With reference to Walther’s Law, describe how changes in sea level can change the vertical and lateral relationship of facies.
Accommodation rate is when the subsidence of a depositional basin allows for the accumulation of more sediments. When the accommodation rate is greater than the sedimentation rate so that there is more space than can be filled by sediment, the shoreline moves inland as the ocean transgresses. A shore consists of interrelated facies of a coarse-grained shoreline gradually fining oceanwards into muds and clays. During a transgression, the sediment sequence shifts inland, creating a retrogradational sequence such that the stratigraphic column fines upwards.
If the accommodation rate is smaller than the sedimentation rate, the sediments swamp available space and extend farther, moving the shoreline oceanward as the ocean regresses. In a progradational sequence, the stratigraphic column coarsens upwards. If the accommodation rate is balanced with the sedimentation rate, it reaches a stable situation where the shoreline maintains its location. The aggradation sequence maintains a fairly consistent stratigraphic column over time. In messy reality, shorelines transgress and regress at irregular intervals, creating a jagged stratigraphic column.
Walther’s Law of Facies is that a vertical change in conformable sediments reflects this lateral changes in facies.