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 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) • Extensive plutonism/metamorphism and reworking of crustal rocks through orogeny
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 • Yet the Phanerozoic, consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, • lasted a comparatively brief 545 million years
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 • 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.
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 • 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 • 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
Trans-Hudson Orogen Figure 7-4 (p. 244)Location of the Labrador Trough and Trans-Hudson Orogen.
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.
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 • 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.
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.
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
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 (= "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?” • 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 • 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. http://www.valdosta.edu/phy/hist_geo_lab/gp_AZ/gp_column/gp_col.html
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 Photomicrograph of spheroidal and filamentous microfossils from the Gunflint Chert of Ontario Canada
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 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 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 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 Doushantuo Formation Metazoans--Eggs & Embryos
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 • The Ediacaran fauna of Australia Tribrachidium heraldicum, a possible primitive echinoderm or cnidarian Spriggina floundersi, a possible ancestor of trilobites
Ediacaran Fauna Pavancorina minchami • Restoration of the Ediacaran Environment
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 • 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 • 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.
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 • 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 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.
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),
Figure 7-37 (p. 263)Correlation of major events in the history of the biosphere, lithosphere, and atmosphere.