Talk:Geology of Minnesota/Sandbox

PreCambrian - Volcanoes, Iron ore formed in Middle Pre-Cambrian Cambrian Paleozoic Mesozoic Quatenary

http://www.dnr.state.mn.us/snas/naturalhistory.html

Geologic Terrains of Minnesota

There are four basic subdivisions to the geology of the central Minnesota area, summarized in the table below. The youngest material, which makes up nearly 100% of the present land surface, consists of unlithified glacial till and glacial outwash. These Pleistocene glacial deposits cover older bedrock units, and are typically in the range of 100 to 200-feet thick but may be as much as 400 feet thick. Cretaceous sedimentary rocks, the next oldest unit, consist primarily of poorly lithified shale, sandstone, and limestone. The Cretaceous rocks in central Minnesota occur mostly as scattered outliers or erosional remnants on top of older Precambrian rocks, but become substantially thicker to the south and west. Paleozoic sedimentary rocks, mainly restricted to southeastern Minnesota, consist of interlayered sequences of sandstone, shale, and limestone that decrease in thickness to the northwest of the Twin Cities. Paleozoic rocks are not present in most of central Minnesota, but are very important aquifers in the Twin Cities and southeastern Minnesota. The counties fringing the northwestern metropolitan area are sites of recharge for these aquifers of the Twin Cities. Precambrian rocks are the first bedrock encountered in most of central Minnesota. The Precambrian is subdivided by age into three major groups-the middle Proterozoic (Keweenawan) (youngest), the Early Proterozoic, and the Archean (oldest). Rocks of the Keweenawan system run along the axis of Lake Superior and continue south along the Minnesota/Wisconsin border. These rocks occur along what is known as the midcontinent rift, and in central Minnesota consist primarily of basalt flows overlain by clean sandstones, which are very similar to overlying Paleozoic sandstones. The next oldest subdivision of the Precambrian, the Early Proterozoic, consists of a wide variety of rocks ranging from slate, schist, and gneiss to granite and gabbro. The northern extent of these rocks is roughly outlined by the Mesabi and Cuyuna iron ranges to the north, and they continue south and southeast beneath Cretaceous, Paleozoic, and Keweenawan rocks. The oldest Precambrian rocks, Archean, are made up primarily of bands of various types of granites situated between linear greenstone belts, and in west-central and southern Minnesota, gneiss.

Pleistocene glacial drift Glacial till- Variable mixture of clay, silt, sand, and boulders. Often described in drillers' logs as clay, boulder clay, or sandy clay, typically red, brown, yellow, gray, or blue. Low water-bearing potential if clay-rich, moderate if sandy. Glacial outwash- Sand and gravel with lesser amounts of silt or clay. Primary source of water throughout central Minnesota.

Cretaceous sedimentary rocks Gray and black shale, yellow sandstone and rare limestone. Low water-bearing potential, especially in shale beds, sandstone beds generally of insufficient thickness to yield substantial water.

Paleozoic Sedimentary rocks Yellowish-brown and white sandstone, yellow to gray limestone, gray shale. Low water-bearing potential in shale beds, high in sandstone and fractured limestone beds. Primary aquifers for the Twin Cities.

Precambrian rocks Weathering residuum- White and green clay, variable proportions of white to gray grit. Forms a cap on top of Precambrian rocks. Low water-bearing potential, except for low-capacity wells in areas where grus is developed, but these are hard to predict.

Keweenawan System- Yellowish-brown sandstone, underlain by volcanic basalt flows with thin brown sedimentary rocks between flows. High water-bearing potential in upper sandstones, moderate in interflow sedimentary rocks. Early Proterozoic and Archean crystalline rocks- Gray to black slate and schist, pink to gray granite, and gabbroic rocks. Prediction of bedrock possible with varying degrees of confidence from published bedrock geologic maps. Very low water-bearing potential, except in areas where the rock is highly fractured.

http://www.geo.umn.edu/mgs/centrlmn.html

Minnesota's Geology, by Richard Ojakangas and Charles Matsch (Minneapolis: University of Minnesota Press, 1982) http://www.tc.umn.edu/~smith213/index.htm GEOLOGICAL SOCIETY OF MINNESOTA