Minnesota's Rocks and Waters

Lecture Outline

Minnesota's Place in Geologic History

Minnesota's physiography

• Physiographic regions of Minnesota
  • Minnesota's landscape can be divided into coherent units based on major drainage basins (the area drained by a major river)
  • Note that the boundaries or divides of the drainage basins often coincide with depositional glacial features called moraines. These are ridges of glacial deposits laid down directly by the ice.
  • Two areas are notable for their general lack of glacial deposits. Here the topography is mainly caused by erosion.
    • North Shore Highlands and Iron Ranges and the Border Lakes in the Superior Basin and Rainy River Basin sections - glacial erosion
    • Driftless area of the Mississippi River Basin, southeastern Minnesota - river erosion
• another map of Minnesota drainage basins

Minnesota's bedrock - Minnesota geologic map

• what is a geologic map?
  • a map showing distribution of different bedrock units at the earth's surface
  • for sedimentary rocks, formations are mapped. These are units defined by distinctive types or interlayered types of sedimentary rocks
  • for igneous and metamorphic rocks, units are mapped according to distinctive rock types, often grouped into packages that are united by a common history
• what information can we get from a geologic map?
  • distribution of rock types in a region
  • distribution of rocks of different ages in a region
  • age distribution reveals pattern of geologic structures in a region
  • rock types and their ages and structural patterns enables us to work out geologic history from a geologic map
  • geologic maps can also be used for land-use planning
• what does the Minnesota geologic map reveal?
  • the northern half of the state is underlain by ancient Precambrian crystalline rocks, mainly metasedimentary rocks and igneous rocks
  • the southeastern quarter of the state is underlain by Paleozoic sedimentary rocks. These rocks are tilted southward into a basin in Iowa.
  • the southwestern quarter of the state is underlain by Mesozoic sedimentary rocks
  • a system of northeast-oriented faults traverses central and northern Minnesota, associated with a less prominent northwest-oriented system
• geologic provinces in the state
  • maps of geologic terranes of Minnesota
    • mapped separately from one another on the basis of the assemblage of geologic materials and their ages
    • note that ages are given for the bedrock in a particular terrane, using terminology from the geologic column and time scale. You should be familiar with the ages, including the use of the two units of the Precambrian (Archean (>2.5 billion years), and Proterozoic (2.5-0.6 billion years)), and the Paleozoic Era and Cretaceous Period. One unfamiliar term is Keeweenawan Rocks. These are late Proterozoic Precambrian rocks including mafic volcanic rocks and sandstones.
  • bedrock terranes including ages
    • mapped only on the basis of bedrock, its rock type and age
    • the notation Ga refers to billions of years
    • we have used the term Phanerozoic previously. Remember that it refers to all time after the Precambrian (600 million years to the present).
    • several terranes use names of rock units, such as the Duluth complex and the Sioux Quartzite
    • other geologic features such as the midcontinent rift system and the iron ranges are also mentioned
    • again, note the antiquity of the rocks in the northern part of Minnesota, the northeast-southwest orientation of the younger Proterozoic midcontinent rift, and the presence of a blanket of Phanerozoic sedimentary rocks in southern Minnesota

Minnesota's bedrock in cross section

• derivation of cross sections from geologic maps - how is it done?
• cross sections
  • by rock type
    • complexly deformed and steeply inclined igneous and metamorphic rocks (crystalline rocks) in central and northern Minnesota
    • gently inclined blanket of sedimentary rocks atop the crystalline basement in southern Minnesota. This blanket is tilted southward toward a basin center in Iowa
    • steeply inclined (dipping) faults are present in crystalline rocks of the central and northern portion of the state. These faults originated by compression
    • steeply inclined (dipping) faults are present in the younger Precambrian volcanic and sedimentary rocks of southern Minnesota, and some of these faults extend upward into the base of the overlying sandstone, dolostone and shale
  • by time-stratigraphic packages of rocks
    • notice that the rocks become generally younger from north at the left to south at the east
  • by geologic terrains
    • the various rock units related to one another by type, age and origin are given names. We shall begin studying these units in more detail as we begin the second portion of the course following exam #1.
    • note the ancient complexly deformed and metamorphosed rocks of the northernmost part of the state, intruded by granite batholiths of different ages.
    • Iron formations and other sedimentary rocks were deposited in basins formed atop the crystalline rocks, and in turn were metamorphosed. Late
    • Precambrian rifting resulted in deposition of the Keweenawan volcanics and sediments, followed by
    • deposition of the blanket of younger Phanerozoic sediments.

