Misplaced Pages

#27972

40-647: The Gai-As Formation is correlated with a series of formations in the Pelotas and Paraná Basins in southeastern Brazil, deposited in a larger basinal area, 150 million years before the break-up of Pangea . The abundance of Glossopteris and Mesosaurus fossils in the underlying Huab Formation are characteristic of the Gondwanan correlation across present-day South America, Africa, Antarctica and Australia. The Gai-As Formation has provided fossil bivalves and an indeterminate stereospondylid . The Gai-As Formation

80-736: A different history due to the Polônio paleohigh until the Late Maastrichtian . The basinal sequence starts with the Imbituba Formation, a unit consisting of basaltic volcanic rocks. Parts of these rocks have been dated at ages between 125 ± 0.7 Ma and 118 ± 1.9 Ma, more recent than the flood basaltic Serra Geral Formation of the Paraná and Etendeka traps to the west in the Paraná Basin . The basalts are exposed in

120-707: A large river is enormous. It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Yellow River 796 million tons, and the Po River in Italy 67 million tons. The names of many rivers derive from the color that the transported matter gives the water. For example, the Yellow River (Huang He) in China is named after the hue of the sediment it carries, and

160-782: A sequence of rift basins bordering the present-day South Atlantic on the Brazilian and southwestern African sides. The Pelotas-Namibia spreading commenced in the Hauterivian , around 133 million years ago, and reached the Santos Basin to the north in the Barremian . Seafloor spreading continued northwards to the Campos Basin in the Early Albian , at approximately 112 Ma. Five tectonic stages have been identified in

200-746: A sharp contact. The basal beds of the formation are ubiquitously represented by fine-grained distal deposits and imply a much larger extent of the formation due to the absence of sediments from marginal environments. The lower part of the succession is formed by approximately 50 metres (160 ft) of thick reddish-violet, slightly calcareous shales . In the lower half of this mainly pelitic succession, intercalations of dark brown, very often concretionary calcareous layers up to 1 metre (3.3 ft) thick, occur. These layers contain siltstone beds which locally exhibit flaser bedding, oscillation ripples and hummocky cross-bedding. The calcareous layers also contain small fossiliferous lenses with fishbones. The upper half of

240-454: A short distance then settling again). If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as wash load . As there are generally a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions. Sediment motion can create self-organized structures such as ripples , dunes , or antidunes on

280-600: A thick layer of salt and the pre-salt layer pinches out just in the north of the Pelotas Basin stratigraphy. Within the Brazilian Atlantic margin, the Pelotas Basin is relatively underexplored. Twenty exploration wells have been drilled in the Brazilian portion of the basin with one ultra-deepwater exploration well drilled on the Uruguayan side in 2016. No hydrocarbon accumulations have been proven in

320-507: Is a mostly offshore sedimentary basin of approximately 346,000 square kilometres (134,000 sq mi) in the South Atlantic , administratively part of the southern states Santa Catarina and Rio Grande do Sul of Brazil and the departments Cerro Largo , Rocha and Treinta y Tres of Uruguay . The Pelotas Basin is one of the basins that formed on the present-day South Atlantic margins of South America and Africa due to

360-768: Is characteristic of a large-scale prolonged transgression that lasted until the Oligocene . As in other areas of the Brazilian continental margin, the Late Turonian is marked by a regional unconformity . The Late Cretaceous to recent sequence is subdivided into the proximal Cidreira Formation, related to the fluvial progradation of the Rio Grande, and a distal part known as the Imbé Formation, consisting of shales , intercalated with sporadic siltstones and fine-grained sandstones . The transgressive series

400-672: Is followed by the volcanics of the Curumim Formation , dated at 113 ± 0.1 Ma, and small incursions in the northern area of Pelotas Basin of the Ariri Formation , the evaporites that form the salt layer in the Santos Basin to the north. The rapid subsidence from the Albian to Turonian created a wide marine platform with in the north elevated areas due to the doming of the Curumim Formation. On this platform,

440-400: Is interrupted by the presence of various Paleogene hiatuses, that appear also in the regressive Miocene to recent beds. The unconformities have been analysed in detail using Sr/ Sr ratios . The Pelotas Basin is relatively underexplored. The Brazilian part of the basin had a total of twenty exploration wells drilled until 2017. The first eight wells were drilled in the onshore section in

SECTION 10

#1732802599028

480-606: Is named after the ruins of a colonial German police station located at a spring in the central Huab area. The formation is a lithological unit with an approximate maximum thickness of 70 metres (230 ft), deposited in the Huab Basin where it overlies the Huab Formation , separated by a significant hiatus, and is unconformably overlain by the Etjo Sandstone , while later authors assign a separate formation to

520-540: Is the most widespread depositional unit within the Huab Basin. The approximately 70 metres (230 ft) thick succession is easily distinguished from all the other units by its characteristic reddish to violet color. The Gai-As Formation gradationally overlies the lacustrine deposits of the Huab Formation in the western part of the basin, but in the east succeeds the fluvial deposits of the Huab Formation with

