Proterozoic Eon Closer to modern

Began 2.5 b.y. ago, ended about 540 m.y.ago 42% of Earth’s history Beginning based on more modern plate tectonics style & sedimentation Broken into three eras –Paleoproterozoic – 2.5 to 1.6 b.y. ago –Mesoproterozoic – 1.6 to 1.0 b.y. ago –Neoproterozoic – 1.0 b.y to 540 m.y. ago

–Proterozoic rocks less altered than Archean rocks, easier to interpret –Still lack abundant fossils

Several large, distinct crustal segments (Precambrian provinces) developed during Archean Eon Some became sutured (“sewn”) together during Paleoproterozic to form large continent called Laurentia –Suture zone – zone of convergence between 2 plates, showing severe folding, faulting, metamorphism, & intrusive activity (mountain building) –Also called orogens – process by which these activities occur is called orogeny –Each orogeny occurs at a specific place during a specific time, and is given a name Laurentia (& other land masses) grew by accretion of sedimentary rocks, microcontinents, & island arcs to their margins

As continents grew, plate motion, rifting, & seafloor spreading increased Wide continental shelves existed –Shallow seas flooded continental interiors (epicontinental seas) –Allowed accumulation of clean sands & carbonates

Paleoproterozoic 2.5 to 1.6 b.y. ago Vigorous early plate tectonics Major mountain-building on all major continents First great ice age Increased oxygen in atmosphere

Example of an orogenic belt at the time –Slave province in Canada’s Northwest Territory –Called the Wopmay orogen (orogenic belt)

Evidence of a Wilson cycle A plate tectonics idea, consisting of three steps –1. Opening of ocean basin –2. Sedimentation along margins of separating continents –3. Closing of ocean basin Sediments folded & faulted during later plate collisions

Another N. American example – Trans- Hudson orogen –Extends southwest from present Hudson Bay –Rocks show evidence of initial rifting, opening of ocean basin, sediment deposition, closure along subduction zone with folding, metamorphism, & intrusive igneous activity

Location of Trans-Hudson orogen

Evidence of an early ice age Rocks located north of Lake Huron Tillites –Unsorted, lithified glacial debris –Boulders, cobbles, sand, and so forth –Larger fragments scratched & faceted by action of ice mass moving across bedrock Varved mudstones –Deposited in lakes adjacent to leading edges of glaciers –Alternating layers deposited during summer & winter Coarser, light colored layers during summer Finer, dark colored layers during winter Each pair indicates one year

Banded iron formations & prokaryotes Rocks found around western shores of Lake Superior Animikie Group –Coarse sandstones & conglomerates, overlain by cyclic cherts, cherty limestones, shales, & banded iron formations –Once a major source of of iron ores for U.S. steel industry Gunflint Chert –Formation within the BIF that contains fossils of cyanobacteria & other prokaryotes (more on them later) –About 1.9 b.y. old

Labrador Trough –East of Superior province –Folded, thrust-faulted, & metamorphosed during Hudsonian orogeny Marks end of Paleoproterozoic

Mesoproterozoic Era 1.6 to 1.0 b.y. ago; a few highlights follow Lake Superior region –Keweenawan rocks Extensive quartz sandstones, arkoses, conglomerates, & basalt lava flows Lava flows are over 25,000 ft thick (> 5 mi) The lava flows in Michigan contain native (pure) copper, with some native silver Most of the source magma did not erupt –Crystallized in subsurface, forming Duluth Gabbro – 8 mi thick & 100 mi wide

Large amounts of lava suggests seafloor –In central, stable region of continent, indicates a rift zone, where the continent breaks apart –The Keweenawan zone developed 1.2 to 1.0 b.y. ago –Extends from Lake Superior into Kansas –Rifting ceased before continent split completely

Grenville orogeny –Major orogeny in eastern North America (Grenville Province) –Atlantic coast of Labrador to Lake Huron, then down eastern U.S. & westward into Texas –Difficult to interpret because of later events that built the Appalachians –Originally carbonates & sandstones, later metamorphosed & intruded by igneous rocks –Occurred 1.2 to 1.0 b.y. ago

Note the missing “divet”

–Grenville orogeny associated with formation of supercontinent Rodinia –Ocean to the west – “proto-Pacific” – Panthalassa Ocean –Rodinia breaks apart about 750 m.y. ago along eastern side of N. Am., forming narrow seaway called Iapetus Sea (proto-Atlantic)

1.0 b.y. to 542 m.y. ago Belt Supergroup –Montana, Idaho, British Columbia –Deposited in depressions related to rifting –Shales, siltstones, sandstones, dolomites –Over 12 km (7.5 mi) thick, but display ripple marks & stromatolites Shallow waters, passive margin Neoproterozoic rocks

