Proterozoic Eon: Plate Tectonics and Cratons

• Lecture 6 explores the Proterozoic Eon, from 2.5 billion to 542 million years ago.

Plate Tectonics

• In the Archean Eon, plate tectonics were "hot," with proto- to microcontinents.
• Plate boundaries were numerous, leading to the "squishing" of proto-continents into microcontinents.
• Accretion, or growth by addition, is how things become larger.
• Planetary accretion occurred in the solar system.
• Continental accretion in the Proterozoic was slower than in the Archean.
• Microcontinents collided and formed stable continental interiors, or cratons.
• Cratons are the stable interiors of continents, found on all major continents.
• The North American continent has a stable interior, the North American Craton composed of the Canadian Shield and Platform.
• Cratons include the Shield, made of crustal (Precambrian) rocks at the surface extending under the craton, and the Platform, with Precambrian rocks covered by Phanerozoic sedimentary rocks.
• There is an unstable exterior to the North American continent.

North American Craton Basement Rocks

• These are the surface upon which other rocks are deposited.
• They are typically igneous or metamorphic, forming the crystalline basement.
• Dated units are called provinces, equivalent to microcontinents formed at different times.

Grenville Province

• This province was added during a continental collision and formed locally
• The Grenville Orogeny occurred 1.2 billion years ago as a Continental Collision Zone. Led to the formation of the first supercontinent.
• Rodinia assembled around 1.2-0.9 billion years ago.
• The Grenville Province is covered by approximately 25,000 feet of Ediacaran rocks since the Proterozoic.
• It was pushed upwards during the formation of the Appalachian Mountains (Blue Ridge Mountains)
• Features include Harper's Ferry (foliated metamorphic rocks like phyllite) and metamorphosed basement (granite and granite-gneiss).
• In the Late Proterozoic, Rodinia broke up, or rifted.
• Lithosphere thinned due to stretching, accompanied by magnetism.
• The East African Rift Zone is a modern analog for the rifting of Rodinia.
• Alluvial fans with gravels interfingering with basalts were present. Metamorphic basaltic lavas show columnar jointing on Blue Ridge.

Proterozoic Life and Environment

• At the beginning, the world was dominated by simple prokaryotes.
• Stromatolites were the dominant life forms on Earth, expanding with bigger continental shelves and shallow water habitats.
• 2.3 billion years ago, O2 began to build up in the atmosphere.
• Pyrite became rare as a sedimentary grain after 2.3 billion years, due to excess O2.
• BIFs (Banded Iron Formations) became rare after 1.85 billion years ago.
• "Red Beds" formed as oxygen levels rose, leading to well-oxidized iron oxide materials.
• Rare before 2 billion years ago, they became common after 1.9 billion years.
• The oldest known macrofossil is a eukaryotic algae from 2.1 billion years ago.
• Eukaryotic organisms are exclusively aerobic.
• Eukaryotes evolved through endosymbiosis.
• Endosymbiosis involves organisms benefiting from living inside one another.
• Bacteria (procaryotes) consumed smaller procaryotes, eventually leading to "plant cells" consuming cyanobacteria -> became chloroplasts
• Heterotrophs became animals, and photosynthetic autotrophs became plants.
• Proterozoic single-celled animals appeared 750 million years ago, resembling modern amoebas.

Ediacaran Fossils

• Preserved in Ediacara Hills sandstone and resemble phyla still existing today.
• The Ediacaran Fauna appeared 570 million years ago and consisted of large, multicellular, soft-bodied animals (mobile and sessile) with no hard parts.
• Requiring only a fraction of modern oxygen levels.
• Some animals were several inches, and over a meter, in size.
• Thin, "quilted" forms increased surface area and absorbed oxygen through the outer membrane.
• They were preserved because there were no predators or scavengers at the time.
• As sandstone compacted, remains became impressions in gelatinous algal mats on the seafloor
• Examples of Ediacaran fossils include Mawsonites (a fossil jellyfish), Dickinsonia (a segmented worm), Kimberella (early mollusk), and Spriggina (a segmented arthropod).

Proterozoic/Phanerozoic Transition

• Marks the transition from early life to visible life.
• "Zoic" means "life."

