Outline of "Minnesota's Geology"

Pages 62-86

Paleozoic Geologc History

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Shallow Inland Seas

• Connected to seas at margins of continent, where episodic sedimentation was continuing from Precambrian into Paleozoic time
• Seas flooded continental platform and retreated in repetitive fashion as the result of
  • worldwide changes in sea level (eustatic changes) and
  • changes in the level of the continents brought about by uplift and subsidence (tectonics)
• about 550 Ma, seas began to flood the craton and new life forms began to evolve
  • seas rich in diverse life including plants (algae) and animals (dominantly invertebrates)
  • an important difference between Cambrian and Precambrian life is the presence of hard parts which facilitate preservation
  • new life forms evolved from Precambrian forms in response to change in environment
    • oxygen content of atmosphere increased to point where it wasn't used up by oxidation of carbon and iron
    • build-up of oxygen permits respiration by animals
    • some oxygen changed to ozone which further shielded evolving life from UV radiation from the sun
    • first animals probably evolved near colonies of plants in water deep enough to shield them from UV radiation.
    • as animals began to migrate to shallower water, they developed hard parts to protect from UV, higher wave and current energy and predators in more crowded niches
    • more space for marine life in shallow epicontinental seas resulted in burst of evolution
    • how shallow? by analogy with present, probably mostly within the photic zone, less than 330 feet (100 meters) deep
    • alternating subsidence and uplift of the craton. together with world-wide changes in sea level, brought about many transgressions and regressions of the seas throughout the Paleozoic and Mesozoic, and with each cycle, new life forms evolved
• Regional structural features of the craton were important to pattern of sedimentation in Cambrian and Ordovician seas in Minnesota
  • Transcontinental Arch - served as barrier to transgression and source of sediment
  • Illinois Basin - served as subsiding center for deposition (depocenter) on south side of arch
• Marine flooding of Minnesota beginning in late Cambrian
  • transgression from continental margins at about a mile every 100,000 years
  • reason for flooding? often difficult to sort out the relative importance of the following mechanisms
    • climatic warming and melting of ice raises global sea level
    • subsidence of craton - not a global change - probably most important
    • uplift of ocean floor, perhaps in response to appearance of new spreading centers, results in global rise in sea level
  • distribution of arches and basins which were involved in subtle vertical movement of the craton controlled the distribution of Paleozoic seas and the resulting sediment
  • only part of Minnesota was flooded by Paleozoic seas.
    • Evidence is in the sedimentary rocks formed from sediment deposited in basins. Areas that stood high in the landscape, on arches, were being eroded as source areas and were not likely flooded by the seas
    • Of course, the possibility remains, that the seas were more widespread, and that sediment formed over a larger area was later removed by erosion during subtle uplift of certain regions. To figure out whether this was the case requires careful mapping and study of the sedimentary rock record.
  • Paleozoic sediment in Minnesota
    • Late Cambrian, Ordovician and Middle Devonian represented
    • Unconformities in the record indicate retreats of the sea, perhaps caused by subtle uplift of the craton
    • maximum thickness ~2,000 feet, mainly in southeastern Minnesota where they are exposed in outcrops along the Mississippi Valley and associated tributaries
    • limited occurrence of Ordovician rocks (~450') in northwestern Minnesota, but not exposed - covered by Mesozoic sediments and glacial drift
  • Cratonic structures specifically involved in controlling pattern of sedimentation in Minnesota
    • Transcontinental Arch
    • Wisconsin Arch
    • Hollandale Embayment of the Forest City Basin (centered in Iowa)
    • Williston Basin
  • Several major rock types occur in repetitive cycles, the pattern of which permits reconstruction of the record of flooding and retreat of the seas
    • quartz sandstone and sandstone with green glauconite (related to mica)
    • mudstone and shale
    • dolostone and subordinate limestone
  • Model for reconstruction of history of marine flooding and retreat is based upon observations of modern environments in which sediments accumulate
    • pure quartz sandstones - higher energy coastal environments rework the sediment, round it, sort it, and may eliminate unstable grains
    • shales and mudstones - lower energy, deeper offshore environments or protected environments where clay minerals in the form of mud can settle from the water
    • carbonates - warm shallow seas in which no siliciclastic mud is present to cloud the waters, so that organisms can secrete calcite required to make limestones (dolostone is formed by a chemical alteration of limestone following deposition).
  • Cycles of sedimentation at Hollandale
    • Austin defined 9 cycles of transgression and regression in Cambro-Ordovician rocks
    • example of single cycle in Twin Cities Basin
      • St. Peter quartz sandstone, overlain by Glenwood shale, overlain by Platteville limestone
      • transition from sandstone to shale represents flooding and reduction of current energy through time as water deepens in one spot
      • transition from shale to limestone represents flooding of source areas, so little siliciclastic debris enters basin and water clears up, permitting carbonate secreting organisms to fluorish
  • While study of vertical sections is useful, lateral extent of rocks must be mapped in order to reconstruct the paleogeography of the region

