Can someone explain and describe cross bedding, graded bedding,bedding planes,ripple marks mudcracks, fossils or flute casts in sedimentary rocks?even if u can only do one or two help is greatly appreciated!!(:-thanks!
cross bedding is where layers of sediment lie at different angels.this is produced when the wind or a river changes direction before depositing more material
fussels- is the remains or signs left behind by a once-living thing. Dinosaurs have fossils that are discovered by scientists. for example water animals die and later their fossil would be discovered.
cross bedding is found in meandering braided rivers as you're usually looking side on to the bedding so you're seeing the bedding building one way and then the other because thats the way the water is flowing. bedding planes are, typically, distinct changes in the colour - thus chemicals - grain size flow direction etc. graded is caused by the slowing down of a river flow...typically caused by sea level change. it doesn't matter which way it goes - coarse to fine or not - although it can help with determining a way up. Mud cracks are called 'desication'. modern examples of how this works is the african savana. the mud dries and cracks and is then infilled by a sediment rich flood, which fills the cracks before the mud has time to become gel like again. if this isn't disturbed then this pattern is recorded in the record.
Cross-bedding that you're talking about refers to inclined sedimentary structures in a horizontal unit of rock. The tilted structures are deposits from bedforms such as ripples and dunes, and they indicate that the depositional environment contained a flowing fluid (either water or wind). Sediment grains bounce up the windward/upstream (stoss) side of a ripple, and then tumble down the lee side. This is an interesting case where original depositional layering is tilted, rather than post-depositional deformation. It is most common in stream deposits (consisting of sand and gravel), tidal areas, and in aeolian dunes. Cross-bedded sediments are recognized in the field by the many layers of foresets, which are the series of layers that form on the lee side of the bedform (ripple or dune). These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities. It can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area. The direction of motion of the cross-beds can show ancient flow or wind directions (called paleocurrents). The foresets are deposited at the angle of repose (~34 degrees from the horizontal), so you can measure dip direction of the cross-bedded sediments and calculate the paleoflow direction. However, most cross-beds are not tabular, they are troughs. Since troughs can give a 180 degree variation of the dip of foresets, false paleocurrents can be taken by blindly measuring foresets. In this case, true paleocurrent direction is determined by the axis of the trough. Paleocurrent direction is important in reconstructing past climate and drainage patterns: sand dunes preserve the prevalent wind directions, and current ripples show the direction rivers were moving. Be sure to look into the links below for more information.
