The beginning of the Penokean orogeny is recorded by the banded iron formations which are sedimentary strata of late Archean and early Proterozoic time. These sediments are also thought ot record the introduction of abundant free oxygen into our atmosphere. This change was very 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, itıs 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.
So how do we know the primal Archean atmosphere was rich in the right chemicals to enable production of life? 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 have persisted from this change 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 Earthıs 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.
In the Great Lakes region during the early Proterozoic, the gneissic block that accreted to the Superior experienced a rifting event as described near the end of the last section. This rifting split the gneissic terrane leaving only part of it attached to the Superior province as the edge of the continental crust. The original extent of the gneissic terrane to the south is not known because of this rifting event. This type of rifted edge is called a passive margin, which develops after rifting as it becomes a slowly subsiding, technically calm continental edge.
During rifting, there are two generations of sediments that collect in the 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 injected along the center rise. Sedimentation generally ceases during this transitional period because of the elevation of the area 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 that is heavy enough that 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.
This rifting scenario is basically what happened to the southern coast of the gneissic terrane and Superior Province. The oceanic sediments associated with the last stage of rifting contain the Banded Iron Formations that were discussed earlier. 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 GLTZ, from Minnesota through upper Wisconsin and Michigan into eastern Canada where they terminate against a later formed tectonic terrane called the Grenville province. We see these two billion year old sediments of sands and silts today as hard sedimentary rocks assemblages that are called by their local formation names such as, the Marquette Range Supergroup in upper Michigan and Wisconsin, Mille Lacs, Animikie, and North Range groups in Minnesota.
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. These deposits were fuel for the industrial development of the United States that has an equally interesting history, but unfortunately is out of the scope of this project.
The sedimentation on this calm coastal passive margin came to a close when the next orogeny began to develop around 1.90 bya. These sediments took on a different character shortly before they were crumpled up during the Penokean orogeny, which is the focus of the next section.
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