The Purcell Supergroup is composed primarily of argillites , carbonate rocks , quartzites , and mafic igneous rocks of late Precambrian ( Mesoproterozoic ) age. It is present in an area of about 15,000 km (5,800 sq mi) in southwestern Alberta and southeastern British Columbia , Canada , and it extends into the northwestern United States where it is called the Belt Supergroup . It was named for the Purcell Mountains of British Columbia by R.A. Daly in 1912. Fossil stromatolites and algal structures are common in some of the Purcell Supergroup rocks, and the Sullivan ore body at Kimberley, British Columbia , a world-class deposit of lead , zinc , and silver , lies within the Alderidge Formation in the lower part of the Purcell.
31-578: Spectacular outcrops of Purcell and Belt Supergroup rocks can be seen in Glacier National Park in northwestern Montana and Waterton Lakes National Park in southwestern Alberta. The Purcell Supergroup consists primarily of argillites, carbonate rocks ( limestone and dolomite ), and quartzites, and includes localized occurrences of igneous rocks ( mafic lava flows, tuffs , pillow basalts , and gabbroic and dioritic sills and dykes ). Sedimentary structures are well preserved in
62-533: A supercontinent called Columbia/Nuna . Deposition occurred during the Mesoproterozoic era , much of it probably between about 1470 and 1400 Ma (million years) ago. In Canada, the Purcell Supergroup is present in an area of about 15,000 km (5,800 sq mi) that reaches from the southern Purcell Mountains in southeastern British Columbia to the southern Canadian Rockies in
93-791: A water gap may occur. In these, erosion from a stream occurs faster than mountain uplift, resulting in a gorge or valley that runs through a mountain range from low-lying country on one side to similar country on the other. Examples of such water gaps include the Manawatū Gorge in New Zealand and the Cumberland Narrows in Maryland . The removal of mass from a region will be isostatically compensated by crustal rebound. If we take into consideration typical crustal and mantle densities, erosion of an average 100 meters of rock across
124-527: A broad, uniform surface will cause the crust to isostatically rebound about 85 meters and will cause only a 15-meter loss of mean surface elevation. An example of isostatic uplift is post-glacial rebound following the melting of ice sheets . The Hudson Bay region of Canada, the Great Lakes of Canada and the United States, and Fennoscandia are currently undergoing gradual rebound as a result of
155-661: A continuous line along the top edge with lines protruding down; outcrops have a continuous line around each area of bare rock. An outcrop example in California is the Vasquez Rocks , familiar from location shooting use in many films , composed of uplifted sandstone . Yana is another example of outcrops, located in Uttara Kannada district in Karnataka , India . Tectonic uplift Tectonic uplift
186-508: A more modest uplift over a large region. Perhaps the most extreme form of orogenic uplift is a continental-continental crustal collision. In this process, two continents are sutured together, and large mountain ranges are produced. The collision of the Indian and Eurasian plates is a good example of the extent to which orogenic uplift can reach. Heavy thrust faulting (of the Indian plate beneath
217-546: A record of relative changes within geologic strata . Accurate description, mapping, and sampling for laboratory analysis of outcrops made possible all of the geologic sciences and the development of fundamental geologic laws such as the law of superposition , the principle of original horizontality , principle of lateral continuity , and the principle of faunal succession . On Ordnance Survey maps in Great Britain , cliffs are distinguished from outcrops: cliffs have
248-463: A relatively small role, and high peak formation can be more attributed to tectonic processes. Direct measures of the elevation change of the land surface can only be used to estimate erosion or bedrock uplift rates when other controls (such as changes in mean surface elevation, volume of eroded material, timescales and lags of isostatic response, variations in crustal density) are known. In a few cases, tectonic uplift can be seen in coral islands . This
279-687: Is thrust onto continental crust. Basically nappes (thrust sheets) from each plate collide and begin to stack one on top of the other; evidence of this process can be seen in preserved ophiolitic nappes (preserved in the Himalayas ) and in rocks with an inverted metamorphic gradient . The preserved inverted metamorphic gradient indicates that nappes were actually stacked on top of each other so quickly that hot rocks did not have time to equilibrate before being thrust on top of cool rocks. The process of nappe stacking can only continue for so long, as gravity will eventually disallow further vertical growth (there