Minnesota's Geographic Position and Climate through Time

• knowing how the rocks of the North American continent were positioned relative to the Earth's magnetic pole at known times in the past (i.e., knowing the apparent polar wandering curves) permits reconstruction of the continent's geographic position, and hence allows us to make inferences regarding climate.
• fossils in sedimentary rocks of various ages give us information about past climate because we know the environment in which different life forms must have lived by analogy with similar forms living today
• certain types of sedimentary rocks give us information about past climate. For example, fossiliferous and oolitic Paleozoic limestones were probably formed in tropical and subtropical seas, much as they are forming today in the Bahamas.
• positions of the North American continent from Late Precambrian time to the present show that during the Paleozoic, Minnesota was located in tropical areas. Note that the accompanying map shows continental positions, but not the true shape of the continent through time.)

Paleozoic Era

• By Late Cambrian time, southeastern Minnesota was covered by an inland sea, and the rest of the state and the Canadian Shield to the north were serving as source areas of sediment, carried to the sea by river systems. Remember that there were no land plants at this time, so the weathering of rocks on land must have been very different from today
• Long continued weathering and erosion by running water and wind, and resulting recycling of sediment, produced detritus that was very rich in quartz grains. Clay was also present as a product of chemical weathering of unstable silicates, and ions were dissolved in solution
• The long interval of weathering also resulted in a gently rolling land surface over which the sea transgressed
• Late Cambrian rocks in southeastern MN and western WI - 600-500 Ma
  • mostly quartz sandstones with subordinate shales and dolomite
  • classic exposures along Mississippi River and St. Croix River - rock of Late Cambrian age in North America are referred to as St. Croixan
  • Seven Formations are mapped on the basis of their lithologic characteristics and are named according to the locality where they were first described and well-exposed.
  • Mt. Simon Formation - oldest in region - ~330' thick
    • light-colored medium-grained quartz sandstone with conglomerate at base
    • well-sorted and rounded and crossbedded - attests to higher energy environment of deposition
    • marine fossils in upper part indicate shallow marine environment
    • lower part, including the basal conglomerate, may represent river deposits
    • Similar to underlying Hinckley sandstone of the mid-continent rift, but lies atop unconformity where weathered zone is often present in the underlying Hinkcley sandstones
  • Eau Claire Formation - ~200' thick
    • varicolored shale, subordinate fine-grained quartz sandstone and greensand
    • contains many burrows created by marine organisms living on the bottom in the sediment
    • lower energy environment than Mt. Simon
      • either deeper water, or
      • protected environments on tidal flat
  • Galesville Sandstone - ~100' thick
    • medium grained light-colored quartz sandstone with minor glauconite
    • higher energy nearshore shallow environment of deposition
  • Ironton Sandstone - ~45' thick
    • quartz sandstone with abundant silt component
    • lower energy environment than underlying Galesville
    • may rest unconformably on Galesville, indicating regression prior to Ironton deposition
  • Franconia Formation - ~100-200' thick
    • greensand
    • glauconite is an iron-bearing silicate mineral that forms in micro-reducing environments in shallow marine settings where sedimentation rate is relatively low
  • St. Lawrence Formation - ~65' thick
    • first of the Cambrian dolostones (rock made of the mineral dolomite - calcium/magnesium carbonate) formed by replacement of original limestone (rock made of calcite - calcium carbonate). The replacement was most likely shortly after deposition, and occure near to the surface of the sea floor.
    • impure, containing clay minerals and silt and sand-sized quartz and glauconite
    • some detritus must have been available in the source and was able to be transported into the basin of deposition. Perhaps the dolostone was formed relatively near to shore, in generally low-energy environments of variable character
  • Jordan Sandstone - ~80-110'thick
    • yellow-tan coarse-grained quartz sandstone
    • well-rounded, well-sorted
    • pebbles and cobbles of dolostone included
    • high-energy environment of deposition near shore, perhaps influenced by episodic storms which broke up underlying rocks along the shore and carried pebbles and cobbles offshore to be deposited in Jordan
  • Summary of Late Cambrian sediments
    • quartz sands deposited in warm shallow seas
    • grain size indicates energy level which often correlates with proximity to shoreline or degree of protection from currents. Therefore, coarser-grained sands deposited nearer shore, finer-grained sands deposited farther from shore, or at least in nearshore protected environments
