Outline of "Minnesota's Geology"

Pages 15-20

Overview of Minnesota's Geologic History

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Minesota's geologic setting

• craton of North America, the stable portion of the continent
• straddles boundary between Laurentian Upland (Canadian Shield) and Interior Plains and Lowlands (with cover of Phanerozoic rocks)
• Topography subdued - low-lying and gently rolling to flat
  • why? for one thing, no mountain building has occurred since two major Precambrian events at 2700 million and 1800 million years b.p. (before the present) and no volcanism since formation of the mid-continent rift about 1100million years ago. Only gentle warping of the crust with occasional earthquakes has occurred during the last billion years, interrupting the advance of seas over the craton.
  • long interval of erosion, punctuated by marine flooding events during the Paleozoic and Mesozoic, has produced the low-relief topogrpahy. While erosion by running water has worn down the once great Precambrian mountains, it has also produced erosional topography in the vicinity of the major rivers.
  • glaciation during the last 2 million years has left a thin cover of glacial sediments over the surface of most of the state, and this cover has resulted in constructional topography in many places (topography created by build-up of deposits of sediment)
• Precambrian mountain building
  • rocks of the Canadian Shield in northern Minnesota, especially the Lake Superior region, contain the record of earth history, from 3,600 million years ago to about 600 million years ago - 3 billion years of geological record!
  • two major events of mountain building and volcanism are recorded in the igneous and metamorphic rocks and the metasedimentary rocks of the region
    • 2,700 million years - Algoman orogeny
    • 1,800 million years - Penokean orogeny
  • major extensional forces formed the mid-continent rift
    • 1,100 million years ago
• Paleozoic marine flooding events
  • Beginning about 525 million years ago and continuing to about 450 million years ago, shallow seas flooded the region whose topography was already subdued by erosion.
  • Flooding was the result of global rise in sea level, and was punctuated by retreats of the sea, probably caused by intermittent uplift of part sof the crtaton.
  • In southeastern Minnesota, flat-lying beds of sandstone, shale, limestone and dolomite with a record of shallow-marine fossils (mostly invertebrates) are the modern record of these ancient seas and are exposed by erosion in the bluffs along the major drainage systems
• Mesozic marine flooding events
  • During the Cretaceous Period, around 135 to about 100 million years ago, shallow seas flooded Minnesota from the western interior of North America.
  • In southwestern Minnesota, sandstones and thick accumulations of black organic-rich shales are the main rocks that record this history. Marine invertebrates, sharks and reptilian remains are preserved in these rocks.
• For the past 100 million years, Minnesota has stood above sea-level and has been influenced by surface processes dominated by river erosion, and of course, glaciation during the great ice age of the past 2 million years.
• Glacial drift (sediment) blankets the surface of much of Minnesota, in some places reaching thicknesses of 450-500 feet in northwestern Minnesota.
  • Maps showing thickness of drift essentially show us the paths of valleys that existed in preglacial time. Deeper valleys were filled in with greater thicknesses of sediment. Thus, we can see what the preglacial landscape might have been like by studying these maps.
  • Drift covers bedrock and makes it difficult to study the rocks that form the foundation of the state. When outcrops (surface exposures) of bedrock are absent, the geologist must use information from wells drilled into the bedrock, or information from geophysical surveys such as magnetic or gravity surveys which are influenced by the density of different rock types at depth.

Geologists important in geologic work in Minnesota

• D.D. Owen
  • Geologist for the U.S. Department of the Treasury, responsible for the first report on the geology of Minnesota (this was before the creation of the U.S. Geological Survey in
• N.H. Winchell
  • compiled first geologic map of MN in 1972
  • revised the geologic map in 1900
  • first director of the Minnesota Geological Survey, 1872 -1900
• W.H. Emmons
  • director of the geological survey, 1911-1944
• F.F. Grout
  • revised the geologic map of Minnesota in 1932
  • director of the geological survey1944-1946
• G.M. Schwartz
  • director of the geological survey 1946-1961
• P.K. Sims
  • director of the geological survey 1961-1973
• Matt Walton
  • director of the geological survey 1973-1986
• Priscilla Grew
  • director of the geological survey 1986-1993
• D.L. Southwick
  • director of the geological survey 1993-present

The Minnesota Geological Survey was non-existent from 1900-1911, during which time the U. S. Geological Survey filled the void; the state survey was reestablished in 1911 as a division of the University of Minnesota.

In 1972, the centennial year of the MGS, a centennial volume was published dealing with the geology of the state. This centennial volume, edited by P.K. Sims and G.B. Morey, is a landmark publication.

Funding from the state legislature for the MGS was meager from 1911-1961, a very odd fact indeed, considering the mining activity and mineral potential in the state, and the need to maintain and utilize continuing supplies of clean water, sand and gravel, building stone and other resources, not to mention the need for wise urban and environmental planning. By 1975 the budget increased to $300,000 annually, and by the early 1980's, was up to more than a million dollars a year.

Curiously, a recent governor of the state tried to abolish the survey as a money-saving scheme. Fortunately, the attempt failed. Why is that fortunate? Because the survey is the only group able and mandated to gather a continuing data base on the geology of Minnesota, a data base that is integral to making enlightened decisions about development of resources and wise land-use in the state.