Selected Geology Stories from Minnesota-Part 2
Earth Science Extras
by Russ Colson
St Louis River branching around an island at Jay Cooke St Park, MN. The channel on the left follows the path of an eroded basaltic dike. Picture courtesy of former student Nick Anderson.
Eastern Minnesota and the Penokean Orogeny
The folded and metamorphosed Thomson Formation is exposed along the St. Louis River at Jay Cooke State Park, MN. Before we examine the folding and metamorphism that provide clues to the uplift of the Penokean Mountains, we are going to look at the deposition of the sediments that later became folded and metamorphosed.
In preparing to interpret the depositional environment for the Thomson Formation, we're going to review (in a greatly simplified way) depositional environments. The puzzle below is based on previous depositional environment lessons. Feel free to refer to your notes or to search online to remind yourself what each of the environments might look like in the real world and what kinds of sediments might be deposited in each.
To interpret the depositional environment of the Thomson Fm we need to consider the types of sediments and features that we find. The Thomson is made up of former shale and graywacke sediments (now become slate and metagraywacke through metamorphism). Features include ripple marks as seen below.
Features also include rip-up clasts of shale (now slate) that are incorporated into the graywacke. Think about this. Which would occur at higher energy--deposition of shale or the coarser-grained graywacke? Does it make sense that chunks of shale might get torn up form the sea floor during periods of higher current flow and become chunks within a graywacke deposit?
Both ripples and holes in the rock where the rip-up clasts (now slate) have weathered out are seen in the picture below. Think--which has to be more easily weathered, the slate or the graywacke?
Ripples and holes where rip-up clasts have weathered out on a bedding plane of Thomson Fm. Image courtesy of former student Nick Anderson.
One of the most important features in the Thomspon formation in our interpretation of its environment is the abundance of graded bedding sets. Graded bedding is illustrated in the picture below. Silt and clay sized particles are symbolized by the dashes, coarser sediments by dots and different sizes of circles. Such a feature can form where an initially high-energy underwater event gradually loses energy and the sediment settles out--largest settling out first. Multiple graded bedding sets are created when this process repeats.
In practice, it's sometimes tough to spot the graded bedding sets in the Thomson Formation without taking samples to examine under a microscope. They are especially hard to see on fresh surfaces of the Thomson Fm. However, sometimes they are easier to see on a water-polished surface (as of this wrriting, you can find a picture of a polished surface at a teacher field trip website from Carleton's SERC center https://d32ogoqmya1dw8.cloudfront.net/images/NAGTWorkshops/mpg/workshop2011/mpg_field_trip_pike.jpg )
or on a weathered surface where the subtle laminations are highlighted by the weathering process, as in the picture below.
Consider the cross-sectional models below showing a variety of different possible depositional environments in which the Thomson Fm might have been originally deposited (labeled A-E). Which does our observational evidence best support?
Clues to the Penokean Orogeny include the metamorphism of the Thomson slate and metagraywacke. Other clues include the observation that the rocks are tilted and folded
Previous students measured the rocks below to be tilted toward the NNE (north north east), toward the right in this picture.
The picture below is looking down the fold axis ("end on") of a famous anticline below Thomson Dam in eastern Minnesota. Previous students measured the trend of the axis of the anticline as roughly WNW (west northwest)--ESE (east southeast).
With the iunformation in the two pictures above, can you infer the type and direction of forces that formed the Penokean Mountains?
Unfolded basaltic dikes are seen in the Thomson formation, such as the dike below (picture courtesy of former student Laurie Eli). Dikes are sheet-like igneous features that cut across rock layers.
In the picture at the top of the page, the left hand channel follows the former path of a basaltic dike. If you turn around and look northeast in the opposite direction, you can see the continuation of the basaltic dike in that direction, as shown in the picture below (look for the straightish path without Thomson Formation rock). Notice that you don't actually see the dike--it has weathered away, meaning that the dike weathers more easily than the Thomson Formation.
Eastern Minnesota and the Continental Rift
We already figured out above, based on cross-cutting relationships, that the basaltic dikes are younger than the Penokean Mountains. Based on radiometric ages, they are quite a bit younger. The rift is added to our growing time line below.
The mid continent rift includes not only basaltic dikes, but other volcanic features as well, like lava flows.
