstone.  These layers are dependent on the depositional environment, which varied by both location and the time of deposition as at the time the region was a shallow marine basin which varied in depth and slope as it was concurrently being squeezed between the Superior Province to the north and the Penokean orogeny to the south.

Figure 4  Formation of the Biwabik Iron Formation showing the constraining regions (Severson, et al.)

Figure 5  The Biwabik Iron Formation was deposited in the Animikie Basin during the time when the Penokean was over-riding the existing Superior Province, please note that this was before the rifting event which created the Duluth Complex and as a result the Iron Range and Wisconsin are ~60 miles closer than they are at present (Severson, et al.).

Geological Environment of Formation

            As is seen in the figures 4 and 5 the Biwabik Iron Formation was deposited in the Animikie basin at the time when the basin was constrained between the foreland bulge in the Superior Province and the over riding Penokean Orogeny (Severson, et al.).  Sedimentation coming off of the raised portions of the Superior Province and the Penokean Orogeny mixed with iron which was precipitating out of the marine environment in the basin which resulted in layers of iron bearing minerals between layers of sediment.  This creates the banded Iron Formation that has been traditionally associated not only with the Biwabik Iron Formation, but with many deposits through out the world, all of which were deposited at approximately the same time.
            The Superior Province was a peneplain, or relatively flattened region bordered by a marine shelf.  The Biwabik Iron Formation has been extrapolated to have been deposited in the deeper part of the near shore marine environment on the continental shelf, below the wave base, but still tidally dominated (Severson, et al.).

Geologic Story

            The years have been rough for the Biwabik Iron Formation, if that would be a good way to describe an area that was formed with assistance of a continental orogeny, terminated by a major meteorite impact, and then partially overlaid by a failed rift.
            The formation itself is largely a result of convenient timing creating a marine shelf when the oceans were starting to precipitate oxides as oxygen was being created by early life for the first time in Earth’s history to an abundance that would allow such an event to occur.  The marine shelf was the result of the foreland bulge in the Superior Province and the headland being forced downward by the over riding Penokean Orogeny (Jirsa et al.,2007). 
            Approximately 1.85 Gya the Sudbury Impact occurred a few hundred miles to the east, although the physical effects were mostly felt in the far north-eastern extent of the Biwabik Iron Formation, the timing of the Sudbury Impact has been often associated with the interruption of the deposition of BIF world wide (Wikipedia, Banded_iron_formation).

Figure 6  Map of the region as it is today showing many of the structures that are inter-related with the Biwabik Iron Formation (Iron Range), including the Superior Province, Animikie Basin and the Duluth Complex (Jirsa et al.,2007).
            Still later there was an attempt by Minnesota to break up with Wisconsin which sadly was stopped by the intervention of the Grenville Orogeny further to the East.  This did result in the addition of ~60 miles of additional separation between the Biwabik Iron Formation and Wisconsin, and also the intrusion of the Duluth Complex which buried a large stretch of the Biwabik Iron Formation creating a separation between its exposed stretches on what is traditionally referred to as the Iron Range and the Gunflint Range further to the north-east.  This can be easily noted when comparing the maps in figures 5 and 6.  The intrusion of the Duluth Complex also greatly metamorphosed the contact zones between the Biwabik Iron Formation and the Duluth Complex which created a host of additional minerals in contact horizons. 
            Much, much later humans showed up with shovels and explosives which has possibly had one of the greatest impacts on the Biwabik Iron Formation, and also has made it famous world wide as a source or iron which by some accounts drove the industrial revolution in the United States.


Cannon et al., 2010, The Sudbury Impact Layer in the Paleoproterzoic Iron Ranges of Northern Michigan, USA, Geological Society of America., Wikipedia, Multiple Authors, Background information of Animikie Group, verified Precambrian dating for Animikie group components., Wikipedia, Multiple Authors, Background information on Banded Iron Formation.

Jirsa, M et al., 2007, Geology of the Biwabik Iron Formation and Duluth Complex, Science Direct.

Severson, M et al., Geology and Stratigraphy of the Central Mesabi Iron Range, Field trip guide 2., Humboldt State University Natural History Museum, Multiple Authors, Cover Photo., Earth Impact Database, Multiple Authors, Date of Sudbury Impact., XKCD, Randall Munroe, Cartoon showing that cartoonists don’t know how to use a rock hammer.

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