Chapter 20

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Environmental Science
Jovan Stefanovic

CHAPTER 20: GEOLOGICAL HISTORY OF CANADA: CANADA: A YOUNG NATION, BUT AN OLD COUNTRY • Canada in existence since 1867; welded together as a economic unit by railway; last to join was Newfoundland • North American continent assembled by plate tectonic processes that brought together many smaller land masses; processes took 4 bil yrs • Construction of north America began 4000 mil yr with the formation of Acasta Gneiss (northwest territories) • What remains today is fragments of many formed land masses that were fused together by plate tectonics, building of North America was complete 65 mil years ago; modern landscape result of recent glaciations from the last 2.5 mil yrs WHAT ARE THE MAIN GEOLOGICAL “BUILDING BLOCKS” OF NORTH AMERICA A GEOLOGIC JIGSAW: • Northern part of CA, underlain by exposed ancient core ( craton)  called the Canadian shield; consists of Archean and proterozoic rocks that are 4-1 bil yrs old and are fossils Geologic provinces: broad regions of similar rocks ; subdivided into subprovinces fault bonded units containing similar rock, structure, mineral deposits identified by broad-sale field mapping • Provinces and subprovinces are terranesdiscrete fragment of oceanic/continental material that were added to craton at a active margin by accretion • North American craton assembled b/w 1-4 bil yrs ago • Outer most layer buried by layers of younfer cover rocks • North American craton largest in the world • Opening of north Atlantic ocean 80 mil yrs THE NORTH AMERICAN CRATON VS THE CANAIAN SHIELD: North American craton: exposed parts of the craton and consists of gently undulating surface that rises Canadain shield: large landfrom called a pepeplain surface of low relied and great area extent/ age ; erosion/ bevelling created this peneplain 800 mil yrs ago • Cover rocks that bury the outer margins are fossiliferous sedimentary strata of paleozoic/ mesizoic age; deposited when outer margins of craton pressed and flooded by shallow seas; occurred during mountain building THE GEOLOGIC JIGSAW OF THE NORTH AMERICAN CRATON: • Rocks get younger as one travelled away from interior of north America to its coast • Earliest gold mines in Nova soctia • Working of plate techtonics by studying the sea floor provided key to origin of the Canadain shield HOW DID THE NORTH AMERICAN CONTINENT EVOLVE? STAGES IN THE EVOLUTION OF THE NORTH AMERICAN CONTINENT: • Geologic province rimmed by intense deformed rocks that from ancient orogen • Provinces are remains of individual continents that have collided • Orogens consist of crushed/ deformed rocks that are remains of mountain belts/ volcanic arcs • Growth of North American continent broke down into 5 stages; each characterized by major plate tectonic events • Process of repeated continental aggradations/ breakup Wilson cycle  development of supercontinents • Each present day continent similar have similar geologic history • Some of the stages timing is uncertain STAGE 1 ARTICA: NORTH AMERICA IN THE ARCHEAN • Formation of central part of north American continent spans the Archean area ( 4-2.5 bil yrs) • Oldest rocks found in slave province ( 3.96-4.05 bil yrs); these rocks form an ancient continent named Artica • Artica began to form 2.5 bil yrs ago ; final assembly 2 bil years after • Artica contained much of present day Siberia • Sub provinces composed of distinct rock types that record particular geologic events • Basalt originally formed on the floor of ancient Arhean oceans • Sub provinces contain banded iron depots that are major source of worlds iron • Metasedimentary sub province: consists of deep-sea Archean ocean sediments • various subprovinces evidence of plate tectonics activity in the past • southern continental margin or Arctica was the site of major glaciations ( gowganda glaciations) - one of the oldest recorded on earth; 2.4 bil years STAGE 2- NENA AND RODINIA: NORTH AMERICAN INT HE PROTEROZOIC • spans entire proterozoic era; 2.5 bil – 570 mil years • southern part of Canadian shield added to Artica 1.9 bil years ago to form larger land mass called Nena • final assembly of nena recorded by penokean orogeny  created himalyan type mountain rand ; range was destroyed by erosion