What!? There are volcanos in Minnesota? Where!?
it's true, after 1100 million years, the volcanos themselves are long gone. But the lava flows remain! How can we be sure that these are, indeed, lava flows and not something else? Let's remember "the present is the key to the past."
Observaton 1: As lava flows out of the ground from modern volcanoes, gases exsolve (undissolve, bubble out) of the lava, making bubbles in the rock called vesicles. It is a characteristic feature of many volcanic rocks. Do we see that in the volcanics of eastern Minnesota?
Yes! Check out the vesicles in the pictures below from the North Shore.
But wait! It doesn't look like lava flows in Hawaii! The flows in Hawaii have smooth wrinkles that formed as the flowing lava froze.
So do flows in Minnesota!
Observation 2: Compare the pahoehoe lava flows in the following pictures, the first from Hawaii, the second from the North Shore (Pahoehoe is ropy lava that forms as the crust cools while it is still flowing, rumpling the surface like a wrnkled rug)
Observation 3: When basaltic lava cools, it shrinks, causing the rock to break apart in polygonal columns, kind of like mudcracks form as mud dries. We see this, for example, in the lava flows in Yellowstone and many other places. We also see it along the North Shore (below).
The images below illustrate two different kinds of igneous rocks that appear in the Duluth area. The rock on the left is part of what is called the "layered series" and the rock on the right is part of what are called basalt and rhyolite flows.
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Consider the cross-sectonal illustration of the rocks of the Duluth area below. Think about cross-cutting relationships, as well as the information above about whether rocks cooled quickly at the surface or slowly at depth in the Earth.
The layered series and the anorthosite are part of what is called a layered intrusive, a large former magma chamber at depth in the crust that gradually cooled and crystallized over a long period of time. In general, denser minerals like olivine--that might be expected to sink in the magma chamber--are concentrated in the lower parts of the magma chamber and less dense minerals that might be expected to float in magma--like anorthite--are concentrated in the upper part of the magma chamber. We can construct a simple model of gradual crystallization with minerals sinking and floating in which we form olivine-rich layers at the bottom of the magma chamber and anorthite-rich layers at the top. In reality, this model is too simple. This was a large, complex system for a long time, with periodic influxes of fresh magma, and periodic eruptions of magma from the chamber (feeding dikes and volcanoes that would have been far above the present location of the layered intrusive and above the present landscape). Thus, we see lots of cyclic variation in the intrusive complex (not just one simple set of layers) and lots of dikes of different composition cutting across it. Since our lesson here is mainly on cross-cutting relationship and telling a geological story in sequence, we are not going to look too much further at the intricacies of the Duluth Complex, but it is a very complex and interesting complex and is one of the largest layered intrusives on Earth. Layered intrusives are a major source for many mineral resources around the world, including chromium (Cr) and platinum group elements (PGE), and sometimes copper and gold.
One of the big takeaways from all the basaltic dikes that cut across each other, the basaltic dikes that cut across earlier lava flows, and a deep-cooling layered intrusive that is younger than the basalt flows that it formed within, is that this period of volcanic activity went on for a very long time, with dikes cutting across older lava flows in order to feed younger lava flows that are no longer seen in the Duluth area (due to the eastward tilt, some of these might be found today far out under the water in Lake Superior). Another clue to the vast amount of time that passed is the presence of sedimentary deposits between lava flows. Notice the feature labeled as interflow sandstone in the cross-section above. Lava flows didn't happen one right after another. Immense time passed between lava flows providing time for old lava flows to be eroded by rivers, and arkose-like sandstone deposits to be formed before being covered by the next lava flow.
The picture below (courtesy of former student David Ahumada ) shows one of these arkose-like cross-bedded sandstone units at Leif Erikson Park in DUluth. It sits on top of one basalt while the basalt that once covered it up has eroded away. Notice the prominent crossbedding in the unit.
Intrusive igneous bodies--and magmatic centers in general--provide heat energy that drives the circulation of hot water through the ground. The hydrothermal water carries dissolved minerals that can crystallize out as conditions change (for example, as the water gets hotter or cooler, more acidic or less acidic, more oxidized or less oxidized, and so on). Thus, the heat drives mineralizing fluids that can cause minerals to be deposited in viens or vesicles in the rock or replace other minerals and create ore deposits.
