Typically when you think of a sediment deposit, you think of a fining upwards sequence. That means that the heaviest/coarsest stuff is going to be at the bottom and the lighter stuff is going to be at the top, ‘cause you know, gravity settling. In laterally migrating streams, we see these fining upwards deposits of sand under silt under clay, but in a prograding delta, we see exactly the opposite pattern.
In these deposits, there’s actually a coarsening upwards trend. Even though sand is denser and coarser than silt and clay, a drill core would show clay under silt under sand. So how does this happen?
First, we have Time A. During this first stage, the sand falls out of suspension first and gets deposited at the top of the slope, then silt and clay fall out later and settle towards the bottom of the slope.
At Time B, sand, silt, and clay get deposited along the slope in the same sequence, but the sand from Time B is now lies on top of the finer silt from Time A.
Finally at Time C, we still have the same sequence of sediment deposition, but the sand from Time C is now on top of the silt from Time B, which is on top of the clay from Time A.
If you were to take a vertical drill core of this deposit, you have sand on top of silt on top of clay, a coarsening upwards sequence!
Emily Zawacki, Ph.D. 