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45-594: The formation forms the basement in the Middle Magdalena Valley and is forming the Serranía de Los Cobardes . The Girón Formation consists of thickly bedded red sandstones , shales , siltstones and conglomerates deposited in a terrestrial alluvial and fluvial to fluvio- lacustrine environment in an extensional basin setting. The formation dates to the Late Jurassic period and has

90-626: A deltaic environment. Towards the top of the stratigraphic unit, the deposition became shallow marine. The overall tectonic regime represented an extensional basin setting. The rifting of the Jurassic and Early Cretaceous, resulting from the break-up of Pangea , caused large variations in thickness of the Girón Formation. The grains composing the Girón Formation have been analyzed using X-ray diffraction , resulting in quartz as principal component (73 to 97%), and clay minerals belonging to

135-407: A terrane was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form underplating , where the new igneous rock forms a layer on the underside of the crust. The majority of continental crust on the planet is around 1 to 3 billion years old, and it is theorised that there

180-459: A weak zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced. In Andean geology the basement refers to the Proterozoic , Paleozoic and early Mesozoic ( Triassic to Jurassic ) rock units as the basement to the late Mesozoic and Cenozoic Andean sequences developed following

225-425: A feature with a dip of 45° and a dip direction of 75°, the strike and dip can be written as 345/45 NE, 165/45 NE, or 075,45. The compass quadrant direction for the strike can also be used in place of the azimuth, written as S15E or N15W. Strike and dip are measured in the field using a compass and with a clinometer . A compass is used to measure the azimuth of the strike, and the clinometer measures inclination of

270-412: A few conventions geologists use when measuring a feature's azimuth. When using the strike, two directions can be measured at 180° apart, at either clockwise or counterclockwise of north. One common convention is to use the "right-hand rule" (RHR) where the plane dips down towards the right when facing the strike direction, or that the dip direction should be 90° clockwise of the strike direction. However, in

315-509: A maximum thickness of 3,346 metres (10,978 ft) at its type locality in the valley of the Lebrija River . The formation was first defined by Hettner in 1892 and named after Girón , close to Bucaramanga , Santander . The type locality was defined by Langenheim in 1954 in the valley of the Lebrija River . The formation was studied by various authors in the 1940s, Oppenheim in 1940, Dickey in 1941 and Trumpy in 1943, who described

360-613: A maximum thickness of 3,346 metres (10,978 ft) at the type locality. On the Cáchira Páramo in Norte de Santander, the thickness is 1,140 metres (3,740 ft). The Girón Formation extends over a distance of about 325 kilometres (202 mi) from Teorama in Norte de Santander in the north to Nobsa in Boyacá in the south, stretching across three departments ; Norte de Santander , Santander and Boyacá . The Girón Formation

405-470: A number (between 0° and 90°) indicating the angle in degrees below horizontal. It can be accompanied with the rough direction of dip (N, SE, etc) to avoid ambiguity. The direction can sometimes be omitted, as long as the convention used (such as right-hand rule) is known. A feature that is completely flat will have the same dip value over the entire surface. The dip of a curved feature, such as an anticline or syncline , will change at different points along

450-473: A plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it causes an upwelling of magma that can cause volcanism along the subduction front on the overriding plate. This produces an oceanic volcanic arc , like Japan . This volcanism causes metamorphism , introduces igneous intrusions , and thickens the crust by depositing additional layers of extrusive igneous rock from volcanoes. This tends to make

495-583: A set symmetrical with the general anticline at NNE-SSW and ESE-WNW. Pervasive cleavage exists in the formation at the Floresta Massif. Magnetization analysis performed in 2005 by Ayala et al. showed clockwise rotation of the formation in the southern area around Paz de Río of 31 ± 18° and Nobsa of 40 ± 18° respectively. The Girón Formation is put into contact with the Cretaceous La Luna Formation in Norte de Santander, and

