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39-521: During the time of the deposition of the Ecca Group, the depositional environment, with some exceptions, was predominantly marine . The Ecca sea was vast but shallow, reaching only around 500 m at its deepest in its west/northwestern and southern facies where the Tanqua and Laingsburg Depocenters are situated respectively. The marine environment ranged from deep pelagic , submarine fan systems in

78-409: A clay platelet has a slight negative charge where the edge has a slight positive charge when two platelets come into close proximity with each other the face of one particle and the edge of the other are electrostatically attracted." Flocs then have a higher combined mass which leads to quicker deposition through a higher fall velocity, and deposition in a more shoreward direction than they would have as

117-519: A depth of −13 m relative to mean sea level at the 9 km point down the transect of the central axis. The predominant storm wave energy has unlimited fetch for the outer harbour from a southerly direction, with a calmer environment within the inner harbour, though localised harbour breezes create surface currents and chop influencing the marine sedimentation processes. Deposits of loess from subsequent glacial periods have in filled volcanic fissures over millennia, resulting in volcanic basalt and loess as

156-415: A distinct kind of sediment for that area or environment. Since its inception in 1838, the facies concept has been extended to related geological concepts. For example, characteristic associations of organic microfossils, and particulate organic material, in rocks or sediments, are called palynofacies . Discrete seismic units are similarly referred to as seismic facies. Sedimentary facies are described in

195-435: A group of "facies descriptors" which must be distinct, reproducible and exhaustive. A reliable facies description of an outcrop in the field would include: composition, texture, sedimentary structure(s), bedding geometry, nature of bedding contact, fossil content and colour. The sequence of minerals that develop during progressive metamorphism (that is, metamorphism at progressively higher temperatures and/or pressures) define

234-467: A particular size may move across the profile to a position where it is in equilibrium with the wave and flows acting on that sediment grain". This sorting mechanism combines the influence of the down-slope gravitational force of the profile and forces due to flow asymmetry; the position where there is zero net transport is known as the null point and was first proposed by Cornaglia in 1889. Figure 1 illustrates this relationship between sediment grain size and

273-506: Is because sediment grain size analysis throughout a profile allows inference into the erosion or accretion rates possible if shore dynamics are modified. Planners and managers should also be aware that the coastal environment is dynamic and contextual science should be evaluated before the implementation of any shore profile modification. Thus theoretical studies, laboratory experiments, numerical and hydraulic modelling seek to answer questions pertaining to littoral drift and sediment deposition,

312-468: Is insufficient bed shear stress and fluid turbulence to keep the sediment moving; with the suspended load this can be some distance as the particles need to fall through the water column. This is determined by the grain's downward acting weight force being matched by a combined buoyancy and fluid drag force and can be expressed by: Downward acting weight force = Upward-acting buoyancy force + Upward-acting fluid drag force where: In order to calculate

351-565: Is purely marine . The rocks contain a complete transition, grading laterally into one other, from basin-floor marine deposits through to channelized submarine slope to shelf , pro-delta and beach environment deposits. The deposits of the western/northwestern facies fall within the Tanqua Depocenter , one of the vast submarine fan systems known from the marine Ecca. Associated formations are listed below (from oldest to youngest): Southern Ecca facies This facies succession

390-421: Is shallow marine in its lowermost and uppermost sections, and then changes to coal -bearing fluvial - deltaic peat swamp settings in its central deposits. The northern facies often overlies unconformably on much older basement rocks unlike the other facies of the Ecca Group. It comprises three geological formations (from oldest to youngest): The lower geological formations of the Ecca Group, particularly

429-405: Is the geological process in which sediments , soil and rocks are added to a landform or landmass . Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment. This occurs when the forces responsible for sediment transportation are no longer sufficient to overcome

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468-440: Is the largest of the three facies succession. Its lowermost formations are deep marine comprising basin floor pelagic sediments and submarine fan systems that grade upwards into channelized submarine slopes to shelf marine and beach environments. The Laingsburg Depocenter is found in this facies succession and include the following formations (from oldest to youngest): Northeastern Ecca facies The northeastern facies

507-522: Is the vertical stratigraphic succession that typifies marine transgressions and regressions . Ideally, a sedimentary facies is a distinctive rock unit that forms under certain conditions of sedimentation , reflecting a particular process or environment. Sedimentary facies are either descriptive or interpretative. Sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments. Generally, geologists distinguish facies by