Minnesota's surficial geology

• map of bedrock versus surficial deposits
  • bedrock is present at the surface in northeastern and southeastern Minnesota. Pleistocene age (2 million years to 10,000 years ago) glacial drift is thin or absent in the northeast, as glacial erosion was the main process
  • drift from the last advance of the ice is absent in the southeast, as the most recent glaciation never reached this area but rather surrounded it like an island in a sea of ice.
  • other areas of bedrock are exposed where erosion by major rivers has managed to strip the surface cover. This is especially obvious in the valley of the Minnesota River
• map of bedrock exposure and drift thickness
  • contour map of thickness of glacial deposits reveals thick occurrences that lie along preglacial river valleys. Glacial sediments were deposited in these low places in greater thicknesses than atop preglacial uplands
  • study the distribution of drift of 60-120 meters in thickness (shaded green on the map). Distribution of this drift suggests that the preglacial drainage over much of the state was to the northwest. The present southeast drainage of the Mississippi River may have been rearranged from this early pattern by the great ice sheets of the Pleistocene.
• map of surficial deposits - mostly of glacial origin
  • two ages of surficial deposits are mapped - Pleistocene (2 million to 10,000 years ago) and Holocene (10,000 years ago to the present)
  • Holocene deposits include deposits of modern rivers (alluvium) and deposits of peat (rich in organic carbon) in lakes
  • Pleistocene deposits include sediments laid down directly by the ice. Pleistocene deposits from the last advance of the ice (Wisconsinan) inlcude sediments from different sources (called provenance). Some of the sediments come from Phanerozoic sedimentary rocks in Manitoba to the north, and some come from Precambrian crystalline rocks in the Superior Basin to the northeast.
  • Pleistocene deposits also include outwash (deposited by rivers draining the ice) and glacial lake sediments (deposited from suspension in glacial lakes) from the last ice advance (Wisconsinan). Note how the outwash (colored purple) follows the pattern of drainage systems. Also note the large glacial lake that existed in the present-day Red River Valley of northwestern Minnesota.
  • Pleistocene deposits also include sediments from glacial advances prior to the Wisconsinan. Because these sediments are older, they are not as widely exposed, either being covered by more recent deposits, or removed by erosion.

Brief Outline of Minnesota's Geologic History (derived from the above maps and cross sections)

• Archean collisions of small pieces of crust made of gneiss with volcanic island arcs to create Kenoraland, the nucleus of the North American Continent
• Early Proterozoic collisions of Kenoraland with an island arc system to the south to create the Penokean Mountains and the continent Laurentia
• Erosion of the Penokean Mountains and deposition of quartz sandstones in the Baraboo Interval
• Middle Proterozoic rifting, intrusion of the Duluth Gabbro, eruption of the North Shore Volcanics, and deposition of sediment in the rift
• ~600 million years of erosion
• Early Paleozoic invasions of seas across eroded continental platform and deposition of sandstones, shales and carbonate rocks
• ~300 million years of erosion
• Late Mesozoic invasions of seas and deposition of sandstones, shales and carbonate rocks
• ~75 million years of erosion
• repeated advances and retreats of Pleistocene ice, erosion and deposition of glacial sediment, creation of glacial landforms
• modern erosion by rivers and groundwater