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540-423: A single three-digit number in terms of the angle from true north (for example, N25°E would simply become 025 or 025°). A feature's orientation can also be represented by its dip direction. Rather than the azimuth of a horizontal line on the plane, the azimuth of the steepest line on the plane is used. The direction of dip can be visualized as the direction water would flow if poured onto a plane. While true dip

585-494: Is characterized by a thick sequence of red feldspathic and micaceaous sandstones and thinly bedded reddish siltstones , conglomerates with quartz and lithic clasts, shales with white spots and purple to dark red claystones . In the south, the formation includes fragments of schists , quartzites , and red and green limonite . The middle part of the sequence contains alternating greyish green sandstones and red to purple siltstones in beds up to 1.5 metres (4.9 ft). In

630-494: Is contrasted to overlying sedimentary rocks which are laid down on top of the basement rocks after the continent was formed, such as sandstone and limestone . The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 5 kilometres (3 mi) thick. The basement rock of the crust can be 32–48 kilometres (20–30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at

675-815: Is measured perpendicular to the strike, apparent dip refers to an observed dip which is not perpendicular to the strike line. This can be seen in outcroppings or cross-sections which do not run parallel to the dip direction. Apparent dip is always shallower than the true dip. If the strike is known, the apparent dip or true dip can be calculated using trigonometry: α = arctan ⁡ ( sin ⁡ β × tan ⁡ δ ) {\displaystyle \alpha =\arctan(\sin \beta \times \tan \delta )} δ = arctan ⁡ ( tan ⁡ α ÷ sin ⁡ β ) {\displaystyle \delta =\arctan(\tan \alpha \div \sin \beta )} where δ

720-726: Is present in the hanging wall of the Bituima-La Salina Fault and cross-cut by several faults, among which the Suárez , Cáchira , and the regional Bucaramanga-Santa Marta Fault . The Girón Formation is found, apart from its type locality in the Lebrija River valley in Santander , stretching from the north around Teorama , Norte de Santander, to the south west of Nobsa in Boyacá . The formation crops out along

765-404: Is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination (or depression angle ) measured downward from horizontal. They are used together to measure and document a structure's characteristics for study or for use on a geologic map . A feature's orientation can also be represented by dip and dip direction , using the azimuth of the dip rather than

810-401: Is the true dip, α is the apparent dip, and β is the angle between the strike direction and the apparent dip direction, all in degrees. The measurement of a linear feature's orientation is similar to strike and dip, though the terminology differs because "strike" and "dip" are reserved for planes. Linear features use trend and plunge instead. Plunge, or angle of plunge, is the inclination of

855-773: The Colombia-Venezuela border in Toledo , Norte de Santander, and the urban center of Hacarí is built on top of the Girón Formation. The formation forms the Serranía de Los Cobardes in the western front of the Eastern Ranges and is found throughout the Chicamocha Canyon , and the Sogamoso River cuts through the Girón Formation. The Girón Formation also crops out along the highway from Bucaramanga to Piedecuesta where frequent landslides of

900-500: The Middle Magdalena Valley (VMM), the Girón Formation has been drilled and proved to comprise rhyolitic , rhyodacitic flows and tuffs . To the southeast of Cáchira , Norte de Santander, andesitic porphyry copper deposits in mineralizations of chalcopyrite and bornite occur in the Girón Formation. The Girón Formation in large areas of its extent overlies the Precambrian Bucaramanga Gneiss , in

945-866: The kaolinite group (2 to 4%) and illite - mica (3-19%). It has been suggested a main component of the provenance for the sediments was the Devonian Floresta Formation . The Girón Formation probably deposited as a result of regional uplift of the Santander and Floresta Massifs in the Late Jurassic. The formation is folded in an anticline around the type locality producing the Los Cobardes Anticlinal that extends to Contratación . Transpressive tectonic movements after deposition produced five sets of fractures; an asymmetrical group of NW-SE, NW-SE, and E-W fractures and