560-531: The Terraia altissima type are transitional to the sand-flat facies which is characterised by sandy, flat and lenticular deposits and occasionally contain abundant root marks. The lowermost part of the formation indicates a fluvio -lacustrine environment. The following fossils have been reported from the formation: The lower part of the formation is correlated with the Serra Alta Formation and

600-711: The Serra Geral National Park as a 700-metre-high (2,300 ft) cliff. The volcanic deposits form seaward dipping reflectors (SDR) in the seismic surveys of the basin. The conglomerates , siltstones and diamictites of the Cassino Formation represent the top of the Gondwana break-up unconformity, dates to the Middle Aptian . The depositional environment has been interpreted as fluvio - deltaic to lacustrine . The formation

640-469: The White Nile is named for the clay it carries. The main kinds of fluvial processes are: The major fluvial (river and stream) depositional environments include: Rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle (drag and lift forces), and the settling velocity of

680-418: The deposits and landforms created by sediments . It can result in the formation of ripples and dunes , in fractal -shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains and the occurrence of flash floods . Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity . In typical rivers

720-450: The river bed . The movement of water across the stream bed exerts a shear stress directly onto the bed. If the cohesive strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities of sediment , this material can act as tools to enhance wear of

760-443: The 1950s and 1960s. After acquiring seismic survey in the 1970s, seven more wells were drilled in the shallow offshore part. Five other offshore wells were drilled between 1995 and 2001. The first and hitherto only well drilled in the Uruguayan part of the basin was drilled in 2016. Hydrocarbon accumulations have yet to be discovered in Pelotas Basin. An exploration area of approximately 15,000 square kilometres (5,800 sq mi)

800-573: The Brazilian basins: While the basins to the north of Pelotas Basin, the Santos , Campos and Espírito Santo Basins , are characterised by a thick sequence of salt and an accompanying pre-salt layer , evaporites are almost absent in the Pelotas Basin. The sedimentary succession of the Pelotas Basin is underlain by an extremely deformed (highly stretched, thinned and faulted) continental crust , covered by grabens that can achieve thicknesses of more than 20 kilometres (66,000 ft). The nature of

840-490: The Gai-As Formation contain very well-preserved former glass shards, now mainly replaced by barite , are rich in potassium feldspar and have a porcelain-like appearance. The Gai-As Formation as a whole displays an upward coarsening sequence which starts with distal shallow lacustrine shales. The occurrence of flaser bedding, oscillation ripples and hummocky cross-stratification in silty intercalations indicates that

SECTION 20

#1732802599028

880-852: The Uruguayan departments Cerro Largo , Rocha and Treinta y Tres . About 40,900 square kilometres (15,800 sq mi) of the basin is onshore. The onshore part of the basin in Uruguay is locally called Merín Basin. The basin is bound by the Florianópolis High , separating it from the Santos Basin in the north and the Polônio High in the south, forming the border with the Punta del Este Basin . The coastal ranges Serras de Sudeste in Brazil and Cuchilla Grande in Uruguay form

920-833: The basal shaly section contains a few layers, 10 to 20 centimetres (3.9 to 7.9 in) thick, which are whitish and analcime -rich and occasionally thin layers of green chert . The basal beds grade into a 20 metres (66 ft) thick succession of silty to fine-grained, and eventually medium- to coarse grained, sandstone. Sections of the uppermost part show various types of intercalations: silt- and sandstone layers grade laterally into sequences of interbedded shale and sandstone with occasional intercalations of stromatolitic layers, carbonate horizons with molluscs , bone beds of fish scales and white layers rich in authigenic feldspar . The shale frequently exhibits desiccation cracks, whereas root marks are abundant in sandstone. Halite crystal moulds have been found locally. The white tuff layers within

960-433: The basin thus far. The basin is named after the city of Pelotas , the hometown of Rodi Ávila Medeiros, the geologist who studied the area. The Pelotas Basin is an approximately 346,000-square-kilometre-large (134,000 sq mi), mostly offshore sedimentary basin, located in the South Atlantic offshore Brazil and Uruguay . It covers the southern Brazilian states of Santa Catarina and Rio Grande do Sul and

1000-421: The bed ( abrasion ). At the same time the fragments themselves are ground down, becoming smaller and more rounded ( attrition ). Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material. For each grain size there is a specific flow velocity at which

1040-470: The break-up of Gondwana in the Early Cretaceous . The sedimentary succession started as the other Brazilian marginal basins with a series of basalts, younger than the Paraná and Etendeka traps exposed in the Paraná Basin to the west, followed by shallow to deeper marine carbonate and clastic sediments. Other than the northern neighbours Santos and Campos Basins , the Pelotas Basin lacks

1080-759: The formation is time-equivalent with the Collingham , Ripon and Fort Brown Formations . The fossil assemblages of Glossopteris and Mesosaurus that occur in the underlying Huab Formation are known from other parts of Gondwana ; the Vryheid Formation of South Africa and coal deposits of the Lower Permian in Australia. Pelotas Basin The Pelotas Basin ( Portuguese : Bacia de Pelotas , Spanish : Cuenca de Pelotas )

