<P> Secondary sedimentary structures are those which formed after deposition . Such structures form by chemical, physical and biological processes within the sediment . They can be indicators of circumstances after deposition . Some can be used as way up criteria . </P> <P> Organic materials in a sediment can leave more traces than just fossils . Preserved tracks and burrows are examples of trace fossils (also called ichnofossils). Such traces are relatively rare . Most trace fossils are burrows of molluscs or arthropods . This burrowing is called bioturbation by sedimentologists . It can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited . On the other hand, the burrowing activity of organisms can destroy other (primary) structures in the sediment, making a reconstruction more difficult . </P> <P> Secondary structures can also form by diagenesis or the formation of a soil (pedogenesis) when a sediment is exposed above the water level . An example of a diagenetic structure common in carbonate rocks is a stylolite . Stylolites are irregular planes where material was dissolved into the pore fluids in the rock . This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions . Concretions are roughly concentric bodies with a different composition from the host rock . Their formation can be the result of localized precipitation due to small differences in composition or porosity of the host rock, such as around fossils, inside burrows or around plant roots . In carbonate based rocks such as limestone or chalk, chert or flint concretions are common, while terrestrial sandstones can have iron concretions . Calcite concretions in clay are called septarian concretions . </P> <P> After deposition, physical processes can deform the sediment, producing a third class of secondary structures . Density contrasts between different sedimentary layers, such as between sand and clay, can result in flame structures or load casts, formed by inverted diapirism . While the clastic bed is still fluid, diapirism can cause a denser upper layer to sink into a lower layer . Sometimes, density contrasts can result or grow when one of the lithologies dehydrates . Clay can be easily compressed as a result of dehydration, while sand retains the same volume and becomes relatively less dense . On the other hand, when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes . The same process can form mud volcanoes on the surface where they broke through upper layers . </P>

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