Final answer:
There are several reasons why a sediment layer might not be deposited at the average rate. One reason could be related to the water dynamics and sediment supply in the environment.
Step-by-step explanation:
Sediment layers may not deposit at average rates due to variable water dynamics and changes in sea level, specifically transgression and regression, which can cause sediment to shift or erode rather than deposit evenly. For example, sediment in a lake can have a greater terminal velocity, allowing it to settle on the bottom over days, which might be faster or slower than the average rate depending on the conditions.
Another reason involves changes in sea level, known as transgression and regression. During transgression, sea level rises and sediment types shift towards the coastline. Conversely, during regression, sea levels lower and this can move sediment facies in the opposite direction. If the sea level drops enough to expose sediment, erosion instead of deposition will likely occur, significantly affecting deposition rates.