1120-543: The grains start to move, called entrainment velocity . However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the Hjulström curve . A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where

1160-420: The largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders . When the stream or rivers are associated with glaciers , ice sheets , or ice caps , the term glaciofluvial or fluvioglacial is used, as in periglacial flows and glacial lake outburst floods . Fluvial sediment processes include the motion of sediment and erosion or deposition on

1200-651: The lower crust below the Pelotas Basin remains uncertain, but by analogy with the Namibian conjugate margin, it may correspond to a high-density igneous crust-mantle interface intruded by the Tristan da Cunha plume . The central onshore boundary of the basin is formed by the Neoproterozoic Pelotas Batholith . The stratigraphy of the Pelotas Basin and the Punta del Este Basin to the south had

1240-564: The marine limestones and siliciclastics of the Porto Belo Formation were deposited. Shales comprise the outer marine deposition of this stratigraphic unit, considered as a potential source rock . Overlying the Porto Belo Formation are the proximal Tramandaí and the distal Atlántida Formations , composed of coarse-grained fluvial to alluvial clastics, intercalated with siltstones and shales. This phase

Gai-As Formation - Misplaced Pages Continue

1280-572: The particle. These relationships are shown in the following table for the Rouse number , which is a ratio of sediment settling velocity (fall velocity) to upwards velocity. Rouse = Settling velocity Upwards velocity from lift and drag = w s κ u ∗ {\displaystyle {\textbf {Rouse}}={\frac {\text{Settling velocity}}{\text{Upwards velocity from lift and drag}}}={\frac {w_{s}}{\kappa u_{*}}}} where If

1320-467: The river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium . Even small streams make alluvial deposits, but it is in floodplains and deltas of large rivers that large, geologically-significant alluvial deposits are found. The amount of matter carried by

1360-658: The section. The overlying succession of sandstones, siltstones, stromatolites and emerged horizons reflects the upward shallowing of the formation. Besides extensive sand flats, large playas, several square kilometres in size, must have existed during the final stages of deposition. The playa deposits are represented by shaly sediments with intercalated feldspar-rich layers (2 to 4 centimetres (0.79 to 1.57 in)), stromatolite horizons (5 to 15 centimetres (2.0 to 5.9 in)) and fish scale bone beds (3 to 5 centimetres (1.2 to 2.0 in)). Fossiliferous stratiform carbonates (10 to 15 centimetres (3.9 to 5.9 in) thick) with molluscs of

1400-420: The sediment, surface must have been above the storm wave base. The remarkably high content of analcime (30 volume percent) in some layers may be indicative of contemporaneous volcanic activity. Early diagenetic formation of analcime from volcanic glass in an alkaline environment has been described. High alkalinity of the lake water may also have been the reason for the almost complete lack of fossils in this part of

1440-422: The upper 35 metres (115 ft) of the Gai-As Formation, named Doros Formation . The formation comprises sandstones , siltstones and mudstones deposited in a lacustrine environment . U/Pb SHRIMP dating of zircons from two of several fallout tuff beds in the Gai-As Formation 14 metres (46 ft) above the stereospondyl find gave weighted mean values of 272 ± 1.8 Ma and 265 ± 2.5 Ma. The Gai-As Formation

1480-668: The upper part with the Teresina Formation of the Paraná and Pelotas Basins in Rio Grande do Sul , Brazil . Due to increased subsidence in the central Paraná Basin, thicknesses of the correlative units are considerably greater than that of the Gai-As Formation, attaining a maximum of 1,200 metres (3,900 ft). In the Karoo Basin of southern Namibia and the Eastern , Northern and Western Cape , South Africa ,

1520-404: The upwards velocity is approximately equal to the settling velocity, sediment will be transported downstream entirely as suspended load . If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (see Initiation of motion ), it will move along the bed as bed load by rolling, sliding, and saltating (jumping up into the flow, being transported

1560-656: The western boundary, underlain by the Sierra Ballena Shear Zone . The onshore part of the basin is from north to south crossed by the Urussanga , Araranguá , Mampituba , Jacuí , Guaíba , Camaquã , Jaguarão/Yaguarón and Cebollatí Rivers . The basins of the South Atlantic margin started forming in the Early Cretaceous with the break-up of Gondwana , the southern part of the former supercontinent Pangea . This tectonic movement resulted in

1600-673: Was offered in the Brazil Bidding Round of 2017.       De Santa Ana, Héctor; Goso Aguilar, César; Montaño, Jorge; Piñeiro, Graciela; Muzio, Rossana; Rossello, Eduardo; Perea, Daniel; Ucha, Nelson (2004). Cuencas sedimentarias de Uruguay - geología, paleontología y recursos naturales - Mesozóico (PDF) . División Relaciones y Actividades Culturales de Facultad de Ciencias . pp. 1–219 . Retrieved 2017-09-11 . Fluvial In geography and geology , fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and

#27972