On this day, April 16, 425,000,000 years ago

The next geologic  period that Michigan experienced was named the Silurian. If you recall, my last post was the 1 st mass extinction possibly caused by global cooling. In the Silurian, the climate again changed and warmed  causing massive melting of glaciers and increase in sea level.    Michigan existed submerged under a shallow sea.  Additionally,  the main phase of the formation  of the Michigan basin occurred, a basin being  formed by subsistence of the interior part of a continent.  In the case of Michigan, a bullseye centered in the lower peninsula.

So picture Michigan as a giant swimming pool and around the perimeter – the edges in Wisconsin, the UP, down through Canada , into Ohio Indiana  and Illinois, fantastic coral reef complexes grew rivaling the present Great Barrier Reef in Australia.  Pinnacle reefs 100 meters high kept pace with the basin’s subsistence such that the tops remained near sea level.  Their skeletons contribute to the thick deposits of limestone and dolostone. These carbonate rocks are quarried along several place along the escarpment  including Drummond Island in particular. The reefs had so altered the water flow cutting off the marine water from the open ocean that the concentration increased to the point where the salts precipitated out, predominately into rock salt.  60 trillion tons are estimated to be contained in the Michigan basin and it is mined in the Detroit and Windsor area today.

Other  marine fauna  found a foothold after the 1st mass extinction and jawed fish found a swimming hole along with the trilobites.  Land animals and plants were just starting to find a place on Earth.  Scorpions and millipedes crawled around  and simple very small plants with just clusters of branches as opposed to true leaves clung to the new ground.  These fossils however are not found in Michigan as it  was still submerged under the sea…

Thanks to http://geomaps.wr.usgs.gov/parks/pltec/sc425ma.html and

Michigan Geography and Geology by R. Schaetzl, J. Darden and D. Brandt




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