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Chapter 9. Precambrian Earth and Life History Proterozoic. Introduction. The Archean-Proterozoic boundary at 2.5 billion years ago marks the approximate time of changes in the style of crustal evolution,.
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Chapter 9 Precambrian Earth and Life History Proterozoic
Introduction The Archean-Proterozoic boundary at 2.5 billion years ago marks the approximate time of changes in the style of crustal evolution, First, the Archean was characterized by the originof granite-gneiss terrains and greenstone belts that wereshaped into cratons, Although these same rock associations continued to form during the Proterozoic, they did so at a considerably reduced rate. The change in style of crustal evolution, the Proterozoic was also an important time in the evolution of the atmosphere and biosphere, as well as the origin of some important natural resources. Oxygen dependant organisms and the types of cells that make up most of today's organisms evolved during this time. Fig. 19.1, p. 494
Proterozoic Continents • Acretion continue during the Proterozoic • Laurentia a North land mass known North America • Greenstone belts continue in Australia but without Komatiites
Proterozoic Earth History The Proterozoic2.5 bya – 545 mya Paleoproterozoic History of Laurentia By 1.8 bya, plate collisions had formed several orogens – linear or arcuate belts of deformed metamorphic rocks intruded by magma forming huge batholiths. The old Archean cratons were sutured together by continental accretion along their margins creating a large landmass known as Laurentia. Laurentia included part of North America and Greenland.
Laurentia • Excellent examples of these craton-forming processes are recorded in rocks of the • The Thelon orogen (1.92-1.96 BYA) in northwestern Canada where the Slave and Rae cratons collided, • The Trans-Hudson orogen (1.82-1.84BYA), in the United States and Canada where the Superior, Hearne, and Wyoming cratons were sutured (Figure 9.2a). • the Penokean orogen formed along the southern margin of Laurentia over tens of millions of years, although the most intense episode of this event was about 1.85 BYA.
Proterozoic Earth History The Proterozoic Paleoproterozoic 2.5 bya – 545 mya History of Laurentia Wilson cycle evidence – The Wopmay Orogen Wilson cycles record the opening and closing of an ocean basin Development of a passive continental margin
Uplift Divergence (spreading) Passive continental margin Divergence (spreading) Convergence (subduction) Convergence (collision) and uplift Convergence and uplift Stepped Art Fig9.3
Wopway Ororgen Northwestern Canada • The Wopmay orogen consist of a suite of sedimentary rocks called a sandstone-carbonate-shale assemblage that forms on passive continental margins. • Passive margin deposits are rare or absent in Archean rocks, but they become common during the Proterozoic and thereafter. • This assemblage of rocks is also well represented in the Penokean orogen of the Great Lakes region of the United States and Canada
Proterozoic Earth History The Proterozoic2.5 bya – 545 mya Paleo- and Mesoproterozoic Igneous Activity Between 1.8 and 1.1 bya extensive igneous activity unrelated to orogenic processes occurred, thickening the continental mass with: Granite and anorthosite plutons Volcanic calderas Vast sheets of ash Hypothesis: Mantle plume
Proterozoic Earth History The Proterozoic2.5 bya – 545 mya Mesoproterozoic Orogeny and Rifting Between 1.3 and 1.1 bya the Grenville orogeny occurred as the final stage of Proterozoic continental accretion. Beginning 1.1 bya, the Midcontinental Rift formed, thought to be a failed rift that did not break Laurentia apart. Detrital sedimentary rocks and basaltic lava flows are found in the central portion of the rift.
Proterozoic Earth History The Proterozoic2.5 bya – 545 mya Meso- and Neoproterozoic Sedimentation In the western United States and Canada, Proterozoic sedimentary rocks are well-exposed. These are mostly sandstones and shales, with some dolostones and stromatolite-bearing carbonates. Indicate shallow marine and fluvial environments of deposition.
