Guide to central Minnesota geology
by Terry Boerboom
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.
All of the Precambrian rocks were deeply weathered at about the same time as deposition of the Cretaceous rocks, resulting in a mantle of decomposed rock of varying thickness (typically 10 to 100 feet) on top of the Precambrian basement. As a result of this weathering the minerals in the original rock have broken down to produce a mixture of mainly green and white clays and angular residual crystals that consist mostly of quartz. In general, clay content is highest at the top of the weathering profile and the proportion of residual grains increases with depth, grading into fresh rock below. The weathering profile varies considerably in thickness, but tends to be exaggerated over areas where the bedrock is more permeable, such as fractures. In places where this weathering product has a high proportion of residual grit and a low proportion of clay it is known as grus. This grus can be locally utilized as an aquifer.
- 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.
Selected references to the geology of central Minnesota
The above maps and publications are available from the Minnesota Geological Survey,
- Subsurface research and scientific drilling in western Minnesota. D.L. Southwick, 1980.
- Scientific core drilling in central Minnesota: Summary of lithological and geochemical results. D.L. Southwick, G.N. Meyer, and S.J. Mills, 1986.
- Minnesota kaolin clay deposits: A subsurface study in selected areas of southwestern and east-central Minnesota. D.R. Setterholm, G.B. Morey, T.J. Boerboom, and R.C. Lamons, 1989.
- Graphite in Early Proterozoic rocks of east-central Minnesota. P.L. McSwiggen and G.B. Morey, 1989.
- Scientific test drilling in west-central Minnesota: Summary of lithological and stratigraphic results 1987-1988, and some preliminary geological conclusions. D.L. Southwick, D.R. Setterholm, and T.J. Boerboom, 1990.
- Pre-Penokean igneous and metamorphic rocks, Benton and Stearns Counties, east-central Minnesota. G.A. Dacre, G.R. Himmelberg, and G.B. Morey, 1984.
- Subsurface till stratigraphy of the Todd county area, central Minnesota. G.N. Meyer, 1986.
- Geologic map (scale 1:250,000) of the Penokean orogen, central Minnesota. D.L. Southwick, G.B. Morey, and P.L. McSwiggen, 1988. (Geologic map).
- Geologic atlas of Hennepin county, Minnesota. N.H. Balaban, editor, 1989.
- Geologic atlas of Washington county, Minnesota. L. Swanson, and G.N. Meyer, editors, 1990.
- Geologic atlas of Ramsay county, Minnesota. G.N. Meyer and L. Swanson, editors, 1992.
- East-central Minnesota, bedrock geology. G.B. Morey, B.M. Olson, and D.L. Southwick, 1981.
- St. Paul Sheet, bedrock geology. R.E. Sloan and G.S. Austin, 1966.
- Quaternary Geologic map of the Minneapolis-St. Paul urban area, Minnesota. G.N. Meyer, 1985.
- Bedrock geologic and topographic maps of the seven-county Twin Cities Metropolitan area, Minnesota. M.A. Jirsa, B.M. Olson, and B.A. Bloomgren, 1986.
- Bedrock geologic map of the Minneapolis-St. Paul urban area, Minnesota. B.A. Bloomgren, 1985.
- Bedrock topography and isopachs of Cretaceous and Quaternary strata, east-central and southeastern Minnesota. J.H. Mossler, 1983.
- Ground-water geochemical atlas for parts of east-central Minnesota. R.S. Lively and G.B. Morey, 1985.
- Geologic map of Minnesota, depth to bedrock. B.M. Olson and J.H. Mossler, 1982.
- Geologic map of Minnesota: bedrock geology. G.B. Morey, 1993.
- Geologic map of Minnesota, Quaternary geology. H.C. Hobbs and J.E. Goebel, 1982.
- Hydrogeologic map of Minnesota, bedrock hydrogeology. R. Kanivetsky, 1979.
- Hydrogeologic map of Minnesota,Quaternary hydrogeology. R. Kanivetsky, 1979.