Chapter 20 ➢ Canada's economic well being is dependent on geological resources and the finding and use of new mineral and energy deposites, such as oil and gas, requires new generations of geoscientists ➢ Growing dependence on geology due to urbanization ➢ A detailed understanding of Canadian geology is required for Safe disposal of wastes Design of foundations for buildings Roads Location of sufficient quantities of construction materials such as sand and gravel The assessment of earthquake risk all require a detailed understanding of Canadian geology ➢ Canada: A Young Nation, But An Old Country Been a nation since 1867 Act of Confederation brought together provinces of Ontario, New Brunswick, Nova Scotia, and Quebec to create a larger and more powerful political entity Welded together by a railway Last province to join was Newfoundland North American continent (almost in the same fashion) was assembled by plate tectonic processes that brought together many smaller land masses Process of CONTINENTAL BUILDING has taken more than 4 BILLION YEARS to accomplish Construction of North America began at least 4 000 million years ago (4 billion) with the formation of the Acasta Gneiss of the Northwest Territories, which now forms part of the Slave Province of the Canadian Shield Acasta Gneiss – Located in Yellowknife, oldest known crustal fragment on earth Important in establishing the early history of continental crust Building of North America completed 65 million years ago! Last ice sheet melted in Labrador 6 000 years ago. ➢ WHAT ARE THE MAIN GEOLOGICAL BUILDING BLOCKS OF NORTH AMERICA Craton Northern part of the continent in canada, is underlain by the exposed part of the ancient core or craton of north america This exposed part is called the canadian shield and consists predominantly of very old Archean and Proterozoic rocks Rocks range from 4 billion to 1 billion years old and have no fossils Cratons composed of assemblage of geologic provinces Geologic provinces: broad regions of similar rocks, usually covering many thousands of square kilometres, with characteristics that differ significantly from rock types present in adjacent areas Further subdivided into sub-provinces = fault-bounded units containing similar rock types, structures, and mineral deposits Terranes New name given to provinces and subprovinces Discrete fragments of oceanic or continental material that have been added to a craton at an active margin by accretion ➢ NORTH AMERICAN CRATON VERSUS THE CANADIAN SHIELD Difference between North American craton and the Canadian Shield North American craton: A craton is formed through the assembly of geological provinces and makes up the core of the North American continent • “A large continent sized block of distinct geology making up the basement of much of North America” Underlying rock: metamorphic Overlying rock: cover strata = sedimentary rock that are fossiliferous from Paleozoic and Meseozoic • These bury the outermost margins of the craton It is the largest craton in the world – assembled btwn 1 and 4 billion yrs ago Greenland contains a portion of it as it was once part of North America before Atlantic Ocean opened up in between Burying of outer margins explained by: • Flooding by shallow seas and mountain-building episodes (orogenies) • depressed by the weight of mountain belts and their thick piles of sediment Many other continents on Earth show the same basic anatomy of a central, ancient craton created by the fusion of many separate geological provinces (microcontinents), buried around its margins by younger sedimentary cover strata . Building of North America • 5 principle building blocks • Original North American continent called = Arctica • Appalachian mountains were added during the formation of Pangea • etc. The Shield Is the exposed part of the craton and consists of a gently undulating surface that rises like an arch, in its centre Canadian Shield forms one of the most extensive and ancient landforms in the world and is remarkable because geologists have little detailed knowledge of how such a landscape formed. Peneplain Canadian Shield is a large landform called a peneplain A surface of low relief and great areal extent and age Created 800 million years ago by erosion and bevelling of craton ancient rocks Forms the unconformity btwn craton below and younger surface rocks above Unconformity can be seen in the Grand Canyon (Arizona, US) • Separating metamorphic rocks of craton from overlying Paleozoic rock ➢ THE GEOLOGIC JIGSAW OF THE NORTH AMERICAN CRATON Before plate tectonics people knew of the term “geological province” Province was used to decribe the many distinct blocks of geology they discovered within the Canadian Shield Sir William