Definition Of Graded Bedding

Definition of Graded Bedding

Graded bedding is a sedimentary structure characterized by a gradual change in the size and composition of sediment particles from the bottom to the top of a bed. It occurs when sediment is deposited from a fluid flow that is gradually losing energy. As the flow slows down, larger particles settle out first, followed by progressively smaller particles. This results in a layering of sediment with the coarsest particles at the bottom and the finest particles at the top.

Graded bedding is a common feature of sedimentary rocks, particularly those that were deposited in a fluvial or marine environment. It can also be found in volcaniclastic rocks that were deposited from pyroclastic flows or lahars. The presence of graded bedding can provide valuable information about the depositional environment and the processes that formed the sediment.

Characteristics of Graded Bedding

Graded bedding is typically characterized by the following features:

Gradual change in particle size from the bottom to the top of the bed
Fining-upward or coarsening-upward sequence
Sharp or gradational contacts between layers
May contain other sedimentary structures, such as ripple marks or cross-bedding

Types of Graded Bedding

There are two main types of graded bedding:

Normal grading: This is the most common type of graded bedding, in which the particle size decreases from the bottom to the top of the bed.
Inverse grading: This is a less common type of graded bedding, in which the particle size increases from the bottom to the top of the bed.

Formation of Graded Bedding

Graded bedding is formed when sediment is deposited from a fluid flow that is gradually losing energy. As the flow slows down, larger particles settle out first, followed by progressively smaller particles. This results in a layering of sediment with the coarsest particles at the bottom and the finest particles at the top.

The rate at which the flow loses energy will determine the thickness of the graded bed. A rapid loss of energy will result in a thin graded bed, while a gradual loss of energy will result in a thicker graded bed.

Significance of Graded Bedding

Graded bedding can provide valuable information about the depositional environment and the processes that formed the sediment. For example, the presence of graded bedding in a sandstone bed may indicate that the bed was deposited by a river or stream. The presence of graded bedding in a volcaniclastic bed may indicate that the bed was deposited by a pyroclastic flow or lahar.

Graded bedding can also be used to determine the direction of flow in a sedimentary basin. The coarsest particles in a graded bed will typically be deposited at the bottom of the bed, and the finest particles will be deposited at the top of the bed. By measuring the orientation of the graded beds, it is possible to determine the direction of flow that deposited the sediment.