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28-602: The rocks are located within the confines between the Church Stretton Fault and the Pontesford-Lindley Lineament . The Wentnor Group is a predominantly sedimentary group with a range of facies attributable to that of a closing ocean. The Wentnor Group overlies the Stretton Group of rocks and although the units are separate, together they show a good geological progression. At the base of the Stretton Group

56-406: A change to the density of the fluid. This is usually achieved by highly turbulent liquids which have a suspended load of fine grained particles forming a slurry . In this case, larger fragments of rock can be transported at water velocities too low to otherwise do so because of the lower density contrast (that is, the water plus sediment has a higher density than the water and is therefore closer to

84-535: A consequence, a slightly different set of sedimentary structures develops in turbidites deposited by high-density turbidity currents. This different set of structures is known as the Lowe sequence , which is a descriptive classification that complements, but does not replace, the Bouma sequence. Turbidites are sediments which are transported and deposited by density flow, not by tractional or frictional flow. The distinction

112-478: A high resolution record of seismicity, and terrestrial storm/flood events depending on the connectivity of canyon/channel systems to terrestrial sediment sources. Turbidites from lakes and fjords are also important as they can provide chronologic evidence of the frequency of landslides and the earthquakes that presumably formed them, by dating using radiocarbon or varves above and below the turbidite. Turbidite sequences are classic hosts for lode gold deposits,

140-416: A mechanism for assigning a tectonic and depositional setting to ancient sedimentary sequences as they usually represent deep-water rocks formed offshore of a convergent margin , and generally require at least a sloping shelf and some form of tectonism to trigger density-based avalanches. Density currents may be triggered in areas of high sediment supply by gravitational failure alone. Turbidites can represent

168-506: A thin deposit), or upslope from the deposition centre and manifested as a scour channel filled with fine sands grading up into a pelagic ooze . It is now recognized that the vertical progression of sedimentary structures described by Bouma applies to turbidites deposited by low-density turbidity currents. As the sand concentration of a flow increases, grain-to-grain collisions within the turbid suspension create dispersive pressures that become important in hindering further settling of grains. As

196-644: Is barren of fossils). This is the top most formation of the Wentnor Group and viz. the Longmyndian Supergroup . The unit grades up from the underlying Bayston-Oakswood Formation. The massive and cross-bedded sandstones of the underlying unit become interbedded with purple siltstones and thin ripple cross laminated sandstones. Thick cross-bedded sandstones are recorded with sharp erosional bases and are interpreted as fluvial channels running upon mud rich alluvial plain deposits. Turbidite A turbidite

224-409: Is called a traction carpet , since it is thought to move as a single unit. At some point, the grains move close enough together that collisions no longer generate enough energy to keep the grains in suspension, and the entire layer freezes to create an S2 layer. This process can then repeat to create additional traction carpets. When grains move closer together and settle out, the water between them

252-400: Is displaced so that it can move upward into the flow, helping to keep grains above the traction carpets in suspension. Because the flow is in motion, this upward movement of fluid quickly becomes turbulent. When the energy of the flow drops low enough that it can no longer sustain turbulence, then the entire flow freezes to create the massive to normally graded S3 layer. Subsequent reworking of

280-457: Is intended to complement, not replace, the better known Bouma sequence , which applies primarily to turbidites deposited by low-density (i.e., low-sand concentration) turbidity currents. The Lowe sequence adds three layers labelled S1 through S3 to Bouma's terminology, with S1 being at the bottom and S3 at the top of a sandy turbidite bed. As with the Bouma sequence, each layer has a specific set of sedimentary structures and lithology . And like

308-499: Is presented oldest to youngest as it makes much more sense in this way. One should read the Stretton Group stratigraphy first in order to get a sense of continuity. The progradational Longmyndian Sequence from oldest to youngest is: Ragleth Tuff Formation; Stretton Shale Formation; Burway Formation; Synalds Formation; Lightspout Formation; Portway Formation; Bayston-Oakswood Formation; Bridges Formation. The latter two units belong to

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336-399: Is representative of strong to waning flow regime currents and their corresponding sedimentation. It is unusual to see all of a complete Bouma cycle, as successive turbidity currents may erode the unconsolidated upper sequences. Alternatively, the entire sequence may not be present depending on whether the exposed section was at the edge of the turbidity current lobe (where it may be present as

364-582: Is seldom used. Initially grains, pebbles and large clasts in a high-density turbidity current (i.e., a high-sand concentration flow), are moved by traction (rolling and sliding) to generate a coarse-grained to conglomeratic, parallel-laminated to cross-laminated S1 layer. However, as grains settle out and move closer together, grain-to-grain collisions begin to generate dispersive pressures that help prevent further settling. This results in smaller grains moving between larger grains and preferentially settling out beneath them. Thus, an inverse graded layer develops that

392-531: Is that, in a normal river or stream bed, particles of rock are carried along by frictional drag of water on the particle (known as tractional flow ). The water must be travelling at a certain velocity in order to suspend the particle in the water and push it along. The greater the size or density of the particle relative to the fluid in which it is travelling, the higher the water velocity required to suspend it and transport it. Density-based flow, however, occurs when liquefaction of sediment during transport causes

420-505: Is the geologic deposit of a turbidity current , which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean . Turbidites were first properly described by Arnold H. Bouma (1962), who studied deepwater sediments and recognized particular "fining-up intervals" within deep water, fine-grained shales , which were anomalous because they started at pebble conglomerates and terminated in shales. This