    • source of sediment in highlands on Wisconsin Arch to north and east and Transcontinental Arch to northwest
    • carbonate sedimentation not generally significant, as deposition of large amounts of siliciclastic sediment makes it difficult for carbonate-secreting organisms to survive in large enough numbers to result in a dominance of carbonate sedimentation. Inorganic chemical precipitation of carbonate is also inhibited by all that detritus. However, to the south in the basin, far from the source areas, the siliciclastic sediments grade laterally into carbonate sediments
  • History of Hollandale Embayment of Forest City Basin interpeted on the basis of isopach maps
    • an isopach map is a contour map of the thickness of sediment in a formation or a larger grouping of formations, or the thickness of sediment belonging to a particular age
    • isopach map of Mt. Simon through Ironton Formations
      • note how the 75 and 90 foot contour lines are cut off or truncated by the zero edge of the formations. This indicates that the zero edge formed by erosion and not because of non-deposition. If it formed by non-deposition, the thickness trends should be parallel to the shoreline which would be the zero-edge.
      • note that the formations thicken toward the east and southeast, not toward the south and southwest. This indicates that the Hollandale Embayment of the Forest City Basin had not yet formed. Subsidence to the south and west was not especially significant.
• Ordovician Period - 500-430 Ma
  • By Ordovician time, the craton, including the area now occupied by Minnesota, was eroded to very low relief and was completely flooded by encroaching seas
  • sources of sand were eliminated, water cleared up and carbonate-secreting organisms together with inorganic chemical precipitation of carbonate became dominant throughout the rest of the Paleozoic
  • Ordovician Sedimentary Rocks
    • Oneota Dolomite - ~165' thick
      • rocks at top of bluffs in Winona area
      • crystalline dolomite with some chert/flint replacement
      • contains oolites in lower part and stromatolites upward
      • extensively burrowed - tube-like burrows preserved in Winona stone, the rock quarried in the Winona area for building stone
      • originally deposited as limestone in shallow water setting, but dolomitized soon after deposition by magnesium-rich waters
    • Shakopee Formation - 300'thick
      • New Richmond Member - well sorted, well rounded quartz sandstone 65' thick
      • Willow River Member 215' thick dolomite with oolites and algal stromatolites much like the Oneota
    • St. Peter Sandstone - ~165' thick
      • light-colored medium-grained quartz sandstone
      • type example of a well-rounded, well sorted sandstone
      • surface textures on some quartz grains show abrasion resulting from wind transport, other surface textures indicate water transport
      • poorly cemented
      • named for St. Peter River (now called the Minnesota River)
      • last gasp of quartz deposition in Minnesota's Paleozoic record
      • extends over a vast area of the craton south of Minnesota
      • purity indicates extensive recycling from Cambrian sandstones
        • during regression following deposition of Oneota-Shakopee, when erosion removed the Ordovician dolomite and exposed the underlying Cambrian sands as source areas.
        • But volume of this sheet of sand reaches 20,000 cubic kilometers! And that seems to require another source.
        • Could the sand come from the shield to the north? Perhaps. But what caused the shield to become exposed at this time? The question is under continuing study.
    • Glenwood Formation - ~20' thick
      • gray-green shale with sand at base, transitional from underlying St. Peter
      • fine grain size indicates lower energy environment, perhaps in deeper water offshore from the St. Peter beaches and dunes
    • Platteville Formation - ~30' thick
      • highly fossiliferous limestone
      • fossils include brachiopods, cephalopods, gastropods, bryozoa, crinoids and trilobites. Some of the cephalopods (related to modern squids) are as large as 15' in length and 10" thick
      • shale interbeds appear near top
      • offshore equivalent of St. Peter and Glenwood
      • shallow marine bank of widespread carbonate sediment - Bahaman platform may be a good modern analogy
      • individual centimeter-thick beds can be traced for 10's of kilometers indicating stability of environments during this time
      • volcanic ash bed an inch or so thick present - comes from mountain-building and volcanic event to the east, in the Appalachians
      • St. Peter/Glenwood/Platteville vertical section represents transgression and associated lateral migration of environments of deposition
    • Decorah Shale - ~90' thick
      • grayish-green shale with thin interbeds of limestone
      • highly fossiliferous, and thin beds made almost exclusively of fossil fragments are present
      • mud deposited in quiet water and shell beds indicate episodic storm or tidal currents
      • source of mud may have been from Transcontinental Arch