Proterozoic Earth History The Proterozoic2.5 bya – 545 mya Proterozoic Supercontinents A continent is a landmass, consisting of a granitic crust and most of its surface above sea level. A supercontinent is larger and composed of several continents that have come together. The first supercontinent that geologists recognize is known as Rodinia. assembled 1.3 to 1.0 bya began fragmenting 750 mya
Proterozoic Earth History • Proterozoic Rocks Sandstone-carbonate-shale assemblages • Deposited on passive continental margins and in intracratonic basins • The most common Proterozoic-aged rocks.
Proterozoic Earth History • Proterozoic Earth History Proterozoic Glacial Deposits • Widespread glaciers were present during the Early and Late Proterozoic. Fig. 19.15b, p. 510
The Evolving Atmosphere • the Archean atmosphere contained little or no free oxygen (see Chapter 8), so the atmosphere was not strongly oxidizing as it is now. • Photochemical dissociation and photosynthesis were adding free oxygento the atmosphere, • but the amount present at the beginning of the Proterozoic was no more than 1%of that present now. In fact, it might not have exceeded 10% of present levels even at the end of the Proterozoic. • Remember that cyanobacteria (blue-green algae), and Stromatolites These photosynthesizing organisms and, to a lesser degree, photochemicaldissociation both added free oxygen to the evolving atmosphere
Proterozoic Earth History Proterozoic Earth History Banded Iron Formations Deposition of widespread banded iron formations between 2.5 and 2.0 billion years ago indicate that free oxygenwas absent in the early atmosphere. Fig. 19.16a, p. 511
Banded Iron Formations (BIF) • BIFs, fully 92%, were deposited in shallow-water shelf environments during the interval from 2.5 to 2.0 billion years These deposits are much more extensive thanthose of the Archean and they have important implications for the evolving atmosphere.
Proterozoic Earth History Proterozoic Earth History ContinentalRed Beds Deposition of continental red beds about 1.8 billion years ago, however, indicate that there now was some free oxygen was present in the atmosphere.
Life-Its Origin and Early History • Life of the Proterozoic • A New Type of Cell Appears • Eukaryotic Cells reproduce sexually, and most are multi-celled • nearly all are aerobic, that is, they depend on free oxygen to carry out their metabolic processes • Therefore, they could not have evolved before at least some free oxygen was present in the atmosphere Fig. 19.21, p. 515
Life on Proterozoic • 6 Kindoms of organisms
Life of Proterozoic • 3 Domains
Life-Its Origin and Early History Life of the Proterozoic Endosymbiosis and the Origin of Eukaryotic Cells Endosymbiosis – a process whereby one cell lives within another, a process practiced by prokaryotic cells was probably responsible for the first eukaryotic cells.
Life-Its Origin and Early History • Life of the Proterozoic • The Dawn of Multi-celled Organisms • It is not known how multi-celled organisms arose from single-celled organisms. • Carbonaceous impressions of Proterozoic multi-celled algae are known from areas, some in rocks more than 2 billion years old. Fig. 19.22, p. 516
Life-Its Origin and Early History • Life of the Proterozoic • Ediacaran Fauna • The Neoproterozoic Ediacaran faunas include the oldest well-documented animal fossils. None had durable skeletons, so their fossils are not common. Fig. 19.25b, p. 517
Why Edicaran 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
Resources in Precambrian Rocks • Archean Resources • More than 50% of the world’s gold has come from Archean rocks in South Africa. • Massive sulfide deposits containing zinc, copper, and nickel are known from Australia, Zimbabwe (Africa) and Canada. These are similar to the black smokers on the seafloor. • About 25% of the world’s chromium reserves are also in Archean rocks, along with platinum deposits. • Archean pegmatites in the African Shield and the Canadian Shield have yielded valuable gem-quality minerals.
Resources in Precambrian Rocks • Proterozoic Resources • 92% of all BIF’s (Banded iron formations), the world’s main source of iron ore comes Proterozoic rocks • Nickel, platinum, and chrome are found on the shields of North America and Africa.
