Early Proterozoic 1.9 to 1.8 bya Granite, metavolcanics and metasediments (WI only)
The Early Proterozoic
(2,500,000,000 -1,600,000,000 years ago)
2.5 - 1.6 billion years ago
Rifting and the Animikean Basin
The supercontinent Kenorland begins to rift or break apart, separating into several smaller continents or terranes. Evidence of this rifting can be seen in an east-west trending zone across northern Minnesota, northern Wisconsin, the Upper Peninsula of Michigan, and southern Ontario. In this rifting zone, the Animikean Basin, the continent began to spread apart and gradually subside. Seawater invaded the rift zone, depositing sediments. In areas of Michigan and Ontario deposits formed by glaciers occur.
In Wisconsin basal sandstone made up of material eroded from the Archean granite and gneiss was deposited in the subsiding rift zone. This sandstone is up to 700 feet thick in the Gogebic Range and has been metamorphosed into quartzite.
Overlying this quartzite is a layer of carbonate rock that has been metamorphosed into marble. This carbonate rock was precipitated out of seawater by cyanobacteria. The cyanobacteria formed large microbial mats. These mats were sticky and easily covered with mud and other sediments. After being covered with mud the bacteria would grow up, through the layer, and form another microbial mat on top of the mud. In this way layers upon layers were deposited. These layered structures, often dome-shaped, are called stromatolites.
Banded iron formations are also present and formed in a similar way as stromatolites. Instead of precipitating carbonate minerals the bacteria precipitated iron directly out of the seawater. These iron layers alternate with quartz layers, the quartz often occurring as red jasper. The formation of these banded iron formation could only have occurred in very low levels of oxygen. Banded iron formations are an important source of iron worldwide. Early iron mining in the Lake Superior region concentrated on enriched iron deposits where the silica (quartz) had been leached out and large crystalline bodies of hematite, illmenite, psilomene, and other iron rich ores formed. As these were mined out, low grade ore from unaltered iron formations called taconite was mined and processed. Other sedimentary and volcanic rocks were deposited in the Animikean Basin including sandstone, shale, greywacke and basalt. These rocks were also metamorphosed under high temperature and pressure to form quartzite, slate, schist and greenstone.