The proterozoic eon
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The Proterozoic Eon. CHAPTER 7. Proterozoic- Last chapter of Precambrian time. The Proterozoic boundary is placed @ 2.5 By, mostly arbitrarily Essentially modern plate tectonic activity Archean cratons provide basis for continental development and attachment (accretion)

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The Proterozoic Eon

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The proterozoic eon

The Proterozoic Eon

CHAPTER 7


Proterozoic last chapter of precambrian time

Proterozoic- Last chapter of Precambrian time

The Proterozoic boundary is placed @ 2.5 By, mostly arbitrarily

  • Essentially modern plate tectonic activity

  • Archean cratons provide basis for continental development and attachment (accretion)

  • Extensive plutonism/metamorphism and reworking of crustal rocks through orogeny


The length of the proterozoic

The Length of the Proterozoic

  • the Proterozoic Eon alone, at 1.955 billion years long, accounts for 42.5% of all geologic time yet we review this long episode of Earth and life history in a single section


The phanerozoic

The Phanerozoic

  • Yet the Phanerozoic, consisting of

    • Paleozoic,

    • Mesozoic,

    • Cenozoic eras,

  • lasted a comparatively brief 545 million years


Proterozoic eon can be divided into three eras

Proterozoic Eon can be divided into three eras

  • Paleoproterozoic Era (2.5 - 1.6 by ago)

  • Mesoproterozoic Era (1.6 to 1.0 by ago)

  • Neoproterozoic Era (1.0 by ago to the beginning of the Paleozoic, 540 my ago)


North american continental developments

North American continental developments

  • Precambrian provinces of North America were welded together to form a large continent called Laurentia during the early Proterozoic.

  • Welding occurred along mountain belts or orogens by ~1.7 by.

  • Laurentia continued to grow by accretion throughout the Proterozoic.

  • Sedimentation on and around the craton consisted of clastic and carbonate sediments deposited in shallow water on broad continental shelves and epeiric seas.


Precambrian provinces of north america

Precambrian provinces of North America


Wilson cycle

Wilson Cycle

  • A complete Wilson cycleinvolves

    • Fragmentation of a continent

    • Opening followed by closing of an ocean basin

    • Finally, reassembly of the continent

  • Many example (more abundant) in Phanerozoic


  • Wopmay orogen

    Wopmay Orogen

    • Some of the rocks in Wopmay orogen show

      • An assemblage of rocks typical of passive continental marginsthat first become widespread during the Proterozoic


    Ocean formation closure

    Ocean formation/closure

    • See examples of tensional faulting- where ocean began after being ripped apart

    • Alluvial fans develop; lava comes up through these faults (interspersed with sediments)

    • Ocean widened

    • Closure of ocean; buckled sediments producing trough for further sedimentation; crumpled rocks as buckled


    Figure 7 4 p 244 location of the labrador trough and trans hudson orogen

    Trans-Hudson Orogen

    Figure 7-4 (p. 244)Location of the Labrador Trough and Trans-Hudson Orogen.


    Labrador trough

    Labrador Trough

    • East of the Superior province are rocks indicating deposition on a continental shelf, slope, and rise of the craton.

    • These rocks were folded, metamorphosed, and thrust-faulted during the Hudsonian orogeny, which separates the Paleoproterozoic from the Mesoproterozoic.


    Satellite image of labrador trough

    Satellite image of Labrador Trough


    Paleoproterozoic ice age

    Paleoproterozoic Ice Age

    • A Paleoproterozoic ice age is recorded in rocks north of Lake Huron

    • Evidence for glaciation includes finely laminated mudstones and tillites (or poorly sorted conglomerates of glacial debris).

      • fine laminations can indicate seasonal deposition


    Aulacogens

    Aulacogens

    • Defined as the inactive rift (or failed rift arm) at a Y-shaped triple junction where three plates are separating.

    • Two of the three rifts are dominant and active, and the third is inactive--later it fills with sediment.