546-607: The Karoo Basin . The rocks of the Ecca Group first appear near Sutherland in its westernmost deposits, and continues east through Laingsburg , Prince Albert , Jansenville , Grahamstown , and up until the coast near Port Alfred . In the central north deposits are found near Britstown , running along the Orange River between Petrusville and Hopetown . In the extreme northeast deposits are found east of Johannesburg past Vryheid , Durban , Pietermaritzburg and all

585-457: The aspect of the rock or sediment being studied. Facies based on petrological characters (such as grain size and mineralogy ) are called lithofacies , whereas facies based on fossil content are called biofacies . A facies is usually further subdivided. The characteristics of the rock unit come from the depositional environment and from the original composition. Sedimentary facies reflect their depositional environment, each facies being

624-723: The subduction of the Palaeo-pacific plate beneath the Gondwanan Plate. This resulted in the rise of the Gondwanide mountain range in what is known as the Gondwanide orogeny . The mountain-building and erosion caused by the growing Gondwanide mountain range was the initial subsidence mechanism acting on the Karoo Basin . Flexural tectonics partitioned the Karoo Basin into the foredeep , forebulge , and backbulge flexural provinces. This resulted in deposition of

663-562: The central axis of the harbour, or if classified into grain class sizes, "the plotted transect for the central axis goes from silty sands in the intertidal zone to sandy silts in the inner nearshore, to silts in the outer reaches of the bays to mud at depths of 6 m or more". See figure 2 for detail. Other studies have shown this process of the winnowing of sediment grain size from the effect of hydrodynamic forcing; Wang, Collins and Zhu (1988) qualitatively correlated increasing intensity of fluid forcing with increasing grain size. "This correlation

702-464: The changes that may occur in those attributes over a geographic area. A facies encompasses all the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock. The term "facies" was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy , which replaced

741-406: The deposition of larger grains on a shore profile. The secondary principle to the creation of seaward sediment fining is known as the hypothesis of asymmetrical thresholds under waves; this describes the interaction between the oscillatory flow of waves and tides flowing over the wave ripple bedforms in an asymmetric pattern. "The relatively strong onshore stroke of the waveforms an eddy or vortex on

780-433: The depth of the marine environment. The first principle underlying the null point theory is due to the gravitational force; finer sediments remain in the water column for longer durations allowing transportation outside the surf zone to deposit under calmer conditions. The gravitational effect or settling velocity determines the location of deposition for finer sediments, whereas a grain's internal angle of friction determines

819-406: The drag coefficient, the grain's Reynolds number needs to be discovered, which is based on the type of fluid through which the sediment particle is flowing, laminar flow, turbulent flow or a hybrid of both. When the fluid becomes more viscous due to smaller grain sizes or larger settling velocities, the prediction is less straightforward and it is applicable to incorporate Stokes Law (also known as

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858-505: The earlier notions of Neptunism . Walther's law of facies, or simply Walther's law, named after the geologist Johannes Walther , states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another. In Russia the law is known as Golovkinsky-Walther's law, honoring also Nikolai A. Golovkinsky . A classic example of this law

897-668: The exception of the Prince Albert, Whitehill , Collingham, and the uppermost Waterford Formations which are found in both the southern and western/northwestern facies , the geological formations of the Ecca Group can only be found in one of the three previously aforementioned facies successions. This is because each of these facies successions represents differing preserved environments that can be observed in their diagnostic geological features. The facies successions, along with their geological formations, are described below: Western/Northwestern Ecca facies This facies succession

936-559: The fines are suspended and reworked aerially offshore leaving behind lag deposits of the main bivalve and gastropod shells separated out from the finer substrate beneath, waves and currents then heap these deposits to form chenier ridges throughout the tidal zone, which tend to be forced up the foreshore profile but also along the foreshore. Cheniers can be found at any level on the foreshore and predominantly characterise an erosion-dominated regime. The null point theory has been controversial in its acceptance into mainstream coastal science as

975-451: The forces of gravity and friction , creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes . For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton , the deposition of which induced chemical processes ( diagenesis ) to deposit further calcium carbonate. Similarly,

1014-456: The formation of coal begins with the deposition of organic material, mainly from plants, in anaerobic conditions. The null-point hypothesis explains how sediment is deposited throughout a shore profile according to its grain size. This is due to the influence of hydraulic energy, resulting in a seaward-fining of sediment particle size, or where fluid forcing equals gravity for each grain size. The concept can also be explained as "sediment of

1053-531: The frictional force, or drag force) of settling. The cohesion of sediment occurs with the small grain sizes associated with silts and clays, or particles smaller than 4ϕ on the phi scale. If these fine particles remain dispersed in the water column, Stokes law applies to the settling velocity of the individual grains, although due to seawater being a strong electrolyte bonding agent, flocculation occurs where individual particles create an electrical bond adhering each other together to form flocs. "The face of

