Early Proterozoic Baraboo Interval and Middle Proterozoic Geologic History of Minnesota Outline of Topic

Introduction

The Middle Proterozoic spanned the interval of time from 1.6 Ga to 1.0 Ga. There are no rocks in Minnesota that belong to the Late Proterozoic (1.0 Ga - 543 Ma). By the begining of the Paleozoic (543 million years before the present), organisms had evolved hard parts and began to appear in great numbers in the fossil record. Bedrock of this age is exposed mostly in northeastern and southwestern Minnesota, together with buried bedrock in a strip from St. Cloud south to Austin and Albert Lea.

Early Proterozoic Baraboo Interval and Middle Proterozoic rocks in Minnesota include, in order from oldest to youngest

See the accompanying cross section for the geologic relationships between these rocks and the older Precambrian record.

The Early Proterozoic to Middle Proterozoic erosion interval and deposition of quartz sandstones

Extensional forces attempt to rift North America

The history of the rifting is very interesting and gives us insight into the way plate movements can change through time. Read this next section and study the accompanying maps.

Here's an edited excerpt from the book "Annals of the Former World" (John McPhee, author) that describes this period of geologic time:

    If you could have traveled westward from the site of Chicago 1.1 billion years before present (BP), you would have traversed, along the route of Interstate 80, the Precambrian basement rocks of the continent. At 1.108 billion years BP, rifting (or splitting) of the North American continent began. At around 1.100 billion years BP, the two parts of the continent were still moving apart and would continue to move apart for 14 million years more. Something under the core of North America was tearing North America apart, threatening its continuing existence as an integral continent. It seems likely that the cause of this Midcontinent Rift was a thermal plume from deep in the mantle, a geophysical hot spot doming the crust and then cracking it.
    Flood basalts filled the rifting valley. At night, above the lava fountains that would have been coming from the rifting area, the whole sky was red. At about 1.100 billion years BP, a "triple junction" plate break occurred under what is now Lake Superior, connecting the southwestern arm of the rift with the arm that ran through Michigan's UP and lower peninsula. If the rifting had gone on long enough, the country between the two active arms---including at least half of what is now called the Midwest---would have departed from North America to end up who knows where and in how many pieces. In the middle of North America, a great bay would have developed, with a shoreline of a thousand miles. But for some reason, the rifting stopped as rapidly as it began.
    Between Lincoln and Omaha, Nebraska, I-80 runs directly over the center of the rift. It gradually slides toward the rift's eastern flank at Des Moines. On I-80, the whole western half of Iowa is over the rift itself or its flanking basins. To follow this cartographically, you would need the Composite Magnetic Anomaly Map of the United States. On the magnetic and gravity maps the Midcontinent Rift is the most prominent feature you see. A quarter-century ago, it was as unknown in scientific mapping. The rift was referred to as "the midcontinent gravity anomaly" or "the midcontinent gravity high."  These names implied no genesis for the gravity anomaly, just that it existed. We now know that the gravity anomaly exists due to the presence of heavier, denser basalts in the rift---which have now become hardened lavas.
    If the rifting had continued even for a couple of hundred million years, as the Mid-Atlantic rifting has done, Lincoln and Des Moines would be as far apart as Jersey City and Casablanca, whose sites were once as close as Lincoln and Des Moines. Yet that did not happen. The midcontinent rift system did not, in the end, play a major role in the evolution of the continent, because the rifting stopped---or was stopped-moreor less abruptly. The rift system's rocks date from 1.108 to 1.086 billion years BP, so the rifting lasted 22 million years---not much by comparison with the Atlantic Ocean, but (to date) about three times the length of time that there has been rifting in the Gulf of California and longer than the rifting of the Red Sea.
    Something seems to have snuffed out the young hot spot, leaving the midcontinent intact. Where crustal blocks had dropped in the middle of the rift as it widened, they now were subjected to a compressional force so great that the middle of the rifts rose up to a position higher than the sides. In the language of geology, grabens were squeezed upward and became horsts. It was as if the Red Sea were to stop widening, while its floor came up to stand higher than the shores.
    The compressional force that stopped the rift in Proterozoic North America is believed to have been the Grenville Orogeny. This name has been given to a continent-to-continent collision, completed by about 1.050 billion years BP, that brought large continental blocks to collide with the eastern and southern margins of North America to create the supercontinent Rodinia, hundreds of millions of years before Pangaea, the most recent of supercontinents. In Grenville time, Africa and South America were neither configured nor juxtaposed as they would be later on.  The edge of this continent-continent collision zone is called the Grenville Front--see the map below for its location.

 

 

Here is a reconstruction of the supercontinent Rodinia that formed as a consequence of the Grenville Orogeny, around 1.0 Ga. Rodinia later broke up to become the individual continents that eventually reassembled into the supercontinent Pangaea. Note that Laurentia (present-day North America), following the breakup of Rodinia, existed as a separate continent until another collision brought it together with all the other continents to form the supercontinent Pangaea, about 300 Ma. When Pangaea broke up, about 180 Ma, present-day North America began its trek to its present position on the globe.

Rocks deposited in the Mid-continent rift are shown below

 



Photographs of the North Shore Volcanic Group along the North Shore of Lake Superior

Lava flows of the Mid-Continent Rift

Duluth Complex - Gabbro Intrusives

Youngest Proterozoic Sedimentary Rocks in the rift

Addendum: Below is a stratigraphic column for the mid-continent rift in northern Wisconsin and the upper peninsula of Michigan. Most of the rocks were deposited in river systems and lakes that filled the depressions on the flanks of the rift. Note that in rift basins such as the mid-continent rift, correlation of sedimentary rocks from one segment of the basin to another will be very complex, because the bodies of sediment are not continuous sheets. Consequently, we find very different stratigraphic interpretations for the Michigan and Wisconsin parts of the basin compared to the Minnesota and Iowa parts of the rift. Thicknesses are also quite variable in different segments of the rift basin.

Notes:

Bessemer Quartzite - The oldest unequivocal rift rocks exposed in northern Wisconsin and upper Michigan are quartz-rich fluvial and lacustrine sandstones of the Bessemer Quartzite. These were deposited in a broad basin in response to initial thinning of the crust on the site of the future rift

Oronto Group - A thick succession of rift-filling clastic sedimentary rocks, the Oronto Group, overlies the rift-filling volcanic rocks of the Powder Mill and Bergland Groups (equivalent to the North Shore Volcanic Group in Minnesota). These rocks represent the change from a period dominated by volcanism to one dominated by sedimentation.

Note that as the heat flow from the mantle plume decayed or died down, subsidence in the rift increased because of cooling and contraction of the crust. This allowed the great thickness of the Oronto Group to accumulate. In fact, there are 8km of sediment that accumulated in the rift at the present site of Lake Superior.

The Oronto Group is subdivided into three formations: 1) the Copper Harbor Conglomerate, dominated by coarse red alluvial fan conglomerates; 2) the Nonesuch Shale, a thin intervening lacustrine gray to black shale mineralized with copper sulfides over a wide area, but with economic concentrations only near White Pine, Michigan; and 3) the Freda Sandstone, composed of fluvial red sandstones.

Next, late in the history of the basin, subsidence rates declined and more mature lacustrine and fluvial (river-deposited) sandstones of the Bayfield Group were deposited across the entire basin.