    • Galena Formation - ~230' thick
      • dolomite, limestone and shale
      • fossiliferous, but not as fossiliferous as Decorah and Platteville
      • upper part of unit has fossil assemblage very different from lower part, which may indicate a change from normal marine water to hypersaline water
      • thin volcanic ash beds are present
    • Dubuque Formation - ~35' thick
      • grayish limestone and shale
      • crinoids (animals often called "sea-lillies" are present)
      • normal shallow marine environment
      • two volcanic ash beds present
    • Maquoketa Formation
      • limestone and dolomite with less shale than in Dubuque
      • upper part of the formation has some quartz sand indicating some uplift along arch?
  • Summary of Ordovician Period
    • generally quiet time
    • low-lying landmass devoid of vegetation was inundated by seas
    • seas were teeming with life
    • occasional outfalls of ash from Appalachian Mountain building events reached the Minnesota portion of the craton
• Silurian Period - 430-400 Ma
  • no Silurian rocks in Minnesota but Silurian seas were widespread
  • Silurian rocks probably deposited in Minnesota, but eroded away in response to uplift of craton after Silurian ended
  • Silurian rocks in north in Canada and south in Iowa are carbonates, similar to Ordovician rocks
  • coral reefs became important
  • vasular plants able to distribute water and nutrients throughout cellular structure did not appear on land until late in Silurian
  • Minnesota during Silurian was most likely an eroded low-lying landmass with no land plants and slow-flowing streams carrying some mud and ions dissolved in solution to the sea
• Devonian Period - 400-350 Ma
  • Transgression of sea from the south into southernmost Minnesota
  • last time that seas covered Minnesota for the next 225 million years
  • Cedar Valley Formation deposited
    • 500" thick in SW Iowa
    • 250' thick in MN
    • carbonate unit
    • top of formation missing in MN, so it was no doubt thicker
    • probably tidal-flat and lagoonal environment
    • cycle of transgression and regression preserved
    • dolomitization has destroyed most of the fossils originally present
  • what was MN like during this time? shallow sea in south, with low-lying land mass to north. By analogy with areas where more Devonian rocks are preserved, we guess that the coastal area probably swampy, with trees as tall as 30'. Primitive amphibians were probably present.
  • Appalachian Mountains were continuing to be uplifted along the eastern margin of the continent.
• Mississippian, Pennsylvanian and Permian Periods (350-225 Ma)
  • no rocks representing these periods are present in Minnesota
  • guesses as to what Minnesota was like during this time are made possible by study of rocks of these periods preserved elsewhere in North America
  • Mississippian Period
    • some geologists think that during the Mississippian, Minesota was covered by warm shallow seas in which carbonate sediments were deposited and that these sediments were removed by later erosion. Others like the authors of the text believe that Minnesota was above sea level and was being eroded throughout this interval of time.
    • elsewhere in North America during, extensive wide, shallow warm seas covered the continent and thick sequences of limestones were deposited. The seas teemed with invertebrate life
    • low-lying landmasses were drained by slow-flowing streams carrying clay and dissolved ions to the seas. Land plants were present, but no flowering plants, which didn't appear until Cretaceous time.
  • Pennsylvanian Period
    • This is the age of coal. Low-lying landmasses in the interior of North America had extensive swamps developed in tropical coastal areas. These swamps were probably hundreds of miles wide and in them, forests fluorished with trees as tall as 100'. The plants became deposits of coal upon burial and lithification.
    • Muddy and sandy rocks associated with the coals suggest that Minnesota and the Canadian Shield were slighly uplifted at this time and provided siliciclastic sediment to the basins, such as the Illinois Basin, where coal was forming
    • Appalachian Mountains were becoming a major feature of the landscape along the eastern margin of the continent which was marked by a subduction zone which began in the Ordovician, and culminated with continental collisions in the Permian
  • Permian Period
    • Major landmass built by this time
    • North America collided with what is now Europe and Africa, building a major mountain chain along the eastern margin of present-day North America
    • Continent tilted westward as a result of the collision and the craton was uplifted over a large area
    • Seas existed in the west and encroached as far eastward as Kansas and Nebraska
    • Many evaporite sediments including various salts were deposited in the Permian seas as the seas became more and more restricted and began to dry up
    • Muddy deposits in Permian rocks southwest of Minnesota suggest that Minnesota was a low-lying source area at this time
    • Reptiles began to dominate amphibians and were probably present in Minnesota