Logan First to write of “geological provinces” - in 1860 Term is still used to describe areas of the shield Geological provinces are broad areas/regions of similar rock types that cover thousands of square kilometers and differ significantly from adjacent areas of different rock types Geological Survey of Canada (1840s) Systematic study of the Geology of Canada by professional geologists started with this A time when few topographic maps existed A need to better understand Canada's geological resources; mostly, coal First director: William Logan A lot of coal in Nova Scotia Logan's map of Geology of Canada published in 1869 (parts of Manitoba, Ontario & Quebec) and a major achievement Even today, much of the Canadian Shielf remains to be mapped in detail Plate tectonics and studies of sea floor provided key to origin of Canadian Shield Plate tectonics is responsible for bringing together the various geological provinces of Canada Uniformitarianism = using modern world processes to explain evolution of North American craton ➢ HOW DID THE NORTH AMERICAN CONTINENT EVOLVE? Stages in the Evolution Geological provinces (within craton) are rimmed by intensely deformed rocks that form ancient orogens In other worlds, geological provinces can be regarded as the remains of individual continents that collided Orogens consist of crushed and deformed rocks that represent the remainsof mountain belts or volcanic arcs formed during collision Wilson Cycle • Growth of North America has 5 stages = each characterized by collision of breakage from other lands • This process of repeated continental aggradation and breakup is known as the Wilson Cycle and has resulted in the development of supercontinents • This explains why present day continents have a broadly similar geologic history Early stages of Canada's geological history is preliminary = still more to learn. Strage 1 – Arctica: North America in the Archaen – AT LEAST 2.5 BILLION YEARS AGO Formation of the central part of the North American continent spans the entire Archaean Era Rocks found in Slave Province are thought to have been part of ancient continent called Arctica Artica included present-day Siberia The Slave Province • Focus of much mineral exploration activity especially diamonds in kimberlite popes Superior Province • Also formed part of Artica is important to Canadians because of it's mineral wealth • More than 50 % of Canada's entire annual gold production & 30% of zinc, copper & silver are mined from this province • Evidence of subprovinces in ancient rocks • Subprovinces • Composed of distinct rock types and reveal particular geological events • PLUTONIC SUBPROVINCE • GRANITE-GREENSTONE SUBPROVINCE • METASEDIMENTARY SUBPROVINCE Gowganda glaciation • The southern continental margin of Arctica was the site of this major glaciation • One of the oldest recorded on Earth • Famous for uranium deposits found near Elliot Lake Stage 2 – Nena and Rodinia: North America in the Proterozoic -1.8 BILLION Next stage spand entire Proterozoic Era (Precambrian) Nena • Southern part of Canadian Shield • Nena was added to Arctica to create larger land mass • Formed during Penokean Orogeny Penokean Orogeny • Records assembly of Nena in Ontario • Created a Himalayan type of mountain near Lake Huron • Destroyed by erosion, but roots remain Stage 3 – The Grenville Orogeny and Formation of Rodinia Grenville Orogeny: result of long-lived collision between South and North America 1 billion years ago Collision of South and North America = RODINIA (late Proterozoic) Rocks accreted and deformed during the orogeny underlie much of sourthern Ontario & Quebec, extending through the Maritimes and into Newfoundland Grenville Province/Orogeny is dominated by banded gneisses, highly metamorphosed sediments, and igneous rock Erosion reduced the mountains to a peneplain RODINIA BREAKS APART • Rodinia is unstable because large size prevents heat from escaping from the Earth's interor and promotes buildup of giant convecting “plumes” in the mantle • These plumes cause the land surface to rise in the form of a dome • Eventually continent tears or rifts apart • Breakup begins with a triple junctions with interlinked grabens that eventually grow and widen into a new ocean basin • Breakup started 750 million years ago and finished 570 mill years ago (during Cambrian) • First teared along western margin and millions of years later, it teared along eastern margin = Europe and Africa drifted off to form ancestral Atlantic Ocean = Iapetus Ocean • Evidence of ancient failed grabens = preserved in North America • Benefits of Iapetus Ocean • Warm water that provided nutrient-rich habitats for marine oganisms • Major stimulus to the proliferations of organisms evident