1.9 Billion Years of the Proterozoic : The Modernization of
Earth
- Modern Plate Tectonic Processes
- Beginning of Proterozoic arbitrarily based on recognition of modern plate tectonics
- Pattern of opening & closure repeated throughout the Phanerozoic.
- The PaleoProterozoic
- Wopmay Orogeny - 2300-2100 MY BP
- First orogenic activity along western margin of Slave province
- Represented by a deformed belt including igneous intrusives
- Modern-style orogeny because fold & thrust belts of shallow shelf to flysch deposits
- WILSON CYCLES
- Interpreted continental margin rifting
- Infill of the rift zone (alluvial fans & fluvial deposits; lava flows followed by passive-margin
marine deposition)
- Development of an ocean basin (rifting between continents continues)
- Reversal of plate movements and closure of the basin (westward subduction of leading edge
of Slave plate)
- Why is the Wopmay Different?
- Presence of stromatolitic dolomites.
- First extensive carbonate shelves resembling those in the Phanerozoic.
- First orogeny with aulacogens
- Aborted rift in a three-rift system in response to a over a hot spot in the crust.
- Two arms separate forming an ocean basin, the third fails and is infilled with
terrestrial sediment.
- 1800 MY Hudsonian Orogeny
- Labrador trough east of Superior Province
- Linear basin with terrestrial deposits underlying shelf deposits (Qtz sandstones,
dolomites, and iron formations)
- Adjacent are pillow lavas, tholeiitic basalts, mafic intrusions & graywackes
- Eastern zone deformed and thrust west.
- Used as event separating PaleoProterozoic from MesoProterozoic.
- Early Proterozoic BIF's
- More abundant in the early Proterozoic
- Thick accumulations of oxidized rocks (indicating some free oxygen); principle iron ores in
Animikie Group
- B.I.F.'s are extensive (>1000 m thick; extend over 100 km
- Gunflint chert (1900 my) preserves first recognized microfossil assemblage (1954).
- Early Proterozoic Glaciation
- Evidence in Gowganda Formation north of Lake Huron
- Conglomerates (tillites) alternate with laminated mudstones (varved)
- Conglomerate clasts are scratched and faceted
- Igneous rocks (2100 my) intrude and Formation rests upon crystalline basement (2600 my).
- Similar rocks in Finland, southern Africa, & India.
- "Snowball Earth?"
- Ice sheets to 11 Equator
- South Africa where lava (2.2 BY) covers tillites
- Magnetic minerals indicate equatorial paleogeographic position
- O2 increase removes methane, reducing greenhouse effect
- Volcanogenic activity released CO2 into atmosphere, warming planet
- Increased CO2 results in High Carbonate Deposition under Greenhouse Conditions
- 12C:13C ratios indicate 13C from Volcanic Source (drop in stable isotope)
- Atmospheric Oxygen Increases
- Moderate O2 (estimated 2% PAO [Present Atmospheric Oxygen]) at 2.0 BY following
deglaciation
- O2 added by photosynthesis and breakup of H2O in atmosphere by lack of ozone layer.
- Timing of O2 Increases
- Timing of life
- Chemical building blocks could not have formed in oxygenated environment
- Amino acid synthesis is inhibited by the presence of Oxygen
- Oxidation state of minerals in Archean
- Rocks > 2000 MY, unoxidized Uraninite and pyrite abundant in terrestrial and marine
settings; rare in younger rocks.
- Red beds are never found older than 2200-2300
- B.I.F.'s abundant until 1800 MY BP.
- MesoProterozoic Events
- Keweenawan Sequence (up to 15,000 m) with native copper resources.
- Clean sandstones and conglomerates mixed with basaltic lava flows in which copper
crystallized
- Most magma remained in crust forming a 12,000 m thick pluton - Duluth Complex.
- Continental Margins
- Complex collisional history with several island arc terranes 1700- 1800 MY BP (southern
Wyoming & western Colorado)
- Magmatic activity in the Middle Proterozoic with emplacement of several plutons
(1500-1400 my) extending from California to Labrador
- Grenville Province
- Youngest middle Proterozoic region
- Last orogeny (1200-1000 mybp)
- Supercontinent Rodinia
- Most eastern U.S. Grenville subjected to later tectonism of Appalachian Orogeny
- Typical rocks are metamorphosed carbonates and sandstones, and igneous intrusions up to
6000 m in thickness.
- About 1100 million years ago, the supercontinent of Rodinia was assembled. Though its
exact size and configuration are not known, it appears that North America formed the core of
this supercontinent
- NeoProterozoic Events
- Deposition around continental margins
- Rifting east of Grenville Orogeny
- Lavas and coarse clastics accumulated before a proto-Atlantic ocean formed.
- This ocean basin provided for the development of Phanerozoic shelf deposits.
- Glaciation occurred (840-600 MY BP)
- Glacial deposits in western U.S., western Canada, Alaska, Greenland, S. Am.,
Scandanavia, & Africa - Globally except Antarctica.
- "Snowball Earth Again?"
- 550-750 MY Earth largely covered in Ice
- Ice Sheets reflected back sunlight, cooling atmosphere
- Weathering of silicates using atmospheric CO2
- Evidence in Namib Desert of low organic Carbon productivity
- Volcanics alternate with low productivity
- Freeze-thaw cycles? (Icehouse-Greenhouse)
- Warm intervals record biodiversity increases triggering metazoan radiation
- More Rodinia
- Rodinia split into 2 halves approximately 750 million years ago, opening the Panthalassic
Ocean. North America rotated southwards towards the ice-covered South Pole.
- The northern half of Rodinia, composed primarily of Antarctica, Australia, India, Arabia, and
the continental fragments that would one day become China, rotated counter-clockwise,
northwards across the frigid, North Pole.
- Continental Margin : Belt Supergroup
- Massive limestones (Glacier National Park) and other nearshore shelf deposits
- > 12000 m thick
- Represent a series of deep basins with eroded sediments from shield
- Correlative with the Grand Canyon Supergroup.
- Eastern Continental Margin
- Smoky Mountains of Tennessee (Ocoee Group)
- 10,000 m sandstone high in feldspar (granite source)
- Overlain by >2000 m quartz-rich sediments (Chilhowee Group)
- Continental shelf built eastward from Grenville mountains.