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990-442: The (typically Precambrian ) crystalline basement is not of interest as it rarely contains petroleum or natural gas . The term economic basement is also used to describe the deeper parts of a cover sequence that are of no economic interest. Strike and dip In geology , strike and dip is a measurement convention used to describe the plane orientation or attitude of a planar geologic feature . A feature's strike

1035-547: The Earth's continents being accreted into one giant supercontinent . Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents. In European geology , the basement generally refers to rocks older than the Variscan orogeny . On top of this older basement Permian evaporites and Mesozoic limestones were deposited. The evaporites formed

1080-746: The Mesa de Los Santos and the Bucaramanga Massif, the formation overlies the Jordán Formation with an angular unconformity dipping 10 to 15°. In the central part it is overlain by the Rosablanca Formation . In the southernmost extent of the formation, along the road from Busbanzá to Corrales , the Girón Formation unconformably rests upon the Floresta Formation . The formation forms the economic basement in

1125-885: The Middle Magdalena Valley. The age has been estimated to be Late Jurassic , ranging from Oxfordian to Tithonian , based on ostracods and palynomorphs . Stratigraphically, the formation is time equivalent with the Buena Vista Breccia of the proximal area of the Llanos Basin and the Arcabuco Formation to the west of the Boyacá Fault . The formation was deposited in an alluvial to fluvial or fluvio- lacustrine terrestrial environment , with fast-flowing braided rivers and in certain parts meandering river deposition to

1170-562: The UK, the right-hand rule has sometimes been specified so that the dip direction is instead counterclockwise from the strike. Some geologists prefer to use whichever strike direction is less than 180°. Others prefer to use the "dip-direction, dip" (DDD) convention instead of using the strike direction. Strike and dip are generally written as 'strike/dip' or 'dip direction,dip', with the degree symbol typically omitted. The general alphabetical dip direction (N, SE, etc) can be added to reduce ambiguity. For

1215-745: The ages is visible as well. The basement rocks of the continental crust tend to be much older than the oceanic crust. The oceanic crust can be from 0–340 million years in age, with an average age of 64 million years. Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at subduction and oceanic rifting areas. The basement rocks are often highly metamorphosed and complex, and are usually crystalline . They may consist of many different types of rock – volcanic, intrusive igneous and metamorphic. They may also contain ophiolites , which are fragments of oceanic crust that became wedged between plates when

1260-407: The attitude of an inclined feature, two quantities are needed. The angle the slope descends, or dip, and the direction of descent, which can be represented by strike or dip direction. Dip is the inclination of a given feature, and is measured from the steepest angle of descent of a tilted bed or feature relative to a horizontal plane. True dip is always perpendicular to the strike. It is written as

1305-416: The crust thicker and less dense, making it immune to subduction. Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcanic arc close to a continent, with which it may collide. When this happens, instead of being subducted, it is accreted to the edge of the continent and becomes part of it. Thin strips or fragments of

1350-407: The dip line on both sides of the strike, and horizontal bedding is denoted by a cross within a circle. Interpretation of strike and dip is a part of creating a cross-section of an area. Strike and dip information recorded on a map can be used to reconstruct various structures, determine the orientation of subsurface features, or detect the presence of anticline or syncline folds. There are

1395-549: The dip. Dr. E. Clar first described the modern compass-clinometer in 1954, and some continue to be referred to as Clar compasses. Compasses in use today include the Brunton compass and the Silva compass . Smartphone apps which can make strike and dip measurements are also available, including apps such as GeoTools . These apps can make use of the phone's internal accelerometer to provide orientation measurements. Combined with

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1440-511: The edge of the continent. There are exceptions, however, such as exotic terranes . Exotic terranes are pieces of other continents that have broken off from their original parent continent and have become accreted to a different continent. Continents can consist of several continental cratons – blocks of crust built around an initial original core of continents – that gradually grew and expanded as additional newly created terranes were added to their edges. For instance, Pangea consisted of most of

