Formation of the Great LakesPart 2Precambrian and Paleozoic History Channel Video Chapter 2 in Grady Chapter 2 in Greenberg
Great Lakes Basin in the Precambrian • Foundation for Great Lakes Basin (GLB) was formed over 3 bya in Precambrian • Canadian Shield • Igneous rocks • Exposed in northern and northwestern part of GLB • Extends below sedimentary rocks deposited in the southern and eastern parts of GLB during Paleozoic era
Canadian Shield • Once Shield formed • Middle of continent stable geologically • Craton • Margins of continent very active • Volcanism creating new land • Orogenies
Great Lakes basin • Remnants of the rocks of these three provinces form the deep basement of the Great Lakes basin. • Smooth • Grey to pinkish • Central Lowlands Province • Lower midwest USA region of the GLB • No shield rock is visible at surface • Region was covered over and over again by shallow sea • Bedrock here is limestone • Lots of evidence of ancient life – good fossils • Basin for lower Great Lakes • Resources: coal, natural gas, petroleum
Precambrian • The Precambrian is actually a segment of time that includes two eras, the Proterozoic and the Archaean that span billions of years. • The oldest rocks on earth formed during this time, as well as the first continents, and the earliest, simplest forms of life.
Archean • 3800 to 2500 mya • The oldest rocks on earth are from this era • During the Archean, the cratons--the masses of rock that make up the basic, initial structure of continents--formed. • The crustal bodies that formed were smaller than today's continents and are referred to as protocontinents. • Today, Archean-aged rocks are found in areas such as the Superior Upland and the Rocky Mountains.
Archean Era • Major Events: • The earth was pummeled with meteors. • Scientists believe that a huge impact tore away a large chunk of the earth and trapped it in orbit, forming the moon. • Seawater and an atmosphere developed. • The oceans became populated with algae, bacteria, and colonies of microorganisms known as stromatolites (first oxygen deposited in atmosphere).
Precambrian • Many of the oldest rocks found on earth today are found in the Superior Upland, a part of the Canadian Shield, the broad swath Precambrian rocks that curves around Hudson Bay. • These rocks make up the "core" of the North American continent.
Precambrian • Other Precambrian rocks are found throughout the continent, in places such as the Adirondack Mountains, the Rocky Mountains, the Llano Uplift, the Black Hills, the Baraboo Range, and the Grand Canyon.
Great Lakes Basin in the Precambrian • Volcanism and sedimentary deposits were the source of the rich mineral deposits found in this region • Early sedimentary and volcanic rocks were folded and heated into complex structures. • These were later eroded and, today, appear as the gently rolling hills and small mountain remnants of the Canadian Shield, which forms the northern and northwestern portions of the Great Lakes Basin.
Great Lakes Basin in the Late Precambrian (cont.) • Central North America experienced repeated transgressions and regressions of shallow, tropical seas during Paleozoic • large areas of tropical coral reefs • seas deposited layers of materials that became sedimentary rocks • limestone, shales, sandstone, gypsum
Two Types of Cells • Prokaryotes – cells without a nucleus, simple cell structure • Bacteria and Archaea • Eukaryotes – cells have a nucleus which contains the DNA, also have complex structure • Includes protozoa, algae, plants, fungi and animals (including us)
Life in the Precambrian • Archaean Era - 3800 to 2500 mya • Oldest sedimentary rocks (3.8 BYA) • Few fossils • First life appears ~3.6 bya • Chemotrophic, anaerobic, asexual • Oldest fossils - 3.55 BYA • Prokaryotes dominate • Eubacteria and Archaea • Cyanobacteria form extensive stromatolite systems • Primitive Eukarya appear • Photosynthesis appears
How old are they? • Oldest known fossil bacteria are about 2.8 billion years old • Look like cyanobacteria suggesting ancient origin for photosynthesis • Cyanobacteria fossils from stromatolites date to 2.8 to 2.5 bya (maybe older) • Filamentous strands of cells resembling modern species of Oscillatoria or Lyngbya • Stromatolites still exist as living fossils
Floating cyanobacterial mat from a hot spring Fossilized cyanobacterial mat
Colonial chroococcalean cyanobacterium from the Bitter Springs chert of central Australia, a site dating to the Late Proterozoic, about 850 million years old.
Palaeolyngbya also from the Bitter Springs chert. Living Lyngbya
Stromatolites • Layered mounds of calcareous material between cyanobacterial cells. • Following slides show fossil stromatolites • Closest place to find these: Upper Peninsula of Michigan • FOSSIL
Doing the backstroke among the stromatolites in a tropical Precambrian sea
Stromatolite Formation • Many layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments. • Photosynthesis by the cyanobacteria depleted carbon dioxide in the surrounding water, initiating precipitation of calcium carbonate. • The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. • As this process occured over and over again, the layers of sediment were created.
Stromatolite formation still occurs today; Shark Bay in western Australia is well known for the stromatolite "turfs" rising along its beaches. • Marine, low latitude, hypersaline environment.
Proterozoic • The Proterozoic was a time in which multicellular algae began to appear. • The continents grew in size • Oxygen began to build up in the atmosphere (from cyanobacteria). • Today, Proterozoic rocks are found in features like the Superior Upland, the Black Hills and the Adirondack Mountains.
Paleoproterozoic (2500 to 1600 mya) • Oxygen present in atmosphere so - • Metabolism began diversification • Earliest Proterozoic life was single-celled and anaerobic – did not use oxygen • Oxygen was toxic to these organisms – 1st max extinction event! • Various ways to breakdown glucose anaerobically • Later, more complex single-celled aerobic life evolved which used oxygen • Use oxygen for the process of respiration
Paleoproterozoic • Peak of stromatolites • Cyanobacteria oxygenated the atmosphere • Oxygen caused “Rusting of the Earth” • Depletion of oceanic iron • Combined with oxygen and precipitated as “rust” • Precipitated iron settled to the ocean floor • Iron found in Banded Iron Formations
Banded Iron Formations • Banded Iron Formations (BIFs) are another type of stromatolite • 1.8 to 2.5 billion years old • Composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert) • Also found in Northern Michigan • How they were made • Cyanobacteria provided the source of oxygen for BIF formation. • FOSSIL
Banded Iron Formations • How they were made #1 • Large amounts of the soluble form of iron were released from the Earth's interior into the Archaean oceans. (reduced ferric iron) • Oxygen in the oceans would have oxidized (rusted) this iron to form insoluble (ferrous) iron oxide which precipitated and formed layers of reddish sediment on the ocean floor. • The layers or banding is assumed to result from cyclic peaks in oxygen production. It is unclear whether these were seasonal or followed some other cycle.
Banded Iron-Formation • Seasonal and/or biological cycles resulted in intervening periods when iron or oxygen were not as available • The black layers are made of chert (micro-crystalline quartz) that was laid down during these intervening periods.
Banded Iron Formations • How they were made #2 • It is also thought that bacteria may have played a role in the formation of these bands of precipitated iron. • Certain bacteria can oxidize reduced ferric iron to insoluble ferrous iron • Economic significance • BIF’s are important sources of iron ore (mining) • Bacteria are capable of precipitating a number of other minerals including manganese and even gold • These processes are being adapted to facilitate mining of these minerals and removal of heavy metal pollutants from soils