We're going to consider a couple of crosscutting relationship that constrain the time period of the hydrothermal deposition of quartz in veins and vesicles throughout the region. Hydrothermal vein quartz is typically white because tiny bubbles in the quartz from the hydrothermal water cause the quartz to look milky (many white things are a result of tiny particles or droplets inside it--think of clouds!). We are going to look at miky quartz veins and vesicles in basalt, and then we are going to look at milky quartz in a sedimentary rock that was deposited in the rift valley as rivers washed down off the high areas on either side of the rift.
Quartz mineralization in basalt on the North Shore. Notice the white quartz in the vein on the left and filling vesicles on the right.
Milky Quartz pebbles in the lower Fond du Lac Formation. This conglomerate is at the very based of the Fond du Lac Formation, laying on top of the Thomson Formation in an angular unconformity.
Below is a cross-sectional illustration that includes the Fond du Lac Formation--an immature arkose-like sandstone that formed as rivers washed sediment out of the surrounding highlands down into the rift valley (with the quartz pebble conglomerate at its base). It also shows the red-colored Glacial Lake Duluth Deposits, muddy sediments deposited from the water of Glacial Lake Duluth during the retreat of the glaciers when low-level outlets were dammed by glaciers (simiilar to how the damming of the Red River by glaciers made Glacial Lake Agassiz). Squiggly lines indicate unconformities. This particular conceptual cross-section is valid in the Jay Cooke State Park area south of Duluth, The Duluth Complex, intruded into the volcanic layers after a great thickness of volcanics had already accumulated, is not seen here, either becaue it was never intruded into this area or because it has since eroded away. Even the immensely thick volcanic deposits are eroded in most areas near Jay Cooke, leaving the late-rift red sandstones of the Fond du Lac to lie directly on the Thomson Fm. Notice that there is a "triple junction" between the basalt flows, Fond du Lac, and Thomson formations. This triple junction can also be seen in map view and is a key clue to the presence of an anugular unconformity.
Three more cross-cutting stories related to the the Duluth Intrusive Complex
The pictures below show the contact between Anorthosite (a coarse-grained intrusive rock and part of the intrusive complex) and a diabase dike in Duluth. Diabase is a fine-grained igneous rock much like basalt except that it formed in a dike instead of a lava flow, and so cooled slightly slower. A student is pointing out the contact between the rocks in the right-hand picture. A contact is an important feature in geology because it often allows us to establish the relationship between different rocks. What does the small-crystal size of the dike and the fact that it cuts across the anorthosite tell us?
The picture below shows anorthosite xenoliths in gabbro within the intrusive complex in Duluth. A xenolith is a chunk of some other lithology that gets caught up in an igneous rock (xeno= stranger, lith = rock). Notice that the blobs of anorthosite have been flattened into flying-saucer shapes.
The two pictures below are from north of Duluth along the shore of Lake Superior in Split Rock State Park. In this area, a large diabase dike cuts across the layers of lava that lie stratigraphically above the layered intrusive in Duluth (Think 'Dike F" in the cross-sectional illustration of the intrusive given above). Within the dike are quite large anorthosite xenolith carried upward in the magma from the top of the layered instrusive that would have been present deep below the surface. Compare this contact with the contact shown within the intrusive complex in some of the pictures above. The magma that fed this dike must have passed through the Anorthoisite layer of the Duluth Complex just like the dike seen above in Duluth.
Northeast Minnesota and Ore Deposits: Iron, Copper, Ni, PGEs and others.
The Banded Iron Formations (BIFs) of Minnesota are chemical sedimentary deposits formed in the time range from around 2.6bya to around 1.8bya. The chemically deposited Fe-oxide deposits that form the ore are interbedded with another type of chemical sedimentary rock--jasper. Jasper is a form of cryptocrystalline (super tiny crystals) quartz that has a reddish color due to presence of iron oxides. Jasper is the same mineral as chert and flint--only the reddish color makes it different. These alternating layers of iron oxide and jasper are what gives the Banded Iron Formations their name. The picture below, courtesy of former student David Ahumada, shows the appearance of the BIF in the Sudan Mine near Tower MN.
The red bands are the jasper, the silver bands are the iron oxides--the ore. In some places, the iron oxides are highly concentrated and these are the areas that were mined out when the mines were active.
