[ ] BS-(Geology)

Walther’s Law of Correlation:

A German geologist Johannes Walther gave this law in 1894 to describe the manner in which a sedimentary sequence of facies develops vertically. In geology, the term facies refers to the bodies of sediment recognizably different from adjacent sediment deposited in a different depositional environment. Walther’s law states that ‘to a vertical progression of facies can be found corresponding lateral facies changes”. This law is based on the following understandings:

Sedimentary rock types record the environment of their deposition. Depositional environments can shift laterally as conditions change. When so, laterally related environments become superimposed. There will be same vertical succession and lateral sequence of facies. Time-transgressive sedimentary formations are the result.

Sedimentary facies are developed in marine environment. This can be related to Walther’s law by the statement that Sedimentary environments that started out side-by-side will end up overlapping one another over time due to sea level change i.e. transgression and regression. So the application of Walther’s law in the two sea-level stages is explained below.

Transgression:

A rise in sea level is called a transgression; it can be caused by rate of sea floor spreading, melting of polar ice caps, or by localized sinking or subsidence of the land in coastal areas. A transgression produces a vertical sequence of facies and represents progressively deeper water environments (deepening-upward sequence). As a result, a transgressive sequence will have finer-grained facies overlying coarser-grained facies (fining-upward from sand at the bottom, and then to silt, and then to shale). This is sometimes referred to as an on lap sequence.

Regression:

A decrease in sea level is called regression; Regression can be caused by a buildup of ice in the polar ice caps, or localized uplift of the land in coastal areas. A regression will produce a sequence of facies representing progressively shallower water environments (shallow-upward sequence). As a result, a regressive sequence will have coarser-grained facies overlying finer-grained facies (coarsening-upward). This is also called an offlap sequence.

AssignmentGeology of Pakistan