310-482: Is an upper limit to vertical mountain growth). Although the raised surfaces of mountain ranges mainly result from crustal thickening, there are other forces at play that are responsible for the tectonic activity. All tectonic processes are driven by gravitational force when density differences are present. A good example of this would be the large-scale circulation of the Earth's mantle . Lateral density variations near
341-679: Is evidenced by the presence of various oceanic islands composed entirely of coral , which otherwise appear to be volcanic islands . Examples of such islands are found in the Pacific, notably the three phosphate islets of Nauru , Makatea , and Banaba , as well as Maré and Lifou in New Caledonia ; Fatu Huku in the Marquesas Islands ; and Henderson Island in the Pitcairn Islands . The uplift of these islands
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#1732788078096372-464: Is hosted in the lower part of the Purcell Supergroup. During the life of the mine, the Sullivan ore body is reported to have yielded 8,412,077 tons of lead, 7,944,446 tons of zinc, and 9,264 tons of silver, as well as significant quantities of tin and other metals. Outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of
403-452: Is known to be sensitive to temperature and rainfall. The magnitude of the exhumation a rock has been subjected to may be inferred from geothermobarometry (measuring previous pressure and temperature history of a rock or assemblage). Knowing the pressure and temperature history of a region can yield an estimate of the ambient geothermal gradient and bounds on the exhumation process; however, geobarometric/geothermometric studies do not produce
434-404: Is proportional to the rate of increase of average surface height. The highest rates of working against gravity are required when the thickness of the crust (not the lithosphere) changes. Lithospheric flexure is the process by which the lithosphere bends under the action of forces such as the weight of a growing orogeny or changes in ice thickness related to glaciation. The lithosphere rests on
465-402: Is rapid and exceeds the weathering rate such as on steep hillsides, mountain ridges and tops, river banks, and tectonically active areas. In Finland , glacial erosion during the last glacial maximum (ca. 11000 BC), followed by scouring by sea waves, followed by isostatic uplift has produced many smooth coastal and littoral outcrops. Bedrock and superficial deposits may also be exposed at
496-450: Is specified. Molnar and England identify three kinds of displacement to which the term "uplift" is applied: This simple equation relates the three kinds of displacement: The term geoid is used above to mean mean sea level and makes a good frame of reference. A given displacement within this frame of reference allows one to quantify the amount of work being done against gravity. Measuring uplift and exhumation can be tricky. Measuring
527-490: Is the geologic uplift of Earth's surface that is attributed to plate tectonics . While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening (such as mountain building events), changes in the density distribution of the crust and underlying mantle , and flexural support due to the bending of rigid lithosphere . Tectonic uplift results in denudation (processes that wear away
558-409: Is the result of the movement of oceanic tectonic plates. Sunken islands or guyots with their coral reefs are the result of crustal subsidence as the oceanic plate carries the islands to deeper or lower oceanic crust areas. The word "uplift" refers to displacement contrary to the direction of the gravity vector, and displacement is only defined when the object being displaced and the frame of reference
589-487: The asthenosphere , a viscous layer that in geological time scales behaves like a fluid. Thus, when loaded, the lithosphere progressively reaches an isostatic equilibrium. For example, the lithosphere on the oceanward side of an oceanic trench at a subduction zone will curve upwards due to the elastic properties of the Earth's crust. Orogenic uplift is the result of tectonic-plate collisions and results in mountain ranges or
620-446: The Earth and other terrestrial planets . Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by soil and vegetation and cannot be seen or examined closely. However, in places where the overlying cover is removed through erosion or tectonic uplift , the rock may be exposed, or crop out . Such exposure will happen most frequently in areas where erosion
651-423: The Earth's surface due to human excavations such as quarrying and building of transport routes. Outcrops allow direct observation and sampling of the bedrock in situ for geologic analysis and creating geologic maps . In situ measurements are critical for proper analysis of geological history and outcrops are therefore extremely important for understanding the geologic time scale of earth history. Some of