Making an ore deposit requires that elements that are present in the earth become concentrated sufficiently to make them mineable at a profit. The concentration of an ore requires that elements of interest be dissolved from areas where they are at low concentration, transported by some process, and then precipitated (deposited) in areas where they become more concentrated. Iron is soluble (and so can be dissolved and transported in water) in its chemically reduced state (low oxygen conditions) and insoluble (so it it will precipitate) in its chemically oxidzised state (higher oxygen conditions). Thus, we can imagine iron ore formation as being the process of dissolving iron from volcanic or weathering activity in areas of low oxygen and transporting it to small areas of high oxygen. If conditions are universally oxidizing, then we can't dissolve and transport the iron. If there are no patches of high-oxygen, then there is no way to precipitate the iron. Thus, it is necessary to have both reduced and oxidizing conditions to get the formation of BIFs.
It is probably no accident that the great BIFs of the world occur in a time period when Earth was presumably transitioning for a low-oxygen atmosphere without life, to a world with photosyntheitic life producing oxygen, although in northeast Minnesota, the ejecta from a giant meteorite impact in ancient times lies directly on top of some of the last BIFs, suggesting that the impact itself may have finally tipped the balance toward too much oxygen in Earth's oceans to transport iron.
Hill Annex Mine State Park
The BIF at HIll Annex Mine was a major iron ore source from 1913 to 1978. The mine is situated within one of the youngest of the BIFs. The BIF is overlain by the Virginia Formation, which is a black shale with graded bedding sequences much like the Thomson Formation and is thought to be a farther-north equivalent of the Thomson Formation that was deposited in the same subsiding sea as the Penokean Mountains built up from the south.
The BIF at Hill Annex Mine is characterized by the presence of oolitic hematite. Oolites, typically made of calcium carbonate rather than iron oxide (hematite), form where wave action turns and tosses the particles of sediment during the ongoing process of chemical deposition, creating round particles with concentric layers of sediments sort of like a pearl. These sediment particles are called oolites. The presence of oolitic hematite at Hill Annex Mine thus suggests which of the following depositional environments?
The scale markings in the image below of an ooloitic hematite sample from the Hill Annex Mine are 1mm.
In addition to the Biwabik BIF, there is a conglomerate unit seen at HIll Annex Mine SP called the Colerain Fm. This conglomerate unit includes clasts eroded from the BIF, incuding clasts of chert (jasper) and hematite.
Which do you think had to come first, the shallow sea in which the BIF was deposited, or the sea whose crashing waves formed the conglomerate?
Right! The BIF had to already be there before the crashing waves could erode it and redeposit it as rounded pebbles in a conglomerate!
The conglomerate also includes fossil shark teeth, marine clams and snails, and other fossils of Cretaceous age. Check out the Timeline above to see where the Cretaceous Interior Seaway falls relative to the deposition of the Biwabik Formation. It was "quite awhile" later!
Below are some fossils found on field trips in the Coleraine conglomerate (alas, no shark teeth!)
Below is a stratigraphic section seen on the wall of the old mine pit at HIll Annex Mine. What stories does it tell?
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Sudan Mine State Park
The BIF at Soudan is significantly older than that at the Hill Annex Mine, taking us back to the early days of the window of time in which BIFs occur. Iron was first deposited during the time when the early formation of continents was still ongoing, a period of time when "cores" of continents, consisting of greenstone-granite provinces, were forming.
Greenstone belts include pillow basalts made of bulbous blobs of lava. The 'pillows' form when lava flows deep underwater. The water rapidly cools the surface of an emerging mass of lava, forming a glassy 'pillow case' around a pillow-shaped blob of magma. The pressure of the water suppresses the exolution of gasses making the lava less vesicular than might be true for surface lavas. A famous example of ancient greenstone-belt pillow basalt is found southeast of Vrignia, MN.
Greenstone belts are long, narrow stretches of mafic rock (such as basalt) that are sandwiched between areas of more felsic rock (like granite). These areas are thought to be fundamental features of the early formation of crust on the Earth. We can imagine that divergent tectonic zones created basaltic crust and convergent zones created more felsic crust, somewhat like today, and the movement of these crustal 'plates' caused the different types of crust to accrete together to make ever-larger continent-like land masses. In the early days of crust formation, plate tectonics was probably quite unlike what we see today, and we are still trying to understand how continents were being assembled in these ancient times, but a simplified illustration of the concept described here is shown below.