Grand Canyon –Three major Precambrian rock units –Vishnu Schist/Zoraster Granite Older units Metamorphosed sediments & gneisses, intensely folded & intruded by granite At least b.y. old (Mesoproterozoic) –Grand Canyon Supergroup Mostly sandstones, siltstones, shales Neoproterozoic age Nonconformity at base Angular unconformity at top, overlain by Cambrian sediments

Grand Canyon rocks

Proterozoic Life At beginning, life not significantly different from late Archean –Photosynthetic cyanobacteria constructed algal mats around ocean margins –Prokaryotes floated in the surface waters of seas & lakes –Anaerobic prokaryotes lived in oxygen- deficient environs, such as deep-sea hydrothermal springs

Stromatolites –Relatively sparse during Archean, but abundant during Proterozoic –Declined by the end of the era Possibly due to grazing by evolving groups of marine invertebrates

Microfossils in the Gunflint Chert –A chert exposed along NW shores of Lake Superior –About 1.9 b.y. old –A number of prokaryote fossils preserved Some resemble living algae Some resemble living iron- & magnesium reducing bacteria Others we have no clue about Host rock also contains organic compounds thought to be the breakdown products of chlorophyll Life abundant by 2 b.y. ago

Diagrams of organisms in the Gunflint Chert. A = Eoastrion ( = dawn star), probably iron- or magnesium-reducing bacteria B = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameter C = Animikiea (probably algae) D = Kakabekia, an organism or uncertain affinity

What some of the critters really look like Kakabekia

The Eukaryotes Arise Reminder – cells w/true nuclei, enclosed within nuclear membranes, having well-defined chromosomes & cell organelles Fossils of them (early ones esp.) rare First appeared 2.7 to 2.2 b.y. ago Earliest evidence base on biochemical remnants of the organism –“molecular fossils”

Did not diversify until 1.2 to 1.0 b.y. ago –Possibly had to await sufficient oxygen levels –Possibly had to await advent of sexual reproduction –Maybe both

Acritarchs –Proterozoic fossils useful for correlation –Unicellular, spherical microfossils w/resistant single-layered walls Walls may be smooth, or ornamented –We aren’t sure exactly what they are, but appear to be a form of phytoplankton

First appear 1.6 b.y. ago Maximum abundance & diversity 850 m.y. ago Few after 675 m.y. ago –Decline coincides with glaciation event near end of Proterozoic –Possible reduction of carbon dioxide & increase in atmospheric oxygen as a result of glacial conditions

A few acritarchs

Protozoan eukaryotes –Probably present –Non-photosynthetic, thus, they gotta eat (heterotrophs)(there weren’t no Checkers) –Modern examples: foraminifera, amoebas, ciliates Many have preservable shells –Proterozoic ones had no shells Little chance of preservation Does not mean they weren’t abundant May have been part of the decrease of stromatolites

Metazoans Multicellular animals –More than one kind of cell –Cells organized into tissues & organs Appear in the Neoproterozoic –Most are only impressions in sediments –Have now been found on every continent

Ediacaran Fauna Large metazoan fossils first found in Ediacara Hills, Australia in 1940’s Later also found in Russia, China, England, & Africa Some members survived into the Cambrian Period Three types: discoidal, frondlike, & elongate

Discoidal group –Shape is flat and circular –Some may represent jellyfish –Others don’t resemble anything we know of Frondlike group –Look like fern fronds, resemble present-day “sea pens”

Artists view of frondlike organisms & jellyfish

Elongate group –Ovate to elongate in shape –Thought to be impressions of large flatworms & annelid (segmented-body) worms

Kimberella A significant Ediacaran fossil –About 550 m.y. old First found poorly preserved in Australia –Resembled jellyfish 30 well-preserved specimens found in Russia in 1993 –These did not resemble jellyfish –Showed evidence of a coelum

–Coelum - a body cavity in which internal organs are suspended –All higher animals possess this feature –Also, bilateral symmetry, dorsal cover, ruffled border (similar to the mantle that secretes the shell in mollusks) –Most important – was an advanced, complex invertebrate that appeared 10 m.y. before the “Cambrian explosion” (….later)

Kimberella reconstruction; up to 10 cm long

Most fossils of this time were soft-bodied Some shell-bearing fossils also found –Cloudina, found in Namibia, Africa Had a tubular, calcium carbonate shell, only a few centimeters long Probably the tube secreted by a worm –Possible primitive mollusks, sponge spicules, hyolithids (tiny tusk-shaped fossils)

Drawings of Cloudina

Ediacaran Period A new geologic period, added to the geologic time scale in 2004 Ranges from 600 to 542 m.y. ago Based on the fossils found in Australia –Russians wanted the claim, based on the exposures in USSR –Often see this time interval referred to as the Vendian (the Russian name)

–Decision based on geologic community principles –Exposures must be freely available for anyone to investigate Remember how USSR used to be (esp. 1940’s to the 1980’s)? Not to mention China

Correlation of events of Precambrian Hadean Ediacaran Period