    Y shaped triple junction

    Y-shaped triple junction


    The mesoproterozoic era

    The Mesoproterozoic Era

    The Midcontinent Rift and the Keweenawan Sequence

    • Extending southward from the Lake Superior region, and overlying Animikian rocks or crystalline basement, the Keweenawan consists of:

      • Clean quartz sandstones

      • Conglomerates

      • Basaltic lava flows

      • Native copper in rocks

    • An old rift zone 1.2 by - 1.0 by old.

      • Extends as far south as Kansas.


    Precambrian provinces of north america note mid continent rift and grenville province

    Precambrian provinces of North America. Note Mid-Continent Rift and Grenville Province.


    The grenville province and rodinia

    The Grenville Province and Rodinia

    • The Grenville Province in eastern North America consists of sedimentary rocks that were metamorphosed and intruded by igneous bodies.

    • Deformation of these rocks occurred during the Grenville orogeny, 1.2 by to 1.0 by ago.

    • This orogeny was associated with the formation of the supercontinent Rodinia (“Proto-Pangea”)

    • Ocean around Rodinia was called Mirovia


    The proterozoic eon

    Figure 7-8 (p. 247)The Neoproterozoic supercontinent Rodinia as it began to break apart. (After Hoffman, P. F. 1991. Science 252: 1409-1412.)


    The neoproterozoic

    The Neoproterozoic

    • The Neoproterozoic (= "new" Proterozoic) ranges from about 1.0 by to 0.54 by (540 my).

    • Neoproterozoic rocks were deposited in basins and shelf areas surrounding the North American craton. Most of these rocks were deformed during the Paleozoic orogenies.


    Glacial deposits in the neoproterozoic snowball earth

    Glacial deposits in the Neoproterozoic - “Snowball Earth?”

    • Tillites or lithified, unsorted conglomerates and boulder beds are found nearly worldwide (on all land masses except India and Siberia-Mongolia) at this time, and are interpreted by many geologists as glacial deposits.

      • Stratified cobbles and dropstones

    • Glacial deposits are so widespread at this time that geologists refer to it as "snowball Earth” (Varangian glaciation)

      • HIGHLY debated


    Precambrian rocks of the grand canyon

    Precambrian rocks of the Grand Canyon

    • Vishnu Schist - the oldest unit. Complex unit of metasediments and gneisses, intruded by granites about 1.4 by to 1.3 by. Topped by an unconformity.

    • Grand Canyon Supergroup - a Neoproterozoic unit consisting mainly of sandstones, siltstones, and shales. Top of this unit is an unconformity overlain by Cambrian sedimentary rocks.


    Vishnu schist grand canyon supergroup and other rocks in the grand canyon of the colorado river

    Vishnu Schist, Grand Canyon Supergroup, and other rocks in the Grand Canyon of the Colorado River.

    http://www.valdosta.edu/phy/hist_geo_lab/gp_AZ/gp_column/gp_col.html


    Late proterozoic fossil record

    Late Proterozoic Fossil Record

    Microfossils of the Gunflint Chert

    • First Precambrian fossils to be discovered were Proterozoic fossils in the Gunflint Chert, NW of Lake Superior. Fossils resemble living photosynthetic organisms and include Cyanobacteria

    • Probably photosynthetic - altered atmosphere.

    • Rock contains organic compounds that are regarded as the breakdown products of chlorophyll


    Gunflint microfossils

    Gunflint Microfossils

    Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada


    Proterozoic fossil record

    Proterozoic Fossil Record

    • Eukaryotes (large cells with nuclei and organelles) appeared by Mesoproterozoic time. Appeared by 1.6 by to 1.4 by. Increased in abundance about 1.4 by ago.

    • Potential for sexual reproduction and increasing variation (evolution)


    Acritarchs

    Acritarchs

    • Acritarchs are single-celled spherical, organic-walled microfossils.

    • Not known what sort of organism they were, but they may have been phytoplankton. First appeared 1.6 by ago. Maximum diversity and abundance 850 my ago. They declined steadily during the Neoproterozoic glaciation, and few remained by 675 my ago. Useful for correlation in Proterozoic strata.


    Origin of eukaryotes

    Origin of Eukaryotes

    Eukaryotes are thought to have arisen from an originally endosymbiotic relationship between two or more prokaryotic cells.