Notice that the mafic area gets folded into a broad syncline-like mass. It will also be metamorphosed, which produces green minerals in the basalt that give the belts their name--greenstone belts.
A cross-sectional illustration of the Soudan ore body is shown below. Notice the big synclinal fold apparent in the cross section. Notice also how the mine shaft follows the main ore body as it dips steeply down into the earth. The steep dip of the layers of the ore body is what required this mine to be an underground mine rather than an open pit mine like at Hill Annex Mine.
Because the Sudan BIF is so much older than the Hill Annex Mine BIF, and because it has been through this folding process, the bands are more complex than the simple tilted layers we see at Hill Annex Mine SP. The picture below, courtesy of former-student Dustin Wenzil, illustrates some of that greater complexity. There are folds, fractures (with some fault-like offset) and the the yellow arrows point out a few of the quartz veins that cut through the rock. The iron oxide (hematite) is the darker or silvery material and the jasper is the lighter reddish material.
The picture below showing a quartz vein in the banded iron formation comes from level 27 within the Soudan mine. This picture is courtesy of former student David Ahumada.
Sulfide mineralization
Whenever you have hot water percolating through a rock, you have the potential for all kinds of mineralization processes. Typically, hot water is drivern by energy from a nearby magmatic or metamorphic event. There have been several of these in Minnesota over the eons, most notably the magmatic activity associated with the continental rift that we examined above. If you also have access to sulfur from some of the rocks, then sulfide minerallization becomes possible. Except for iron, most metal ores on Earth, such as copper, are associated with sulfide deposits.
The picture below is of a Cu-sulfide deposit on level 27 in the Soudan Mine, courtesy of former student David Ahumada.
So, did this Cu ore form at the same time as the iron ore, as a chemical sedimentary deposit in an ancient Archaen sea or was it the product of later hydrothermal transport and deposition?
Probably the latter, occuirng several hundred million years after the original iron deposition, although the deposition of the copper sulfides might have happened at or near the same time that hydrothermal activity was concentrating the iron ore (by dissolving and transporting away the unwanted jasper deposits).
Minnesota is home to one of the largest deposits of Cu, Ni, and Platinum Group Elements (PGEs) in the world, mostly in sulfides! The picture below shows sulfide mineralization in a core sample from the Poiymet property southeast of Virginia MN. The gold and silver colors are characteristic of different sulfide minerals. The sulfides in this area contain significant Copper, Nickel, Platinum, Palladium and other metal resources.
The material surrounding the sulfides is anorthositic mafic intrusive igneous rock that is part of the large intrusive complex associated with the rift that formed in eastern MN. Most of the ore deposits are found where this intrusive material (supplying heat and perhaps water to drive hydrothermal circulation) makes contact with the black shale of the Virginia Formation (supplying low oxygen and high sulfur). The combination of a source of low oxygen, high sulfur and a source of hot water, create conditions for sulfide mineralization.
You might be wondering why, if this is one of the largest mineral reserves in the world, it hasn't already been mined. One reason is that the deposits, although large, are not as highly concentrated or accessible as some other deposits in the world, thus it is cheaper to get the ore elsewhere. However, eventually the price and politics will be right so that people start to mine this immense resource.
The sulfide deposits formed quite differerently from the iron ore deposits. The sulfides formed by hydrothermal activity, the iron ore deposits by chemical sedimentary processes (perhaps concentrated by later hydrothermal activity). There is another issue related to sulfide deposits that is different from iron mining of the past. Iron oxides are very insoluble in water and do not react with the atmosphere to create problematic pollutants. Not so with sulfides. Sulfides often contain very environementally-unfriendly elements such as arsenic and cadmium, and also create acidic runoff with makes many problematic elements soluble (and therefore mobile). The acidic water itself can be an environmental issue. The production of acidic water is an inescapable by product of suflde mining. Pyrite, a very common sulfide, reacts with oxygen in the air and rainwater to produce sulfuric acid after a reaction of the sort:
1/2FeS2 + 1.5O2 + H2O → H2SO4 + 1/2Fe
Preventing acidic water and dissolved pollutants that will inevitably be produced by a suflide mining operation from getting into surface and ground water is a major pre-requisite for utilizing this large ore reserve in Minnesota.
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last updated 5/15//2020. Images from other sources, such as from student field trip participants ,are indicated in the text. Other text and pictures are the property of Russ Colson.