proterozoic eon closer to modern. began 2.5 b.y. ago, ended about 540 m.y.ago 42% of earth’s...

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 1.4-1.3 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 bacteriaB = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameterC = 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