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34-581: The base of the Beacon Supergroup is marked by an unconformity and is composed of the Devonian Taylor Group, a quartzose sandstone sequence; and the Late Carboniferous to Early Jurassic Victoria Group, consisting of glacial beds, sandstone, shale , conglomerate , and coal. The Beacon Sandstone was named by Hartley T. Ferrar during Scott's Discovery Expedition (1901–1904), using

68-406: A meandering river , which has a single sinuous channel. It is also distinct from an anastomosing river , which consist of multiple interweaving semi-permanent channels which are separated by floodplain rather than channel bars; these channels may themselves be braided. The physical processes that determine whether a river will be braided or meandering are not fully understood. However, there

102-542: A meandering profile. A stream with cohesive banks that are resistant to erosion will form narrow, deep, meandering channels, whereas a stream with highly erodible banks will form wide, shallow channels, preventing the helical flow of the water necessary for meandering and resulting in the formation of braided channels. Braided rivers occur in many environments, but are most common in wide valleys associated with mountainous regions or their piedmonts or in areas of coarse-grained sediments and limited growth of vegetation near

136-404: A dendritic system, or of cohesive sediments with no bedload transport. Meanders fully develop only when the river banks are sufficiently stabilized to limit lateral flow. An increase in suspended sediment relative to bedload allows the deposition of fine erosion -resistant material on the inside of a curve, which accentuated the curve and in some instances, causes a river to shift from a braided to

170-406: A kind of relative dating . A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in the rock record. A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when

204-426: A straight channel. Also important to channel development is the proportion of suspended load sediment to bed load . An increase in suspended sediment allowed for the deposition of fine erosion -resistant material on the inside of a curve, which accentuated the curve and in some instances, caused a river to shift from a braided to a meandering profile. These experimental results were expressed in formulas relating

238-402: A variation in sediment load, provided the amount of water carried by the river is unchanged. A threshold slope was experimentally determined to be 0.016 (ft/ft) for a 0.15 cu ft/s (0.0042 m /s) stream with poorly sorted coarse sand. Any slope over this threshold created a braided stream, while any slope under the threshold created a meandering stream or – for very low slopes –

272-874: Is deposited against older strata thus influencing its bedding structure. A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols , or beds of pebbles derived from the underlying rock. Braided channel A braided river (also called braided channel or braided stream ) consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots . Braided streams tend to occur in rivers with high sediment loads or coarse grain sizes, and in rivers with steeper slopes than typical rivers with straight or meandering channel patterns. They are also associated with rivers with rapid and frequent variation in

306-704: Is found in the South Victoria Land region between the Arena Sandstone and the overlying Devonian Aztec Siltstone. It is well sorted and cemented, with medium to coarse grain sizes and trough cross-beds, with Haplostigma and Beaconites remnants. The Aztec Siltstone (125–220 m) is found both in South Victoria Land and the Darwin Mountains. The siltstone includes interbedded sandstones, fish-bearing shales, conchostracans , and paleosols implying subaerial periods within an alluvial plain sequence. Within

340-408: Is poorly sorted at the base, with influxes of coarser material. Coarseness is laterally variable, with pebbles in places and sands in others at the same horizon. The conglomerate includes planar beds, trough cross-bedding , flaser bedding , mud-drapes on some ripples , U-shaped burrows and escape structures, with fining-up cycles topped by desiccation cracks in places. The depositional environment

374-651: Is probably that of an alluvial fan , though unidirectional flow and sheet-like deposition point to braided channels . Equivalent strata in South Victoria Land include the Wind Gully Sandstone (80 m), the Terra Cotta Siltstone (82 m), and the New Mountain Sandstone (250 m), which are separated from the overlying Altar Mountain Formation (235 m) and Arena Sandstone (385 m) by a disconformity. Within

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408-403: Is wide agreement that a river becomes braided when it carries an abundant supply of sediments. Experiments with flumes suggest that a river becomes braided when a threshold level of sediment load or slope is reached. On timescales long enough for the river to evolve, a sustained increase in sediment load will increase the bed slope of the river, so that a variation of slope is equivalent to

442-703: The Beacon Heights survey points as reference. Glossopteris fossils dated the sandstone to the Permian and linked the lithology to similar sequences on neighboring continents. Generally flat lying, the supergroup is up to 3.2 km thick and is fairly continuous from south Victoria Land to the Beardmore Glacier along the Transantarctic Mountains . The Urfjell Group in Dronning Maud Land and

476-972: The Lower Permian Fairchild Formation (220 m) arkosic sandstone, the Upper Permian Buckley Formation (750 m), the Middle- Lower Triassic Fremouw Formation (650 m), the Upper- Middle Triassic Falla Formation (530 m), and the Jurassic Prebble Formation (0–460 m) volcanic conglomerate , tuff and tuffaceous sandstone. The Aztec sandstone contains units bearing body fossils of fish: Phyllolepid placoderms , and thelodonts , abundant in fish beds, and conchostracans . The presence of Scoyenia ichnofacies implies freshwater. Also present are charred wood remnants and

510-579: The Rakaia and Waitaki Rivers of New Zealand are not aggrading, due to retreating shorelines, but are nonetheless braided rivers. Variable discharge has also been identified as important in braided rivers, but this may be primarily due to the tendency for frequent floods to reduce bank vegetation and destabilize the banks, rather than because variable discharge is an essential part of braided river formation. Numerical models suggest that bedload transport (movement of sediment particles by rolling or bouncing along

