Define Graded Bedding In Geology

Graded bedding is a sedimentary structure characterized by a gradual decrease in the size of clasts or grains from the bottom to the top of a bed. This change in grain size is a result of the sorting of sediments by gravity as they are deposited from a fluid. The coarser grains are deposited first, followed by the finer grains. The process of graded bedding is common in sedimentary environments where there is a change in the velocity of the transporting fluid, such as in a river or stream. Graded bedding can also be found in sedimentary rocks that have been deposited by glaciers or landslides.

Characteristics of Graded Bedding

The key characteristics of graded bedding include:

A gradual decrease in the size of clasts or grains from the bottom to the top of a bed
A sharp contact between the graded bed and the underlying bed
A gradual or gradational contact between the graded bed and the overlying bed

Formation of Graded Bedding

Graded bedding is formed when sediments are deposited from a fluid that is flowing at a variable velocity. The coarser grains are deposited first, followed by the finer grains. This is because the coarser grains are more difficult to transport than the finer grains. As the velocity of the fluid decreases, the finer grains are able to settle out of the fluid and be deposited on top of the coarser grains. This process can be repeated multiple times, resulting in a graded bed with a distinct layering of different grain sizes.

Significance of Graded Bedding

Graded bedding is a valuable tool for geologists because it can provide information about the depositional environment of a sedimentary rock. The thickness of a graded bed, the size of the clasts or grains, and the degree of sorting can all be used to infer the velocity of the transporting fluid and the distance that the sediments were transported. Graded bedding can also be used to identify the direction of flow of a transporting fluid, such as a river or stream.

Examples of Graded Bedding

Graded bedding is found in a variety of sedimentary rocks, including sandstone, shale, and limestone. Some of the most famous examples of graded bedding include the Bouma sequences found in turbidites, which are sedimentary rocks that are deposited by turbidity currents. Graded bedding is also found in sedimentary rocks that have been deposited by glaciers or landslides.