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Sea Urchin Holaster subglobosus
Sedimentary structures are created during the formation of the sediment. It may involve certain layering and structures in the stratigraphy that can help to reconstruct the depositional environment. Structures may also be used to determine whether or not the layers are still in their original position, or have overturned by tectonics. Here are a few examples of sedimentary structures.
- Wave ripples are symmetrical and occur in coastal or near-beach deposits.
- Current ripples are asymmetric and occur for example in river sediments. The direction of flow can be derived.
- Shrinkage Cracks in mud originated in an area that was previously wet and then dried out.
- Organisms can also can make sedimentary structures in the form of burrows, slide traces and bioturbation. Burrows and bioturbation are more common in shallower water, siled traces are more common in deeper water.
- Impressions of raindrops
- Cross stratification is formed by wind or rivers, but also occurs in near coastal sediments. Due to the deposition of sand in ridges or dunes in an area with net sediment supply, successive layers occur with cross stratification. The base of the ridges or dunes is preserved as oblique layering. With successive layers with cross stratification and changing directions these may be tide deposits. Wind deposits can be distinguished by the well sorted granules.
- Graded layers are layers which is finer towards the top. The most common form of such a deposit is a turbidite. The submarine avalanche deposits increasingly finer material by the decreasing flow.
- Tool marks occur when an object is dragged into the flow if it is too heavy to move in suspension. The groove is filled by the above- lying sediments.
- Flute casts are caused by an object just too heavy to move in suspension. The object bounces therefore regularly on the surface. The remains are an asymmetrical pit where the object touched the ground. Flute casts are common in turbidite deposits. The pointed part of the pit structure is pointing in the direction of the original flow.
Recent wave ripples on the North Sea beach . Photo : Kevin Nolis
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