1	The Precambrian
2	The Precambrian Common Name for the Cryptozoic Eon (“ obscure life”) 4.5 billion to ~ 700 million years ago (80% of Earth’s history) Fossils from this time are rare Understanding of cryptozoic events is poor
3	Divisions of Precambrian
4	Important Geological & Biological Events of the Precambrian Earth formed (been there, done that) Continents formed, plates moved, continents grew Atmosphere became enriched in oxygen Eukaryotic cells evolved & 1^st multi-cellular animals
5	End Precambrian
6	Development of Continental Crust Original crust was thin, and grew by accretionary tectonics Began in Archean Subduction-related volcanism Collision of microplates Isotopic age provinces
7	Isotopic Dating & Continental Accretion Theory Isotopic date provinces terrains that collided with the protocontinent Show ‘younging outward’ of age Shield & platform = nucleus (craton)
8	Possible Precambrian Scenario for Continent Accretion in NA (Laurentia) Archean- small granitic shields originally form Proterozoic- 2 billion years ago: small continents colliding & accreting 1.5 billion years ago: Convergence Siberia (maybe Australia & Antarctica) to west Baltica (Europe) to east Mazatal continent to south 1.3-1.4 billion years ago: 1st supercontinent, Rodinia
9	Typical Rocks of the Precambrian Archean: Greenstone Belts Gneiss Belts Proterozoic Metamorphosed pure-quartz sandstones, graywackes Stromatolite-bearing limestones
10	Precambrian Rocks of North America: The Canadian Shield Largest area of Archean rocks in the world 2/3 of eastern Canada, parts of WI, MI & MN Oldest rocks in Shield: Acasta Gneiss, near Great Slave Lake (4.03 Ga) Greenstones, Michigan UP: 2nd oldest in MI, (3.5-3.7 Ga) Valuable metal resources
11	Earliest Atmosphere Atmosphere- Lacked oxygen Methane, carbon-dioxide rich Greenhouse gases Climate- evidence of climatic extremes Hot- mud cracks, desert dunes “Snowball Earth” Theory Late Proterozoic- Widespread glacial deposits Found at all latitudes, including subtropical Suggests worldwide chilling of climate May have precipitated “Cambrian Explosion”
12	Earliest Oceans Lacked oxygen Sulphur & iron-rich Evidence Most metal resources from Archean rocks Lack oxidation Oxidized metals- ‘1^st’ appear in Proterozoic Red beds, banded iron ores
13	Source of Oxygen Red beds start appear in Proterozoic rocks Suggest appearance of abundant oxygen Theory: cyanobacteria (stromatolites) photosynthesis
14	Stromatolites: cyanobacteria of Proterozic Seas
15	What was life like during Precambrian? Single-celled organisms dominated Earth from 3.5 Ga to ~560--600 million years ago Cyanobacteria 600 ma- Bacteria, green algae, & acritarchs were common in the seas 600 ma- 540 ma- Vendian/Ediacaran Fauna 1^st multi-cellular organisms
16	Vendian Period Cyclomedusa most common and widespread Vendian fossil benthic (bottom-dwelling) polyp, somewhat like a sea anemone White Sea region of Russia
17	Vendian Period Pteridinium Exactly what it was, or how it lived, is open to question. May belong to a phylum- level group that is now extinct. Had an elongated, ribbed body usually found squashed flat
18	Vendian Period Dickinsonia south Australia and north Russia annelid worm or cnidarian polyp
19	Vendian Period Kimberella one of the most fascinating Vendian fossils White Sea region of Russia appears to be somewhat like a mollusc uncertain which group of modern animals is most closely related to this