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Michigan, USA
Introduction
The State of Michigan is unique in that it is separated into two geographically distinct regions that are, for the most part, geologically different as well. The State is divided into an Upper and Lower Peninsula, separated by the Straits of Mackinaw between Mackinaw City and Saint Ignace. The state is divided by geology as well and features two very distinct areas. The first area includes all of the Lower Peninsula and approximately one-half (the eastern) portion of the Upper Peninsula, which consist mostly of Paleozoic aged sediments (mostly limestone and dolomite). The second area is the western half of the Upper Peninsula which is dominated by Archean and Proterozoic Precambrian aged gneiss, granites, basalts, and sandstones.
The boundary between the sediments to the east and mostly Precambrian rocks to the west extends from approximately Marquette in the north to Iron Mountain in the south. The following is a very brief description of the geology of Michigan. For a more detailed read, please see the many books published on the subject.
Precambrian Geology
The Precambrian rocks that dominate the western half of Michigan’s Upper Peninsula are very diverse and include both intrusive and extrusive varieties, as well as sediments and their metamorphic equivalents. Rocks of this period include those of the Canadian Shield, the Banded Iron Formations, and the Portage Lake Volcanics. Outcrops of these rocks can be found scattered throughout the area; however, in most cases, the rock record is either fragmentary and incomplete, or buried under overburden left by the glaciers.
Since their formation, much has happened to these rocks. The oldest, the Watersmeet Gneiss, has been dated to 3.56 billion years old, making it some of the oldest rock in North America. The Watersmeet Gneiss, as well as many other of the oldest rocks, have been subjected to at least three major crustal deformations and mountain building events. In addition, there have been at least four smaller scale deformation events in the area. These events were accompanied by the intrusion of granitic rocks deep within the crust that were later brought to the surface by uplift and erosion. In addition to the granitic intrusions, at least four periods of volcanism have been discovered that produced basalt rocks and pyroclastic deposits, as well as dikes and dike swarms composed of diabase and gabbro.
Since their formation, metamorphism of these rocks has produced a multitude of new and reworked rocks and minerals. The greenstone, gneiss, quartzite, marble, slate, and schist found throughout the western Upper Peninsula can all trace their roots to much older rocks. The youngest rocks in this group are the Portage Lake Volcanics, which include basalts, conglomerates, and sandstones. These packages of rocks have remained largely unchanged since their formation, except for very shallow burial metamorphism of the basalts to the prehnite/pumpellyite grade. Rocks of the Portage Lake Volcanic series are associated with the Midcontinent Rift, a great tear in the Earth’s crust that nearly separated North America in two. The Rift can be traced from near Detroit northwest through the Lower Peninsula into Lake Superior, then arching back southwestward across the Keweenaw Peninsula and into Wisconsin.
Many of the metallic mineral deposits in Michigan come from the Precambrian rocks of the western Upper Peninsula, including the world renowned native copper deposits on the Keweenaw Peninsula and the banded iron formation deposits of the Marquette, Menominee, and Gogebic Ranges. Other minerals of note to come from the Precambrian rocks include silver, prehnite, garnet, goethite, datolite, gold, and the famous Lake Superior agate.
Bedrock geology map (MDNR)
Paleozoic Rocks
Rocks of the Paleozoic are represented by formations that underlie all of the Lower Peninsula and the eastern half of the Upper Peninsula. All of these rocks are sedimentary that were deposited in a shallow sea over a period of about 370 million years beginning in the Cambrian and lasting through the Pennsylvanian. During this time, approximately 14,000 feet of mostly limestones, dolomites, sandstones, and shales were deposited on top of the Precambrian basement rocks; the same rocks that are exposed in the western Upper Peninsula. As these sediments were being deposited, the entire region began to downwarp into a bowl-shaped depression. This depression, called the Michigan Basin, dominates the geology of the Lower Peninsula. The basin dips inward from the edges at approximately 2 degrees until reaching the center of the basin near the geographic center of the Lower Peninsula. Most of the Paleozoic rocks in the basin are derived from marine sediments; however, a few fluvial sandstones and coal deposits are exceptions.
What is unique about the entire sedimentary package is that they have not been metamorphosed or subjected to igneous intrusion or volcanism. Structurally, the rocks have not been subjected to major deformation except for minor folding and faulting near the edges of the basin. In essence, these rocks have remained sedimentary rock, modified only slightly by weathering and the action of groundwater. The rocks of the Paleozoic do not represent a complete record of sedimentation in the Michigan Basin. Several times during the sedimentation, uplift interrupted the sinking of the basin which is characterized by erosion, or at least non-deposition, and has created several time gaps, or unconformities. While very few metallic minerals are found in the Paleozoic rocks, almost all of Michigan’s oil and natural gas reserves are found in the Michigan Basin rocks. Also found in the basin are large economic deposits of limestone and dolomite, including the world’s largest limestone quarry located near Rogers City, and large salt deposits under Detroit, Michigan.
