Geology Notes Week 6
Geology Notes Week 6 GEOL1005
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This 8 page Class Notes was uploaded by Kate Notetaker on Friday February 19, 2016. The Class Notes belongs to GEOL1005 at George Washington University taught by Catherine A. Forster in Winter 2016. Since its upload, it has received 50 views. For similar materials see Historical Geology in Geology at George Washington University.
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Date Created: 02/19/16
February 16, 2016 Textbook Notes for the Quiz Laurentia o Name given to the combination of Greenland and North America in the Proterozoic and most of Phanerozoic period o Gained territory by continental accretion Found this through Rocks obtained from wells that penetrate the Phanerozoic cover Exposed rocks of the Canadian Shield Precambrian rocks that have been elevated by Phanerozoic mountain building in the American West Evidence appeared decades ago Recognition of structural trends and regional occurrences of rock units Uranium-lead techniques Dates obtained from crystalline rocks o Grew rapidly during Proterozoic time as other cratons became sutured to it o First stage Assembly of at least five micro continents into a sizable craton Micro continents had formed during Archean time Took place within about 100 million years Between 1.95 billion and 1.85 billion years ago o Superior Province Largest of the Archean terranes Crops out as far south as Wisconsin Separated from smaller Archean terranes to the west by a broad zone of rocks that formed between 2.0 billion and 1.8 billion years ago Trans-Hudson Belt o Comprises both deep-sea sediments squeezed up between the converging cratons and crystalline rocks produced by an igneous arc o South of the Archaen terranes between 2. Billion and 1.8 billion years ago Series of terranes whose addition expanded Laurentia eastward Clastic example of continental accretion Grenville orogeny Ended about a billion years ago Three successive orogenies Produced the Appalachian Mountains during the Paleozoic Era Added more continental crust to eastern North America Sudbury impact o 1.85 billion years ago o Extraterrestrial event o Produced one of the largest bodies of metallic ores ever discovered o Impact of an asteroid crater in southern Canada Originally about 150 kilometers in diameter o Shattered rocks to form breccias Some contain such large clasts that they are called megabreccias o Pressure also shattered rocks in cone-shaped radial patterns Shatter cones Point toward the impact site o Tsunami deposits (some cross bedded) indicate that the impact happened in a marine setting o Blasted chunks of rocks out of the seafloor Ejecta Some objects flew as far as 800 kilometers from the impact site Spherical sand-sized grains that were originally glassy Formed by the rapid cooling of droplets of molten rock Larger, elongated clasts Formed by the cooling of airborne globs of lava o Sudbury ore deposits Rich in copper, nickel, platinum, palladium and gold Derived from bedrock Heavy metals segregated from other molten materials when the heat of the impact melted a huge volume of rock Banded iron formations in the Lake Superior region o A lot of iron for the production of steel o Deposition ended about 1.85 billion years ago o Sudbury impact stirred up the ocean, changing the concentration of oxygen near the seafloor and temporarily terminating iron deposition Greatest disturbance of the central North American craton o Last 1.4 billion years o Spreading center formed beneath the craton begins to rift apart o Lavas poured into elongate basins along a belt that extended from the Great Lakes region to Kansas o Rifting ceased before the continent was split o Left its mark in enormous volumes of mantle-derived lavas o Lavas produced basalts and felsic volcanic rocks o Had the crescent-shaped zone of rifting extended to the southern margin of the craton Eastern US would have drifted away o Keweenawan Supergroup Configuration and composition indicate the presence of failed midcontinent rift Rocks that formed within rift include hardened lavas known as Keweenawan basalts Exposed near the southern border of the Canadian Shield Associated with red siliciclastic rocks and alluvial-fan conglomerates Contain ore deposits of native copper o Mineral consisting of elemental copper uncombined with other elements Presence is associated with feature known as “midcontinental gravity high” o Local increase in Earth’s gravitational field as measured from the surface Grenville Orogeny o Another step in the accretion of Laurentia Added a belt that stretched from northern Canada far down into the eastern United States o Stabilized a large volume of sediments that had accumulated along the margin of eastern North America o Crystalline rocks Best exposed in the Canadian portion of the Grenville Province o Grenville event collision of eastern Laurentia with a landmass that would later become northern South America Remnants of mountain systems that are the same age as the Grenville orogenic belt The Canadian Shield o Large portion of the North American craton o Precambrian rocks underlie the interior of the continent farther south Overlain by a relatively thin veneer of Phanerozoic sedimentary rocks Lecture Proterozoic o 2.5 billion years ago to 542 million years ago o Divided into three Eras Paleo Meso Neoproterozoic o Earth has developed both oceanic and continental crust as well as oceans o Tectonics is in full swing o Simple, unicellular life has evolved Archaea Cyanobacteria o Atmosphere Richer in free oxygen evolution of cyanobacteria Beginning of the Proterozoic atmospheric oxygen still low By