CRYSTALLINE ROCKS OF THE WESTERN UP

As thousands of feet of sediments --- clean sands, clays, silts, lime muds, and mixtures of any two or all of them --- were eventually spread on the Precambrian ocean’s floor, thousands of feet of stratified (sedimentary) rocks were formed. As they became thicker their weight upset the balance between the high lands and ocean troughs. Then nature readjusted the pressures by shaking, lifting, thrusting, the hardened sediments above sea level, building mountain ranges by pushing molten rocks from beneath the crust near to the surface, in places the flat lying sediments were lifted, wrinkled, folded, crumpled, and broken into arches and troughs, making new mountains and rearranging the ocean basins.  Nature changes rocks from one form to others, making metamorphic rocks out of the older igneous or sedimentary rocks.  Granites are metamorphosed to gneisses or schists, sandstones to quartzites, limestones to marbles, shales to slates.
    So upon the earth we have three types of rocks: igneous, sedimentary, metamorphic. The metamorphic rocks tell this tale of change but in them are many other records buried and lost. The western UP has a variety of Precambrian rocks formed through igneous or metamorphic activity: crystalline rocks.  Many of these are NOT sedimentary rocks, such as are found in the Michigan Basin.

Source:  Photograph courtesy of Randy Schaetzl, Professor of Geography - Michigan State University  


Note the locations of Precambrian rocks in Michigan---in the western UP.

Source: Geology of the Lake Superior Region,  LaBerge.

As the thickened, Precambrian crust emerged above sea level, it was stripped by erosion and the resulting sediments were redeposited in adjacent depressions. As exposed in the Canadian Shield, the thickened crust consists of multifarious granitic intrusions and subordinate enclaves of folded and faulted volcanic and sedimentary rocks. The enclaves, known as "greenstone belts," contain minable concentrations of gold, silver, copper, zinc and lead.   The rocks are called "greenstones" because of the green hue that the chlorite minerals within impart to the rocks.  Greenstones form in tightly folded and metamorphosed "belts", which in North America often run east-west.  Between the belts are large blocks of folded and metamorphosed gneiss, and then later, as molten material intruded into these rocks, granites formed amidst the gneisses and greenstones.  The map below shows the abundance of greenstone belts in Canada, on the Candiamn shield.

Source: Geology of the Lake Superior Region,  LaBerge.

Greenstones are thought to have formed at failed rift zones, as lavas poured out into the tectonically "stretching" crust.  Some of these lavas have since been metamorphosed, by compression, to greenstones.  After the failure of the rift, the rift then closed again, causing the lavas/greenstones to fold into synclines (the folding created the heat and pressure for the metamorphism).  Greenstone belts are often intruded between (younger) granite rocks, as shown below.

                        A cross-section through a greenstone belt.

Source: Geology of the Lake Superior Region,  LaBerge.

Next, during part of the Precambrian era, a 250 million year-period of quiet, which we name the Huronian from its record north of Lake Huron, thick sediments were laid down in a shallow sea trough that covered the Lake Superior region. In places thick sand was deposited; in other fine muds, and in other places pure lime, accumulated in the shallow but slowly deepening sea. Over the sand great masses of iron minerals accumulated, either by chemical action or by the work of iron forming bacteria, or by both and perhaps other means, until vast thicknesses of sand and iron sediments were built up.  The world’s largest iron deposit was in the making....in Minnesota, Wisconsin, and Michigan.  In that far ago time the foundations of Michigan’s wealth and the automobile industry were laid in the old Huronian sediments we now find in the iron ranges of Marquette, Baraga, Iron, Dickinson, Menominee, and Gogebic counties.
    Although one often thinks of copper and iron ore when Precambrian rocks of Michigan are mentioned, other rocks are also found in the PC formations. For example, there is the green verde antique and white marbles, graphite, slate, talc, and asbestos of the Huronian rocks. The white marbles of Dickinson County are too badly shattered for use as a building stone, but are in use for stucco, small ornamental work, and paint filler. Green marble near Ishpeming was formerly quarried for use as terrazzo, but it is possible that blocks large enough for building purposes may be produced. Graphite used for paint was once produced from graphitic slate quarries east of L’Anse. The fine black slate quarries are on the northwestern side of the Huron Mountains near Arvon, Baraga County. When the quarries were in operation the slate was used for roofing shingles. Some of the Ford Museum buildings at Dearborn are roofed with this slate. Talc and asbestos are in the rocks north of Ishpeming. Gold has been found in quartz veins of the rocks north of Ishpeming. The most famous and productive mine was the Ropes Mine, from which over $625,000 worth of gold was taken. Exploration may reveal more gold-bearing veins. All these mineral resources await future development and use.

Some of the images and text on this page are from LaBerge's Geology of the Lake Superior Region.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.