544-704: The Beardmore Glacier region, the Devonian Alexandra Formation (0–320 m), which constitutes the entire Taylor Group, is a quartz sandstone to siltstone. The Victoria Group begins with a diamictite -bearing unit known as the Metschel Tillite (0–70 m) in South Victoria Land and continues with the Darwin Tillite (82 m) in the Darwin Mountains, the Pagoda Tillite (395 m) at the Beardmore Glacier,

578-843: The Darwin Mountains region, the Junction Sandstone (290 m) overlies the Brown Hills Conglomerate, with abundant Skolithos . This is followed by the Hatherton Sandstone (330 m), with brachiopod and bivalve shell fragments in places. Trough cross beds and current rippling are present, with abundant ichnofauna . Drainage was to the north east, with the depositional environment presumed to be marine, though also present are subaerial features such as desiccation cracks, rain drop impressions, surface run-off channels, muddy veneers, and redbeds, besides river-like features such as small channels. The Beacon Heights Orthoquartzite (330 m)

612-701: The Neptune Group in the Pensacola Mountains have been correlated with the Taylor Group. Macrofossils and palynomorph assemblages age date Devonian, Late Carboniferous– Early Permian , Late Permian and Triassic strata . The Ferrar Dolerite intrudes at various levels, while the Mawson Formation and Kirkpatrick Basalts within the Ferrar Supergroup cap the Beacon Supergroup. The location of

646-774: The Scott Glacier Formation (93 m) on the Nilsen Plateau , and the Buckeye Tillite (140–308 m) in the Wisconsin Range and Ohio Range . The glacial beds are valley fill or occur as sheets. This is followed by the Misthound Coal Measures (150 m) in the Darwin Mountains and the Permian Weller Coal Measures (250 m) in South Victoria Land. A disconformity separates the Misthound Coal Measures from

680-437: The amount of water they carry, i.e., with " flashy " rivers, and with rivers with weak banks . Braided channels are found in a variety of environments all over the world, including gravelly mountain streams, sand bed rivers, on alluvial fans , on river deltas , and across depositional plains. A braided river consists of a network of multiple shallow channels that diverge and rejoin around ephemeral braid bars . This gives

714-418: The arthropod tracks (less than 91 cm) is taken to imply that water must have been required for support. Unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of

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748-426: The critical slope for braiding to the discharge and grain size. The higher the discharge, the lower the critical slope, while larger grain size yields a higher critical slope. However, these give only an incomplete picture, and numerical simulations have become increasingly important for understanding braided rivers. Aggradation (net deposition of sediments) favors braided rivers, but is not essential. For example,

782-649: The early Jurassic as a consequence of the breakup of Gondwana 180  million years ago . The rock is low in phosphorus . The Taylor Group is separated from the overlying Victoria Group by a disconformity called the Maya Erosion Surface. Taylor Group formations in the Darwin Mountains region include the Brown Hills Conglomerate (34 m), which overlies pre-Devonian plutonic rocks of igneous and metamorphic nature, with over 30 m of erosional relief, and igneous and metamorphic clasts . The conglomerate

816-539: The formation in a cold, desert environment and the lack of nutrients or soil (due to the purity of the sandstone) has led to the Beacon Sandstone being considered the closest analogue on Earth to Martian conditions; therefore, many studies have been performed on life's survival there, mainly focusing on the lichen communities that form the modern inhabitants. The supergroup originated in a shallow marine sedimentary depositional environment . The well-sorted nature of

850-715: The overlying Ellis Formation (177 m), consisting of conglomerate, sandstone and siltstone. The Pyramid Erosion Surface separates the Mitschell Tillite and the Weller Coal Measures, which are overlain by the Feather Conglomerate (215 m) and the Triassic Lashly Formation (520 m). The Pagoda Tillite is overlain by the MacKellar Formation (140 m) of interbedded black shales and fine sandstones,

884-523: The overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions . In these cases, the hiatus in deposition represented by

918-465: The plants Glossopteris and Haplostigma . The wood bears clear growth rings , indicating a seasonal environment, and is large enough to represent a temperate climate, though glacial just before Beacon deposition. Trace fossils are sparse below, but become common in the Hatherton Sandstone. They change from Skolithos -dominated facies to wide diversity and abundance, including vertical and horizontal burrows and huge arthropod trackways . The size of

952-546: The river a fancied resemblance to the interwoven strands of a braid . The braid bars, also known as channel bars, branch islands, or accreting islands, are usually unstable and may be completely covered at times of high water. The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events. When the islets separating channels are stabilized by vegetation, so that they are more permanent features, they are sometimes called aits or eyots. A braided river differs from

986-414: The river bottom) is essential to formation of braided rivers, with net erosion of sediments at channel divergences and net deposition at convergences. Braiding is reliably reproduced in simulations whenever there is little lateral constraint on flow and there is significant bedload transport. Braiding is not observed in simulations of the extreme cases of pure scour (no deposition taking place), which produces

1020-432: The sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, then the plane of juncture is a nonconformity. An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with

1054-399: The sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region or were subsequently eroded before the next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus . It is

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1088-526: The unconformity may be geologically very short – hours, days or weeks. A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems . A buttress unconformity also known as onlap unconformity, occurs when younger bedding

1122-475: The unit suggests that it was probably deposited close to the shoreline, in a high-energy environment. Features such as the presence of coal beds and desiccation cracks suggest that parts of the unit were deposited subaerially, though ripple marks and cross bedding show that shallow water was also commonly present. Heat from burial is modest, though the rock could have been heated to over 160 °C by intrusion of dolerite sills , dykes and lenses during

1156-505: The younger layer, but the term is used to describe any break in the sedimentary geologic record . The significance of angular unconformity (see below) was shown by James Hutton , who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless

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