Geological Time Scale

• Hadean, Archean, Proterozoic, Phanerozoic
• "Protero" means "first."
• Sedimentary structures should be the basic understanding
• Phaneritic means visible
• The "Great Oxidation Event" occurred about 2.4 billion years ago
• Initially, ocean iron oxidized, then oxygen accumulated in the atmosphere.
• After available iron had begun to oxidize, oxygen began to build up in the atmosphere; there are maxmimum and minimum levels now, but today approxomatley 0.2 P02
• Stage 1: Very little O2; the oceans were anoxic.
• Stage 2: O2 rose, but was mostly chemcially absorbed.
• Stage 3: O2 levels stabilised with the production/oxidation of minerals.
• Stages 4-5: O2 builds in the atmosphere.
• Rising oxygen levels drove evolution in late Proterozoic Eukaryotes.

Fossils

• Body fossils provide direct evidence of ancient life or considered a direct "evidence" of life.
• They include hard parts (shells, bones, teeth) and soft parts (soft tissues, skin, organs, plants). Trace fossils indicate the preservation of activity (tracks, trails, burrows) (waste products) and are classified by shape.

Trace Fossils

• Trace fossil gradients show nearshore to offshore environments
• Show alignment w/ environment, horizontal/stable and vertical/shifting sediments
• Oldest known burrows: simple and fine laminae with no burrows
• The first skeletonized complex parts appeared near the end of the Proterozoic, such as Cloudina shells from Namibia.
• The skeletons and shells may have evolved as structural supports and/or defenses against predation.
• Direct evidence of predation includes borings in shells, which are trace fossils and are similar to modern clams

Geological Eras

• The Phanerozoic Era, meaning "Visible life," is divided into the Paleozoic (Ancient Life), Mesozoic (Middle Life), and Cenozoic (Recent Life) eras.
• Trace fossils offer insights into more complex animal behavior.
• defines the base of the Cambrian period where the tracemaker is unknown.
• Earliest hard parts are "Small Shelly Fossils" with simple structures
• Trilobites were marine arthropods that attained maximum diversity ("The bugs of the sea").
• Archaeocyathids formed mineralized reef builders.
• Burgess Shale Outcrop in B.C. Canada contains fossils
• No oxygen = more preservation and is associated w/ rapid burial
• Soft bodied, high diversity and arthropod dominated
• Anomalocaris was the largest predator of the Cambrian
• Pikaia has a notochord (spinal cord)
• Ancient vertebrate

Sedimentary Environments

• Sedimentary processes aren't always linear. Sediments can deposit anywhere and take the location's characteristics.
• A depositional environment/facies is a place where sediment deposits. Facies characterize environments through appearance. Rivers and deserts differ.
• Facies definition: Sediment, fossils and sedimentray Structure
• Analog: Interpertation, and the key for future.
• Beach sediment, look at lithology, and the sedimentary
• Data also shows sedimentray structure (like the one of "Sand")
• Marine trace fossils
• Okay, lets visit an outcrop (sandstone)
• Grain sizes indicate energy.
• Lighter: travel further, more easy
• Heavier: travel shorter, require enrgy
• Energy: Breccia/Conlglomerate and Claystone/shale
• Brachiopods fossil
• Brachiopods live
• 2.40 min

Depositional Environments

• Most brachiopods prefer warm waters and the Sand helps high energy water.
• Wave ripples and direction
• General clue = Sand and it shown in deserts, rivers and beaches, while glacial will just specidicly point it out to a particaular glacier area.

Individual Environments

• 3 types: Continental Marine
• Depostional Model meaderngstream
• Upper block is short term, while the sides it is a more
• Preservation requries tectonic subsidence
• sand and mud are like associations.

Walter's Law

• The stream moves across the floodplain

• Pennsvylania point deposits at summersville

• Dunes will also have cross beds with curent

• Repple will featres feature, and cross laments

• Floodplians has stack of each other in records

• Law of ancient in environment

• Vertically seperated

• Beaches tidal deltas flat

• Apply the law also!

• Reffered

• Facies will shift landward!

• Sea level gets higher!

mountaining building: Appalachian.

Super: small constent

• super contitent

Mountain Building

Rodinia and Early Cabrian

Passive has nothing and is pasive.

Mountains Building and Strata Correlation

• Continental cllision is product

correlation

• This river in enviorment is freshwater! -Facies that stack is implied

• Mearndering insert!

• Marine has nothing

Fossils are dum

Sea levels are change

Correlation and Rock

• The name of the type is the geogrpahic/functonal of what it does...

They al change!! and you have unit

Paleozoic

• They were inland!
• Coals are unique
• Expansions

Swamp will conatin

Trees

• It is how carbo comes
• Burual cause regression.

High transgrassion

They make sea levels do change. What cycles! And then do what cycles!!!

Cycled

Ice and Eath orbit

Eccentric:

It will cuase transion