    • Symbiosis - two or more organisms living together, where each organism usually derives some benefit froom the relationship.

    • Endosymbiosis - One organism lives inside the other, e. g. bacteria in stomachs of cattle.


    Origin of eukaryotes1

    Origin of Eukaryotes

    • Organelles (Mitochondria, plastids) were once free living bacteria, that entered or were engulfed by another prokaryote. Eventually, relationship became mutually beneficial.

      • E. g. host cell provided proto-mitochondrion (primitive oxidizing bacteria) with plenty of food, mitochondrion performed oxidation and released energy for a bacterium that previously could only ferment. Eventually proto-mitochondrion gives up trying to reproduce.

      • E.g. host cell waste products (carbon dioxide etc.) used by a cyanobacterium (proto-plastid) to photosynthesize. Plastid leaks energy to host cell. Eventually gives up reproducing on its own.


    Neoproterozoic metazoan life

    Neoproterozoic Metazoan Life

    Doushantuo Formation Metazoans--Eggs & Embryos


    The ediacaran fauna

    The Ediacaran Fauna

    • In 1947, an Australian geologist, R.C. Sprigg, discovered impressions of soft-bodied animals in the Pound Quartzite in the Ediacara Hills of South Australia


    Ediacaran fauna

    Ediacaran Fauna

    • The Ediacaran fauna of Australia

      Tribrachidium heraldicum, a possible primitive echinoderm or cnidarian

    Spriggina floundersi, a possible ancestor of trilobites


    Ediacaran fauna1

    Ediacaran Fauna

    Pavancorina minchami

    • Restoration of the Ediacaran Environment


    Represented phyla

    Represented Phyla

    • Three present-day phyla may be represented in the Ediacaran fauna:

      • jellyfish and sea pens (phylum Cnidaria),

      • segmented worms (phylum Annelida),

      • and primitive members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others)

  • One Ediacaran fossil, Spriggina, has been cited as a possible ancestor of trilobites

  • Another might be a primitive member of the phylum Echinodermata


  • Distinct evolutionary group

    Distinct Evolutionary Group

    • However, some scientists think

      • these Ediacaran animals represent

      • an early evolutionary group quite distinct from

      • the ancestry of today’s invertebrate animals

    • Ediacara-type faunas are known

      • from all continents except Antarctica,

      • are collectively referred to as the Ediacaran fauna

      • were widespread between 545 and 670 million years ago

      • but their fossils are rare

    • Their scarcity should not be surprising, though,

      • because all lacked durable skeletons


    Latest proterozoic kimberella

    Latest Proterozoic Kimberella

    • Kimberella, an advanced metazoan in the Ediacaran fauna, which is the first known organism to have a coelom or body cavity. Possibly mollusc-like. Specimens found in Russia in 1993. 550 my old.


    The proterozoic eon

    Figure 7-32 (p. 261)Reconstruction of Kimberella.(After Fedonkin, M. A., and Waggoner, B. M. 1997. The Late Precambrian fossil Kimbrella is a mollusk-like bilateral organism. Nature 388(28): 868-871).


    Soft bodies

    Soft Bodies

    • All known Proterozoic animals were soft-bodied, but there is some evidence that the earliest stages in the origin of skeletons was underway

    • Even some Ediacaran animals may have had a chitinous carapace and others appear to have had areas of calcium carbonate

    • The odd creature known as Kimberella from the latest Proterozoic of Russia had a tough outer covering similar to that of some present-day marine invertebrates


    Small shelly fauna the origin of hard parts

    Small Shelly Fauna: The Origin of Hard Parts

    • Small fossils with hard parts or shells (mostly a few mm long ) appeared in the Vendian

    • Cloudina, an organism with a small (few cm long), tubular shell of calcium carbonate is interpreted as a tube-dwelling worm. It is the earliest known organism with a CaCO3 shell.


    The proterozoic eon

    Geologic time scale across the Precambrian-Cambrian boundary, showing the Vendian, and indicating the occurrence of the Ediacaran fauna, the small shelly fauna (first known hard parts),


    The proterozoic eon

    Figure 7-37 (p. 263)Correlation of major events in the history of the biosphere, lithosphere, and atmosphere.


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