STAGE 3- THE GRENVILLE OROGENY AND FORMATION OF RODINA: Grenville orogeny: result of the long lived collision b/w south & north America (1.3-1 bil year) • result of collision supercontinent Rodinia formed • grenville orogenic belt dominated by banded gneisses, metamorphosed sediments, igneous rocks; by 800 mil years ago force of erosion tuned the mountains into a peneplain • canadain shield one of the most extensive/ancient land forms on earth • grenville province made from smaller terranes remnants of island arcs, micro- continents, pieces of ocean floors • last phase: intrusion of many granite plutons and dikes welded together RODINA BREAKS APART: • supercontinents are unstable, with time they self destruct; large extent prevents heat from escaping from the earth’s interior and promotes the build up of giant convection plumes; plumes cause land surface to rise like a dome, convection force below dome enough to tear supercontinent apart • continental break up commences with a triple junction ; interlinked grabens that grow/ widen into oceans basin • break up of Rodina – 750 mil years ago; finished: 470 mil years ago • first break occurred along the western margin of north America ( Antarctica and Australia) to form panthalassic ocean ; last break occurred eastern margin of north America ( Europe and Africa drift off) to from Iapetus ocean • failed grabens called aulacogens • Iapetus ocean resembles modern Atlantic ocean ; areas of warm water provided nutrient rich habitat for organism major stimulus for Cambrian explosion STAGE 4- PANGEA: NORTH AMERICA IN THE LATE PALOZOIC AND SOME OF THE MESOZOIC • Iapetus ocean was not long lived ; by 480 mil years was beginning to close • Baltica ( land mass consisting of Europe) collided with estern sea board of North America ( laurentia) to form laurasoa • Collision of the 2 land masses causes a major orogenic event called tectonic orogeny • Ash beds provide important marker horizons within the thick sedimentary strata • Weathered ash is called bentonite can be finger printed to its unique chemical characteristics • Closure of the iapetus ocean resulted in the bukeling of the outer margins of the north American craton and the formation of downwrapped sedimentary basins in the continents interior called intracratonic basins form on cratons and are underlain by continental crust • Delta’s drained the north slope of the tectonic mountains and spread fossiliferous sediments inland • When lil sediment was being transported to the basins, tropical reefs hosting s great wealth of marine organisms flouring in the warn clear shallow waters • CA first oil strike= 1858 petroila • Oil is derived from the break down of organic matter in early Palaeozoic shale of Devonian age and migrated upward to the ground surface as oil seeps HOW WERE THE ATLANTIC PROVICES ADDED TO CANADA? • Tectonic orogeny marks the beginning of the construction of another super continent pangea • Pangea completed in the late carboniferous when lauraisa docked with gondwana • Docking occurred in several stages; initial stage devonianage collision known as the Acadian orogeny and later carboniferous collision called Allegehenian orogeny Appalachian mountains remnants of these events • Large geological blocks were added to North America as micro-continents thrust/ accreted onto the continent during the Taconic & Acadian/Alleghanian orogenies  how Atlantic provinces came to being • During the late carboniferous, the alleghanian orogeny produced extensive maritimes basin composed of many small interconnected basins separated by active faults • Thick peat accumulated in these swamps if compressed/ hardened to form coal • Coals are associated with fluvial & shallow marine deposits • Repeated cycles of coal, fluvial and marine deposits are called cyclothems • Clearest evidence for plate collision during the closure of lapetus ocean/ break up of pangea is in Newfoundland • Contact between the crust & mantle known as mohorovicic discontinuity • Central portion of Newfoundland is made of rocks from the floor of iapetus ocean • Modern Atlantic ocean is the result of the breakup of pangea and is young WHERE DID BRITISH COLUMBIA COME FROM? STAGE 5 – CANADA IN THE MESOZOIC: PANGEA BREAKS APART
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