Be certain to place the Early Paleozoic history of Minnesota in the context of earth's Early Paleozoic history

Here is a great source of concise information on the Paleozoic Era, from the University of California, Berkeley

Here is a great source of concise information on the Cambrian Period from the University of California, Berkeley

Here is a great source of concise information on the Ordovician Period from the University of California, Berkeley

Evidence in the Late Cambrian sediments in southeastern MN and western WI

• 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.
• Notice that rock units such as formations are named on the basis of observable characteristics, while the assignment of age (i.e., St. Croixan) is an inference and is based on fossils which permit correlation to standard sections of known age.
• marine fossils are found in all the Cambrian and Ordovician rocks from the middle of the Mt. Simon, upward in the pile, indicating that these rocks were deposited in a shallow sea.

• The royalty of the Cambrian fossils was the trilobite. These organisms, extinct since the end of the Paleozoic Era, have been used to date the various time slices of the Cambrian period. Trilobites are present in the Cambrian as well as Ordovician rocks of southeastern Minnesota.
• Paleozoic Fossils
  • Trilobite
  • Gastropod (snail)
  • Dome-shaped Stromatolite
  • Cephalopod
  • Giant cephalopod (front)
  • Giant cephalopod (rear)
  • Graptolites from Decorah Shale
  • Algae (Fisherites)
• 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 - interpreted on the basis of finer grain size
  • may rest unconformably on Galesville, indicating drop in sea level relative to the land, and retreat of shoreline toward the center of the basin (regression) prior to Ironton deposition
• Franconia Formation - ~100-200' thick
  • quartz and greensand
  • glauconite is an iron-bearing silicate mineral that forms in micro-reducing environments in shallow marine settings where sedimentation rate is relatively low - it's the glauconite that gives the sand it's green color (reduced iron)
  • lots of cross bedding, indicating current activity in relatively shallow water
• St. Lawrence Formation - ~65' thick - outcrop near Homer, MN
  • 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 occured near to the surface of the sea floor.
  • impure, containing clay minerals and silt and sand-sized quartz and glauconite
  • burrowed by bottom-dwelling organisms
    • horizontal burrows
    • vertical burrows
  • some beds contain ripple marks
  • 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
  • cross-bedding includes "bundles" of laminations, and reversing directions - indicates tidal currents were active
  • pebbles and cobbles of dolomitic sandstone are present
    • 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 -
  • The fossils and the grain sizes and nature of cross bedding in the various formations tell us that the shallow Cambrian sea had a variety of environments present within it, ranging from quieter deeper water offshore, below the influence of the waves, to relatively shallow environments just offshore from the beach.
    • In some of these environments, tidal currents are inferred by the opposing directions of currents indicated by the cross beds.
    • In other environments, storm waves must have been present, as indicated by the ripping up of already partly solidified sediment from the bottom, and redepositing it offshore
    • one of our tasks is to work out the history of that sea, especially documenting the rise and fall of sea level relative to the land. This rise and fall alternately flooded the coastlines, and enable the coast to advance toward the sea.
  • quartz sands were deposited in warm shallow seas - note that Minnesota was located practically on the equator during this time. We know this by reconstructing the movement of plates back through geologic time.
  • 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.
  • isopach map of Cambrian sediments in North America
    • reveals larger pattern of depositional basins and arches
    • note presence of elongate basins marginal to the N. American continent - cratonic basins just beginning to be defined on the basis of patterns of thickness of sediment

• Ordovician Period
  • 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
  • A diverse marine invertebrate fauna had evolved by this time, and inhabited the shallow sea floors
  • Brachiopods became a significant part of the Ordovician fauna, and are very important index fossils in assigning ages to the time slices of the Ordovician
  • Ordovician Sedimentary Rocks
    • Oneota Dolomite - ~165' thick
      • rocks at top of bluffs in Winona area and at top of bluffs in Whitewater State Park
      • rock is soluble and will dissolve to form small caverns
      • crystalline dolomite with some chert/flint replacement
      • contains oolites in lower part and stromatolites upward
      • oolitic layers also have quartz grains in them, and the oolites and quartz grains were transported on the bottom of the sea by currents, forming ripple marks
      • extensively burrowed - tube-like burrows preserved in Winona stone, the rock quarried in the Winona area for building stone - you can see these burrows especially well in the steps at the front of Somsen Hall, and in the benches near the clock tower on the mall
      • 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. Abundant cross bedding like the cross bedding we see in sand dunes along the sea shore
      • Willow River Member 215' thick dolomite with oolites and algal stromatolites much like the Oneota. These stromatolites were probably formed by ancient algae, not unlike the formation of stromatolites in the Precambrian by cyanobacteria
    • 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, called Stewartville, is burrowed
      • 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

Principle of Facies: Interpretation of Facies in Terms of Paleogeography

• When sediments are deposited at the same time, side-by-side in different depositional environments, they will take on different aspects or characteristics depending on processes at work in that environment. These different types of sediments are referred to as facies.
• Maps of facies over a region can be made by mapping sedimentary rock types of the same age across that region.
• Facies maps can then be interpreted in terms of paleogeography. We can interpret the processes of deposition that have formed the characteristics of a given sedimentary rock. For example, high energy currents in a shallow sea will form well-sorted, well-rounded quartz-rich sandstones. Using these interpretations, we then simply translate each sedimentary rock type in the facies map to an environment of deposition in which that rock formed. This results in a paleogeographic map.

Interpretation of the Paleozoic Sedimentary rocks in Minnesota

• 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
• 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
    •  
  • 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).
    • When depositional environments which lie side-by-side at a particular time begin to migrate laterally, then the sediment formerly deposited side-by-side in the environments will stack up vertically, reflecting the horizontal distribution of the environments. This is a statement of Walther's Law. This principle is extremely important, because it permits us to take a vertical section of rock and interpret what the lateral distribution of environments might have been at a particular instant in time.
  • 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 (note that the map shows continental positions, but not the true shape of the continent through time.)
• During the Cambrian and Ordovician, Minnesota was located in tropical areas.

Shallow Inland Seas - General comments

• Connected to seas at margins of continent, where periodic 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

• 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