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