Post Pennsylvanian and Pleistocene Geology
This period is marked by the fact that very few rocks are found with an age between the end of the Pennsylvanian and the Pleistocene glacial deposits. This “Lost Interval” in geologic time is represented only by a small area of red beds located near the center of the Basin which are completely buried by glacial deposits and are only known from drill core data. The age of these red beds has been interpreted as youngest Jurassic based on fossil plant spores extracted from the drill cores. Beyond these rocks, no rock record is preserved from this period; this is likely due to a change from deposition to a period of slow erosion on the landscape until the Pleistocene Epoch and the arrival of the glaciers from Canada. It is known that at least four major glaciations took place during the Pleistocene, but only the youngest of these (the Wisconsinan) are preserved in Michigan. The surficial deposits from this epoch include clastic sediments, drift, gravels, sands, and clays. Surface features found scattered across both peninsulas that can be directly attributed to glacial activity include kettle lakes, eskers, lateral and end moraines, and kame deposits. The sediments carried in by the glaciers, while not loaded with economic minerals that can be mined, are very rich in nutrients that make them excellent for growing crops that supply food to millions of people around the world.
One additional rock type that deserves mention are the kimberlite deposits found in Upper Michigan. These kimberlites are thought to have been emplaced during the Cretaceous Period between 150 and 100 million years ago, during the time dinosaurs roamed the earth! First discovered in 1971, these deposits have generated much interest both popular and scientific. To date, no economic diamond deposit has been discovered in Michigan. It has been suggested that Michigan did not have the correct tectonic setting favourable for a highly diamondtiferous kimberlite. It is also believed that the Mid Continent Rift had a very destructive influence on the lithospheric mantle, thus destroying any diamonds being stored in the mantle prior to the kimberlite eruptions. Currently there is no active diamond exploration taking place in Michigan; however, there are a number of untested targets and placer deposits of indicator minerals occurrences in the state. Maybe one day, the state of Michigan will produce the next “diamond rush”?
The State of Michigan is unique geologically, and to truly understand its complexity the rocks must be seen in person and not simply read about in a textbook or trade magazine. Not many places on Earth can boast areas where a person can stand on 3.6 billion year old Archean gneiss then take one step and stand on glacial deposits just 7,000 years old; that is quite an amazing thought!
[Paul Brandes 2011]
Mineral List
Mineral list contains entries from the region specified including sub-localities475 entries listed. 341 valid minerals. 5 type localities (valid minerals).
Localities in this Region
The above list contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to
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References
Carlson, S.M., and Floodstrand, W., 1994. Michigan Kimberlites and Diamond Exploration Techniques (abstract), in Institute on Lake Superior Geology Proceedings, 40th Annual Meeting, Houghton, Michigan, v.40, part 4.
Dorr, J.A., and Eschman, D.F., 1970. Geology of Michigan: University of Michigan Press, 476 p.
Dyl, S.J., and Wilson, M.L., 1992. Michigan Copper Country: Mineralogical Record, v.23, no.2, p. 24-28.
Heinrich, E.W., 1976. The Mineralogy of Michigan: Michigan Department of Natural Resources, Geological Survey Division, Bulletin 6, 225 p.
LaBerge, G.L., 1994. Geology of the Lake Superior Region: Geoscience Press, 313 p.
Robinson, G.W., 2004. Mineralogy of Michigan: Michigan Technological University, 252 p.
Dorr, J.A., and Eschman, D.F., 1970. Geology of Michigan: University of Michigan Press, 476 p.
Dyl, S.J., and Wilson, M.L., 1992. Michigan Copper Country: Mineralogical Record, v.23, no.2, p. 24-28.
Heinrich, E.W., 1976. The Mineralogy of Michigan: Michigan Department of Natural Resources, Geological Survey Division, Bulletin 6, 225 p.
LaBerge, G.L., 1994. Geology of the Lake Superior Region: Geoscience Press, 313 p.
Robinson, G.W., 2004. Mineralogy of Michigan: Michigan Technological University, 252 p.
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Locality Updated: Spray mine, Zeehan, Zeehan district, Tasmania, AustraliaFrom Rod Martin, 19th May 2013 09:41:10