Proterozoic Eon Notes

WILSON CYCLE

• Opening of ocean basin along rift (divergent boundary).
• Subduction zone forms on one margin (convergent boundary).
• Ocean basin closes.
• This process repeats through time in approximately the same areas.
• Normal faults are observed in rift zones.
• Marine sediment infills over terrestrial sediments in these areas.

TECTONICS AND OROGENY

• Tectonic activity: Plate movement of any kind.
• Orogeny: Mountain-building tectonics.
• Orogen: Rocks affected by orogeny.
• Major Laurentian Proterozoic orogeny: Calderian Orogeny.
• Resulted in the Wopmay Orogen in the Northwest Territories of Canada.

PRECAMBRIAN ASSEMBLY OF NORTH AMERICA

• Began in the late Archean.
• Algoman orogeny (Kenoran orogeny):
  • Late Archean episode of mountain building in North America.
  • Superior province, South Dakota to Lake Huron.
  • Occurred during the late Archean Eon ($2700-2500$ Ma).
• Wopmay orogeny:
  • Mountain-building event in northern Canada.
  • Along the western edge of the Canadian Shield ($2100-1900$ Ma).
• Trans-Hudson orogeny (Hudsonian orogeny):
  • Mountain-building event in North America.
  • Extends from Hudson Bay west into Saskatchewan, then south through the western Dakotas and Nebraska.
  • Resulted from the collision of the Superior craton with the Hearne craton and the Wyoming craton during the Proterozoic Eon ($2000-1800$ Ma).
• Nagssugtoqidian orogeny:
  • Late Paleoproterozoic mountain-building event in Greenland ($1910-1770$ Ma).
• Ketilidian orogeny:
  • Collision at the southern margin of the North Atlantic Craton during the late Paleoproterozoic Era ($1850-1720$ Ma).
• Penokean orogeny:
  • Affected Wisconsin, Minnesota, Michigan, and southern Ontario ($1850-1840$ Ma).
• Great Falls orogeny (Big Sky orogeny):
  • Proterozoic collision between the Hearne craton and the Wyoming craton in southwest Montana ($1770$ Ma).
• Ivanpah orogeny:
  • Mojave region, southwestern U.S.
• Yavapai orogeny:
  • Mountain-building event $1.7$ billion years ago in the southwestern United States ($1710-1700$ Ma).
• Mazatzal orogeny:
  • Mountain-building event in North America.
  • Mid to southwestern U.S. ($1675-1650$ Ma).
• Picuris orogeny:
  • Mid to southwestern U.S. ($1430-1300$ Ma).
• Grenville orogeny:
  • Mesoproterozoic mountain-building event.
  • Worldwide, during the late Proterozoic Eon ($1300-1000$ Ma).
  • Associated with the assembly of the supercontinent Rodinia.
  • Formed folded mountains in eastern North America from Newfoundland to North Carolina ($1100-1000$ Ma).

WOPMAY OROGEN

• Named after Canadian WWI flying ace Wilfred Reid “Wop” May.
• Collision of Hottah volcanic terrane with Slave craton approximately $1.9$ Ga.
• Now the westernmost part of the Canadian shield.
• First “modern-looking” orogen accessible for study.
• Shows fold and thrust belt geometry like Phanerozoic orogens.

ATMOSPHERIC OXYGENATION

• Rise in atmospheric $O_2$ (Great Oxidation Event) began in the late Archean or early Proterozoic.
• Changed Earth’s atmosphere from weakly reducing to oxygenated, although free $O_2$ took a long time to build up.
• The atmosphere developed as photosynthetic life developed.
• More abundant $O_2$ = more aerobic life but also caused mass extinction among anaerobic life.
• The oxygenation of the earth is considered to have been started by the oxygenic photosynthesis of cyanobacteria.

EVIDENCE FOR PALEOPROTEROZOIC $\text{O}_2$

• Pre-GOE rocks have mineral grains that cannot be preserved with free $\text{O}_2$ present.
• GOE and more recent rocks have iron oxides (hematite), which require $\text{O}_2$.
• Banded iron formations (BIFs) are alternating layers of hematite and chert.
  • Mainly late Archean and early Proterozoic age.
  • Show oxygenation of oceans.
  • Photosynthetic oxygen reacted with dissolved minerals to precipitate them out of seawater.
• Continental red beds (sandstones and shales with grains covered by iron oxide) appeared approximately $1.8$ Ga.
• Atmospheric $\text{O}<em>2$ concentrations approximately $1-2\%$ present level, but UV radiation likely contributed O and $\text{O}</em>3$, which are more powerful oxidizers.

PALEOPROTEROZOIC GLACIATION

• Glacial rocks of the same age are found in North America, Finland, South Africa, and India = Huronian Glaciation; about the same age as GOE.
• The buildup of atmospheric $\text{O}_2$ weakened the greenhouse effect and allowed glaciation.
• Glacial rocks north of Lake Huron are laminated mudstones from glacial meltwater (varved light and dark for summer and winter deposition).
• Also tillites: unsorted glacial debris that was lithified.

FOSSILS

• Famous fossil locality of the Gunflint Chert ($1.88$ Ga) preserves stromatolites and evidence of earliest photosynthesis at the time of its discovery in the 1950s.
• Also tiny rods, spheres, and filaments that were the first studied unicellular fossils in the 1960s.
• Microbial life and photosynthesis now have much older evidence ($3.5$ Ga +).

GRENVILLE OROGENY

• The last major Proterozoic tectonic event affecting North America ($1.2-1.0$ Ga).
• Grenville rocks are present from the Atlantic coast of Labrador to Lake Huron in North America.
• Associated with the formation of supercontinent Rodinia.

RODINIA

• The assembly of supercontinent Rodinia closes out the Mesoproterozoic (~$1.1-0.9$ Ga).
• Laurentia rotated clockwise ~$150$º vs. present day and was mostly in the Southern Hemisphere.
• The red band represents the Grenville orogen.

EUKARYOTIC LIFE

• Eukaryotic fossils are present by the Mesoproterozoic ($1.6-1.0$ Ga), but the origin may be earlier.

NEOPROTEROZOIC

• Several episodes of widespread and extensive (global scale) glaciation.
• Rodinia began to break up ($750-633$ Ma), and then its fragments reassembled into Pannotia ($633-573$ Ma).

GLACIATION

• May have covered the entire Earth = “Snowball Earth”.
• Widespread glacial striations, tillites, and dropstones (chunks of rock released from melting ice), and varved clay deposits: even in tropical latitudes.

RODINIA BREAK UP

• Fragmented into large continents.
• Antarctica split from Laurentia to the west = opened Panthalassa (proto-Pacific).
• Gondwana split from Laurentia to the east = opened Iapetus (proto-Atlantic).

FIRST METAZOANS

• Metazoan = multicellular (non-colonial) animal.
• The first metazoans were probably sponges (phylum Porifera).

EDIACARANS

• Late Neoproterozoic Ediacaran period ($635-541$ Ma).
• First large multicellular animals.
• What most of them are is still poorly understood: may not represent members of modern phyla.
• Frond, disc, elongate, quilted, non-bilateral symmetry body plans.