Sedimentary Structures

Cross BeddingSand will be moved by wind, river or marine currents in high energy environments. The sand particles are deposited to form dunes or sand bars. The currents are unidirectional - they flow in one main direction over a period of time. The result is a gentle slope on the windward/upstream side and a steep slope on the other side where the sand grains avalanche down the slope to settle at a maximum angle of 37 degrees from the horizontal. This is the maximum angle of rest.Dunes or bars constantly migrate down current, but only the lee side or slip face is preserved. A new dune migrating on top of an original dune cuts off the first set and produces a new cross bedding set. Wind blown sand in deserts is found in large dune structures so the cross bedding is measured in metre. Beach and shallow sea sand bars produce cross bedding that it about 10 to 15cm high, while the smallest scale is in rivers at about 3 to 10cm high. Cross bedding can be used as a palaeocurrent indicator, as the cross bedding planes always dip down in the direction of current flow.Cross bedding is the right way up when it is concave upwards. The cross bedding pattern flattens out at the base but is cut off at the top. This is due to erosion, which can only occur at the top.Graded BeddingThe largest and heaviest particles are on the bottom of the bed and finer particles are at the top. So in each layer, the grains become progressvely finer towards the top but there is an abrupt change in grain size at each bedding plane. This makes graded bedding useful as a way up structure. Graded bedding forms when sediment settles out under calm, low energy conditions where there is little or no current, such as on the abyssal plain. Turbidity currents that flow onto the abyssal plain from the continental slope or rivers that flow into lakes, bring sediment into the calm water where it settles out to form a distinct bed. Graded bedding is found in sandstones, greywacke or conglomerates.Imbricate StructuresPebbles are rolled along a stream bed and then pile up against each other. Flat pebbles stack against each other so that their long axes are roughly parallel. This orientation provides maximum resistance to movement. The pebbles are inclined in an upstream direction, the tops of pebbles pointing downstream. Salt PseudomorphsCubic halite crystals grow at the surface of a bed due to evaporation of salty water in arid environments. The crystals may become partly embedded in sediment deposited by evaporating saline water. The sediment dries out in the heat. When the dried up lake fills with water again halite crystals are dissolved leaving cubic shaped moulds. These are then infilled by sediment, which take on the shape of the halite crystals.Ripple MarksRipple marks form as sand grain are transported by saltation in high energy conditions: symmetrical ripple marks are formed by oscillating currents, so they are most common on beaches where tidal action moves sediment up and down the beach. asymmetrical ripple marks are formed by currents in one direction, so they are common in river, shallow sea and desert environments where the wind acts in the same way as water. These ripple marks can be used as palaeocurrent indicators. Desiccation CracksThese cracks only form in clay rich sediments. Loss of water due to evaporation by solar heating causes mud to contract forming polygonal blocks separated by cracks. Each crack has a V-shaped cross section with a wide top and narrow base, which makes them good way up indicators, The cracks are wider at the top because of the evaporation of water from the mud is greatest near the surface, which receives the most solar heat energy.Cracks are only preserved if they are infilled by sediment, usually silt or mud of a different colour. Desiccation cracks are good palaeo-environmental indicators of aird areas with high evaporation rates, such as playa lakes.Flute CastsA flute cast is a structure found at the base of a bed. Flute casts are formed as a result of erosion caused by turbulent flow, often associated with powerful turbidity currents. They can form in any environment where water flows with high energy over soft mud. They form parallel to the current and are deeper and pointed at the upstream end. This makes them good palaeocurrent indicators. They are infilled by overlying sediments.PalaeocurrentsMeasurements made on sedimentary structures can be used to show palaeocurrent information in the form of a rose diagram: this method works well for flute casts, by measuring with a compass the orientation of each structure. The direction of flow tells us the direction of the turbidity current that formed the structures. cross-bedding dip directions can also be measured as the sediment is only preserved on the leeward slope of a dune or sand bar. Measuring sets of cross bedding over a large area gives data that can then be plotted to show the direction of current. The wind direction that formed the sand dunes of the Triassic can be identified using this method. Way-up IndicatorsWay-up indicators are needed where rocks have been inverted by folding, making it difficult to identify the bottom and top of a bed.

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