1092-451: The individual fine grains of clay or silt. Akaroa Harbour is located on Banks Peninsula , Canterbury, New Zealand , 43°48′S 172°56′E  /  43.800°S 172.933°E  / -43.800; 172.933 . The formation of this harbour has occurred due to active erosional processes on an extinct shield volcano, whereby the sea has flooded the caldera, creating an inlet 16 km in length, with an average width of 2 km and

1131-413: The lee side of the ripple, provided the onshore flow persists, this eddy remains trapped in the lee of the ripple. When the flow reverses, the eddy is thrown upwards off the bottom and a small cloud of suspended sediment generated by the eddy is ejected into the water column above the ripple, the sediment cloud is then moved seaward by the offshore stroke of the wave." Where there is symmetry in ripple shape

1170-490: The lower deposits which grade steadily north-eastwards to shallow marine deposits including shelf ( continental ) marine and marginal marine facies , and finally to beach deposits in younger successions. Coal -bearing fluvial - deltaic , and peatbog settings are also well known from the Ecca Group. The Ecca Group was deposited in a vast retroarc foreland basin . This foreland system was caused by crustal uplift ( orogenesis ) that had previously begun to take course due to

1209-478: The main sediment types available for deposition in Akaroa Harbour Hart et al. (2009) discovered through bathymetric survey, sieve and pipette analysis of subtidal sediments, that sediment textures were related to three main factors: depth, distance from shoreline, and distance along the central axis of the harbour. This resulted in the fining of sediment textures with increasing depth and towards

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1248-755: The nearby deposits of the Huab Basin. Abroad, Ecca-aged deposits are known from the Paraná Basin of Brazil and the Petolas Basin of both Brazil and Uruguay where fossils of Mesosaurus and Glossopteris have also been recovered. Finally, geological dating has also proven the lower Ecca formations to correlate with the Barnett Shale and Marcellus Formation of the United States . Deposition (geology) Deposition

1287-565: The results should not be viewed in isolation and a substantial body of purely qualitative observational data should supplement any planning or management decision. Facies In geology , a facies ( / ˈ f eɪ ʃ ɪ . iː z / FAY -shih-eez , US also / ˈ f eɪ ʃ iː z / FAY -sheez ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation) and

1326-664: The rocks of the Whitehill Formation , correlate in age with the Huab Basin of northwestern Namibia , and lower formations of the Kalahari Basin found in Namibia , Botswana , and Zimbabwe . Near the small town of Khorixas in Namibia there is a locally well-known national monument called the Petrified Forest . Petrified logs were brought into the area and are considered to have been sourced from

1365-438: The theory operates in dynamic equilibrium or unstable equilibrium, and many fields and laboratory observations have failed to replicate the state of a null point at each grain size throughout the profile. The interaction of variables and processes over time within the environmental context causes issues; "a large number of variables, the complexity of the processes, and the difficulty in observation, all place serious obstacles in

1404-679: The vortex is neutralised, the eddy and its associated sediment cloud develops on both sides of the ripple. This creates a cloudy water column which travels under the tidal influence as the wave orbital motion is in equilibrium. The Null-point hypothesis has been quantitatively proven in Akaroa Harbour, New Zealand, The Wash , U.K., Bohai Bay and West Huang Sera, Mainland China, and in numerous other studies; Ippen and Eagleson (1955), Eagleson and Dean (1959, 1961) and Miller and Zeigler (1958, 1964). Large-grain sediments transported by either bedload or suspended load will come to rest when there

1443-420: The way down to Port St. Johns in the southeast. The Ecca Group comprises sixteen recognized geological formations . These individual formations have been grouped into three geographical areas, which are the southern, western/northwestern, and northeastern facies successions. In the east of South Africa there are deposits of as of yet undifferentiated mudstone sequences attributed to the Ecca Group. With

1482-405: The way of systematisation, therefore in certain narrow fields the basic physical theory may be sound and reliable but the gaps are large" Geomorphologists, engineers, governments and planners should be aware of the processes and outcomes involved with the null point hypothesis when performing tasks such as beach nourishment , issuing building consents or building coastal defence structures. This

1521-544: Was demonstrated at the low energy clayey tidal flats of Bohai Bay (China), the moderate environment of the Jiangsu coast (China) where the bottom material is silty, and the sandy flats of the high energy coast of The Wash (U.K.)." This research shows conclusive evidence for the null point theory existing on tidal flats with differing hydrodynamic energy levels and also on flats that are both erosional and accretional. Kirby R. (2002) takes this concept further explaining that

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