448-399: The petroleum industry makes strenuous efforts to predict the location, overall shape, and internal characteristics of these sediment bodies in order to efficiently develop fields as well as explore for new reserves. Lowe sequence The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents . It

476-463: The Bouma sequence, the layers become finer grained from bottom to top. The layers are described as follows. As previously mentioned, the Lowe sequence is intended to complement, not replace the Bouma sequence. Fine-grained turbidites resulting from low-density turbidity currents, in which the Bouma A through Bouma E terminology applies, are referred to in the Lowe classification as Ta through Te, in which

504-470: The T acronym derives from "Traction". By contrast, because the S1-S3 terminology describes sand-rich turbidites deposited by high-density turbidity currents, the S acronym derives from "Sandstone". Lastly, R1-R3, which uses the same descriptive criteria as S1-S3, applies to conglomerates, wherein the R acronym derives from "Rubble". In practice, the S1-S3 terminology is widely used, Ta-Te, is used sometimes, and R1-R3

532-525: The Wentnor Group. Below we carry on from the underlying Portway Formation (Stretton Group). This is made up of fine to medium grained cross-stratified sandstone with mudstone rip-up clasts and sub-rounded lithic clasts. There are interbeds of cross-laminated finer grained sandstones with apparent upward fining successions. The formation also contains matrix and clast supported conglomerate members with subrounded lithic clasts and subangular sedimentary clasts. These are interpreted as braided fluvial deposits. (Unit

560-658: The density of the rock). This condition occurs in many environments aside from simply the deep ocean, where turbidites are particularly well represented. Lahars on the side of volcanoes, mudslides and pyroclastic flows all create density-based flow situations and, especially in the latter, can create sequences which are strikingly similar to turbidites. Turbidites in sediments can occur in carbonate as well as siliciclastic sequences. Classic, low-density turbidites are characterized by graded bedding , current ripple marks , climbing ripple laminations, alternating sequences with pelagic sediments, distinct fauna changes between

588-449: The effect of sea level fluctuations, regional tectonic events, sediment supply type, sediment supply rate, and sediment concentration. Autogenic controls can include seafloor topography, confinements, and slope gradients. There are about 26 submarine fan models. Some common fan models include the classical single-source suprafan model, models depicting fans with attached lobes, detached lobes fan model, and submarine fan models relating to

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616-506: The eventual depositional environments of turbidite deposits. They are aimed at providing insights into the relationships between different geologic processes and turbidite fan systems. Geologic processes influencing turbidite systems can either be of allogenic or autogenic origin and submarine fan models are designed to capture the impact of these processes on reservoir presence, reservoir distribution, morphology, and architecture of turbidite deposits. Some significant allogenic forcing includes

644-462: The prime example being Bendigo and Ballarat in Victoria, Australia , where more than 2,600 tons of gold have been extracted from saddle-reef deposits hosted in shale sequences from a thick succession of Cambrian-Ordovician turbidites. Proterozoic gold deposits are also known from turbidite basin deposits. Lithified accumulations of turbidite deposits may, in time, become hydrocarbon reservoirs and

672-433: The response of turbidite systems to varying grain sizes and different feeder systems. The integration of subsurface datasets such as 3D/4D seismic reflection, well logs, and core data as well as modern seafloor bathymetry studies, numerical forward stratigraphic modeling, and flume tank experiments are enabling improvements and more realistic development of submarine fan models across different basins. Turbidites provide

700-422: The rocks are of basinal oceanic facies and as time goes a coarsening occurs with increased terrigenous input from the continent. Turbidites are observed and deltas form latterly with alluvial plains with occasional marine washovers. This creeps up into the Wentnor Group where alluvial plains occur latterly with fluvial and alluvial deposits noted in the uppermost (youngest) Bridges Formation. The information below

728-424: The top of this new deposit by overlying remnant currents, or by new currents unrelated to the original flow can create laminations that resemble the Bouma B layer. When reworking stops, suspension settling may deposit massive mudstone (Bouma E) directly on top of the laminated layer. Alternatively, if new sediment is introduced during this reworking phase, or if sediment is sufficiently remobilized and transported, then

756-766: The turbidite and native pelagic sediments, sole markings , thick sediment sequences, regular bedding , and an absence of shallow-water features. A different vertical progression of sedimentary structures characterize high-density turbidites . Massive accumulations of turbidites and other deep-water deposits may result in the formation of submarine fans . Sedimentary models of such fan systems typically are subdivided into upper, mid, and lower fan sequences each with distinct sand-body geometries, sediment distributions, and lithologic characteristics. Turbidite deposits typically occur in foreland basins . Submarine fan models are often based on source-to-sink [S2S] concepts linking sediment source areas, and sediment routing systems to

784-593: Was anomalous because within the deep ocean it had historically been assumed that there was no mechanism by which tractional flow could carry and deposit coarse-grained sediments into the abyssal depths. Bouma cycles begin with an erosional contact of a coarse lower bed of pebble to granule conglomerate in a sandy matrix, and grade up through coarse then medium plane parallel sandstone; through cross-bedded sandstone ; rippled cross-bedded sand/silty sand, and finally laminar siltstone and shale. This vertical succession of sedimentary structures , bedding, and changing lithology

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