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#1732788078096682-524: The Eurasian plate) and folding are responsible for the suturing together of the two plates. The collision of the Indian and Eurasian plates produced the Himalayas and is also responsible for crustal thickening north into Siberia . The Pamir Mountains , Tian Shan , Altai , Hindu Kush , and other mountain belts are all examples of mountain ranges formed in response to the collision of the Indian with
713-815: The Eurasian plate. The Ozark Plateau is a broad uplifted area which resulted from the Permian Ouachita Orogeny to the south in the states of Arkansas , Oklahoma , and Texas . Another related uplift is the Llano Uplift in Texas, a geographical location named after its uplift features. The Colorado Plateau which includes the Grand Canyon is the result of broad tectonic uplift followed by river erosion . When mountains rise slowly, either due to orogenic uplift or other processes (e.g., rebound after glaciation ), an unusual feature known as
744-744: The Purcell is not exposed in Canada, but it is inferred to rest unconformably on the Canadian Shield . The Purcell is unconformably overlain by the Neoproterozoic Windermere Supergroup in most areas, or by younger Cambrian or Devonian formations where the Windermere is absent. The now-closed Sullivan Mine at Kimberley, British Columbia, worked a world-class sedimentary exhalative (SedEx) deposit that
775-544: The Purcell rocks despite their great age. In the southern Canadian Rockies (Waterton Park area), the supergroup is subdivided as follows: In the southern Purcell Mountains ( Cranbrook area ), the supergroup is subdivided as follows: The Purcell Supergroup was probably deposited in subsiding deltaic to marine environments along the margin of the North American craton , possibly in an intracratonic basin where North America and another landmass were joined in
806-491: The earth's surface) by raising buried rocks closer to the surface. This process can redistribute large loads from an elevated region to a topographically lower area as well – thus promoting an isostatic response in the region of denudation (which can cause local bedrock uplift). The timing, magnitude, and rate of denudation can be estimated by geologists using pressure-temperature studies. Crustal thickening has an upward component of motion and often occurs when continental crust
837-451: The melting of ice sheets 10,000 years ago. Crustal thickening, which for example is currently occurring in the Himalayas due to the continental collision between the Indian and the Eurasian plates, can also lead to surface uplift; but due to the isostatic sinking of thickened crust, the magnitude of surface uplift will only be about one-sixth of the amount of crustal thickening. Therefore, in most convergent boundaries , isostatic uplift plays
868-540: The southwestern Alberta. It extends southward into the United States (western Montana , northern Idaho , northwestern Washington , and western Wyoming ) where it is called the Belt Supergroup . It reaches a maximum thickness of more than 10,000 metres (33,000 feet) in the Purcell Mountains. The Purcell Supergroup is equivalent to the Belt Supergroup of the northwestern United States. The base of
899-417: The surface (such as the creation, cooling, and subduction of oceanic plates ) also drive plate motion. The dynamics of mountain ranges are governed by differences in the gravitational energy of entire columns of the lithosphere (see isostasy ). If a change in surface height represents an isostatically compensated change in crustal thickness, the rate of change of potential energy per unit surface area
930-468: The types of information that cannot be obtained except from bedrock outcrops or by precise drilling and coring operations, are structural geology features orientations (e.g. bedding planes, fold axes, foliation ), depositional features orientations (e.g. paleo-current directions, grading, facies changes), paleomagnetic orientations. Outcrops are also very important for understanding fossil assemblages, and paleo-environment, and evolution as they provide
961-690: The uplift of a point requires measuring its elevation change – usually geoscientists are not trying to determine the uplift of a singular point but rather the uplift over a specified area. Accordingly, the change in elevation of all points on the surface of that area must be measured, and the rate of erosion must be zero or minimal. Also, sequences of rocks deposited during that uplift must be preserved. Needless to say, in mountain ranges where elevations are far above sea level these criteria are not easily met. Paleoclimatic restorations though can be valuable; these studies involve inferring changes in climate in an area of interest from changes with time of flora/fauna that