the Mesoproterozoic oxygen levels rise to 15% of the present atmospheric level o Length of days has not always been 24 hours Earth’s spin slows by about 20 seconds per day every million years Days are getting progressively longer Days were much shorter in the Proterozoic o Daily growth lines in fossils Animals that build their skeleton from calcium carbonate secrete a thin band of skeletal material on a daily basis Daily growth lines can be counted and registered with seasonal (annual) variations Can estimate the number of days per year through time o Tidal rhythmite deposits Rhythmic sediments laid down by tides Deposited daily by tides and reflect the lunar cycle Each bundle represents two weeks worth Thick lines represent the neap tides When the difference between low and high tide is smallest o We know that the Neoproterozoic day was about 18 hours long and there were about 470 days per year Proterozoic known for o Many orogenies o Continued volcanism o Metamorphism o Continent building Supercontinent o Three times when continents assembled/sutured into a single landmass o Nuna All the continental landmasses were united Through continental collisions and suturing of continental masses to one another Fully assembled by 1.7 BY and lasted around 400 million years Laurentia North American craton Southern end of NUna Sutured to what will become the northern part of South America Collision between Laurentia and Amazonia resulted in a broad mountain building event Grenville Orogeny Grenville Orogeny Produced sequence of folded and faulted mountains along eastern margin or Laurentia Nuna began to rift apart One of the attempted rifts failed on the western side of Laurentia Filled with both marine water and sediment o Lots of sediment mainly mudstones Also sandstones and limestones o 16000 m of sediment are preserved in this rift o Abundant stromatolites o This deposit is called the Belt Supergroup The appearance of the first eukaryotes happened around the time of Nuna Unicellular and simple multicellular organisms o Protists and algae o Rodinia 900 MY Included nearly all continental masses Straddled the equator Lasted until about 630 MY after increased volcanism and rifting Broke it into two continental blocks Two blocks separated, rotated and then moved south Reassembled once again into Pannotia o Pannotia One of the last Proterozoic supercontinents around 600 MY Didn’t last long Rifting began Left the “southern” continents of Africa, South America, Australia, India and Antarctica These sutured continents is called Gondwana o Cryogenian Time span of Rodinia and Pannotia Means “cold birth” Time of enormous glaciations that expanded and contracted in at least two large and one small pulse Sturtian 750-700 MY Marinoan 675-635 MY Gaskiers 570 MY Widespread glacial deposits through these times Not only found at high latitudes o Also in areas that were at mid-latitudes and even at or near the equator Proterozoic glacial depositis o Tillite Glacial debris scraped up by glaciers is called till Glaciers contain dirt, dust and rocks Poorly sorted conglomerates (diamictites) o Dropstones Chunks of ice break off melting glaciers and fall into oceans and float away Carry with them some of the rocks and dirt (till) Ice eventually melts and the till carries drops to the ocean floor Unlike other marine sediment o Glacial striations As glaciers scrape across the ground they grind over bedrock Till carried in the ice can gouge into bedrock Leaves glacial striations Grooves and flattened surfaces Lines are parallel to direction of glacier movement How did life survive through such extensive glaciation? o Widespread ice sheets and low global temperatures Metazoa (animals) o Between the Marinoan and Gaskiers glaciations o Sudden and widespread appearance of many complex multicellular organisms o Appear 635 million years ago Ediacarans Ediacarans o Come from the Ediacara Hills in Australia where they were discovered in 1946 o Known from impressions and trackways of more than 40 different species o Macroscopic o Soft-bodied organisms and their trackways o Preserved as impressions on fine-grained rock o Lived in cold, shallow oceans around the margins of the continents o Known from more than 40 different taxa o All lived on the surface of the ocean floor No burrowing ediacarans o Some of the Ediacaran organisms may be ancestral to present day organisms Some are unique and apparently have no later descendants Sponges o Deposit a mineralized skeleton between its two cell layers o Minerals appear as tiny silica spikes called spicules o Fossils of 580 million year old silica sponges Contain silica spicules and were part of the Ediacaran fauna o 650 million year old fossils that may be early sponges in Australia Lived during the Marinoan Glaciation This pre-dates the Ediacarans Found in shallow marine carbonates Just before Cambrian animals evolved another unusual innovation o Hard skeletons Slowly lightly mineralized animal parts Tiny plates, cones, spikes, plate and tubes began to appear in fossil record Fauna of small, shelly metazoans Small Shelly Fauna o The last of the Ediacarans overlap with the first of the SSF metazoans o The SSF extend well into the early part of the Cambrian o Minerals used to build their shells are very restricted Silica Calcium carbonate Calcium phosphate o Mineralization is biologically expensive It takes a high physiological toll to build and carry a heavy shell o Why just before the Cambrian? Earth is warmer animal metabolism has speeded up O 2evels have risen dramatically to 75% present atmospheric level larger and more metabolically active animals
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