Menarik juga yah, turbidity current di meandering river, nanti kalau sempat saya ingin juga mencari referensi turbidit jenis ini (kalau sudah ada yang punya mohon kesediaannya untuk berbagi).
Sementara itu saya "copy & paste"-kan catatan dari Britannica Online tentang arus turbid ini di bawah. Salam Minarwan ++++++ Density currents in the oceans > Turbidity currents Density currents caused by suspended sediment concentrations in the oceans are called turbidity currents. They appear to be relatively short-lived, transient phenomena that occur at great depths. Turbidity currents are thought to be caused by the slumping of sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons (see below Continental margins: Submarine canyons). Slumping of large masses of sediment creates a dense sediment-water mixture, or slurry, which then flows down the canyon to spread out over the ocean floor and deposit a layer of sand in deep water. Repeated deposition forms submarine fans, which are analogous to the alluvial fans found at the mouths of many river canyons. Sedimentary rocks that are thought to have originated from ancient turbidity currents are called turbidites. Although large-scale turbidity underflows have never been directly observed in the oceans, there is much evidence supporting their occurrence. This evidence may be briefly summarized: (1) Telegraph cables have been broken in the deep ocean in a sequence that indicates some disturbance at the bottom moving from shallow to deep water at speeds on the order of 20 to 75 kilometres per hour, or 10 to 40 knots. The trigger for this phenomenon is commonly, though not exclusively, an earthquake near the edge of the continental slope. The only disturbance that seems capable of being transmitted downslope at the required speed is a large turbidity current. The best-known example of such a series of cable breaks took place in the North Atlantic following the 1929 earthquake under the Grand Banks of Newfoundland, but other examples have been described from the Magdalena River delta (Colombia), the Congo delta, the Mediterranean Sea north of Orléansville and south of the Straits of Messina, and Kandavu Passage, Fiji. (2) Cores taken from the ocean bottom in the area downslope from cable breaks reveal layers of sand interbedded with normal deep-sea pelagic or hemipelagic oozes (sediments formed in the deep sea by quiet settling of fine particles). In the case of the cable breaks south of the Grand Banks, a large-diameter core taken from the axis of a submarine canyon in the continental slope contained 1 centimetre of gray clay underlain by at least 20 centimetres of gray pebble and cobble gravel. Cores farther south showed a graded layer about one metre thick of coarse silt and fine sand. The presence of these gravel and sand layers is consistent with the hypothesis that they were deposited by the turbidity current that broke the cables. (3) Coring has revealed layers of fine-grained sand or coarse silt at many other localities in the abyssal plains of the oceans. These layers are generally moderately well sorted and contain microfossils characteristic of shallow water that are also size-sorted. In some cases the layers are laminated and arranged in a definite sequence. It is clear that the sand forming these layers has been moved down from shallow water, and in many cases the only plausible mechanism appears to be a turbidity current. (4) At the base of many submarine canyons there occur very large submarine fans. Deep-sea channels on the fan surfaces extend for many tens of kilometres and have depths of more than 100 metres and widths of one kilometre or more. Submarine levees are a prominent feature, and these project above the surrounding fan surface to elevations of 50 metres or higher. The gross characteristics of such channels suggest that they were formed by a combination of erosion and deposition by turbidity currents. (5) Thick deposits of interbedded graded sandstones and fine-grained shales are common in the geologic record. In some cases there is good fossil evidence that the shales were deposited in relatively deep water, perhaps as much as several thousand metres deep. Relatively deepwater deposition is also suggested by the absence of sedimentary structures characteristic of shallow water. The interbedded sandstones, however, contain shallow-water fossils that are sorted by size, have a sharp basal contact with the shale below and a transitional contact with the shale above, and display a characteristic sequence of sedimentary structures. The structures include erosional marks made originally on the mud surface but now preserved as casts on the base of the sandstone bed (sole marks) and internal structures including some or all of the following: massive graded unit, parallel lamination, ripple cross-lamination or convolute lamination, and an upper unit of parallel lamination. This combination of textural and structural features can be explained by deposition from a current that slightly erodes the bottom and then deposits sand that becomes finer grained as the velocity gradually wanes. The properties inferred from these ancient sandstone deposits are consistent with the properties of turbidity currents inferred from laboratory experiments. In spite of the convincing nature of the evidence, there are still some objections to the turbidity current hypothesis. Most geologists and oceanographers accept that such currents exist and that the currents are important agents of erosion and sediment deposition, in both modern and ancient seas, but researchers believe that the turbidity current hypothesis has been overworked. There is evidence, for example, which suggests that currents flowing parallel to submarine contours exist in many ocean basins. These bottom currents have been observed in a few cases, and velocities as high as 20 to 50 centimetres per second have been recorded. These currents can produce some of the features that previously had been attributed to turbidity current action. Moreover, nearly all features of sands that are produced by turbidity currents can be formed by shallow-water action, such as fluvial processes. Hence the problem of discriminating between deposits formed by turbidity currents and deposits formed by other current types is quite complex and requires a careful assessment of all lines of evidence in each case. Some ancient sandstones have been interpreted as "fluxoturbidites" because the sedimentary structures and other properties suggest a transporting agent intermediate between turbidity currents and large-scale slumping and sliding of sediment. Gerard V. Middleton On 3/28/06, johnson achmad paju <[EMAIL PROTECTED]> wrote: > biasanya proses turbidit terjadi adalah karena adanya perbedaan kemiringan > (slope) dari regim aliran dan salah satu syaratnya adalah dari confined ke > unconfined, jadi dari saluran sempit menuju ke saluran yang lebih lebar, > salah satunya di meandering atau braided system adalah bobolnya tanggul alam > (leeve) menuju dataran banjir (flood plain), mungkin dimensinya tidak > sebesar yang ada di prodelta atau sub-marine fan, tetapi proses yang terjadi > kurang lebih sama > -- Blog at http://decartenz.blogspot.com Help GeoTUTOR at http://www.geotutor.tk Jiwa Merdeka at http://jiwamerdeka.blogspot.com --------------------------------------------------------------------- To unsubscribe, send email to: iagi-net-unsubscribe[at]iagi.or.id To subscribe, send email to: iagi-net-subscribe[at]iagi.or.id Visit IAGI Website: http://iagi.or.id Pembayaran iuran anggota ditujukan ke: Bank Mandiri Cab. 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