1485-422: The feature and be flat on any fold axis . Strike is a representation of the orientation of a tilted feature. The strike line of a bed , fault, or other planar feature, is a line representing the intersection of that feature with a horizontal plane. The strike of the feature is the azimuth (compass direction) of the strike line. This can be represented by either a quadrant compass bearing (such as N25°E), or as

1530-407: The feature measured downward relative to horizontal. Trend is the feature's azimuth, measured in the direction of plunge. A horizontal line would have a plunge of 0°, and a vertical line would have a plunge of 90°. A linear feature which lies within a plane can also be measured by its rake (or pitch). Unlike plunge, which is the feature's azimuth, the rake is the angle measured within the plane from

1575-530: The formation as a lateral equivalent of the La Quinta Formation of northern Colombia and western Venezuela . Julivert (1958) assigned a thickness of 2,500 metres (8,200 ft), while Navas in 1963 observed a thickness of 2,690 metres (8,830 ft). Cediel in the same year defined the thickness at its type locality of 4,650 metres (15,260 ft) and divided the formation in seven units. Research by Osorio and Velandia, published in 2015, concluded

1620-514: The fractured rocks occur in the rainy season. Basement (geology) In geology , basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The basement rocks lie below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In

1665-732: The northern area the Arboleda Granite , and the Carboniferous to Permian Diamante Formation in the area around Toledo , Norte de Santander . In this part, the formation is overlain by the Río Negro and Tibú-Mercedes Formations . In the central area at the type locality in the Lebrija River valley, the formation unconformably overlies the Bocas Formation and is overlain by the Tambor Formation . On

1710-738: The onset of subduction along the western margin of the South American Plate . When discussing the Trans-Mexican Volcanic Belt of Mexico the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero terranes respectively. The term basement is used mostly in disciplines of geology like basin geology , sedimentology and petroleum geology in which

1755-426: The same way, the sediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover". Crustal rocks are modified several times before they become basement, and these transitions alter their composition. Basement rock is the thick foundation of ancient, and oldest, metamorphic and igneous rock that forms the crust of continents , often in the form of granite . Basement rock

1800-540: The strike line. On geologic maps , strike and dip can be represented by a T symbol with a number next to it. The longer line represents strike, and is in the same orientation as the strike angle. Dip is represented by the shorter line, which is perpendicular to the strike line in the downhill direction. The number gives the dip angle, in degrees, below horizontal, and often does not have the degree symbol. Vertical and horizontal features are not marked with numbers, and instead use their own symbols. Beds dipping vertically have

1845-418: The strike value. Linear features are similarly measured with trend and plunge , where "trend" is analogous to dip direction and "plunge" is the dip angle. Strike and dip are measured using a compass and a clinometer . A compass is used to measure the feature's strike by holding the compass horizontally against the feature. A clinometer measures the feature's dip by recording the inclination perpendicular to

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1890-520: The strike. These can be done separately, or together using a tool such as a Brunton transit or a Silva compass . Any planar feature can be described by strike and dip, including sedimentary bedding , fractures , faults , joints , cuestas , igneous dikes and sills , metamorphic foliation and fabric , etc. Observations about a structure's orientation can lead to inferences about certain parts of an area's history, such as movement, deformation, or tectonic activity . When measuring or describing

1935-618: The surface. Basement rock is visible, for example, at the bottom of the Grand Canyon , consisting of 1.7- to 2-billion-year-old granite ( Zoroaster Granite ) and schist ( Vishnu Schist ). The Vishnu Schist is believed to be highly metamorphosed igneous rocks and shale , from basalt , mud and clay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through

1980-414: The underthrusting oceanic plate may also remain attached to the edge of the continent so that they are wedged and tilted between the converging plates, creating ophiolites . In this manner, continents can grow over time as new terranes are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages. As such, the basement rock can become younger going closer to

2025-572: Was at least one period of rapid expansion and accretion to the continents during the Precambrian. Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted into continental crust . It is possible for oceanic crust to be subducted down into the Earth's mantle , at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust. When

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