Understood on the chemistry, but it appears that layer B could be associated with dredge material more than the clay layer?
Its funny that you posted this because I was just going to post:
"Elutriation is a process for separating particles based on their size, shape and density,..."
When mixed with water heavy things tend to settle at the bottom ... lighter things toward the top. The wet viscous dredging spoils would have followed this principle to some degree. Lighter material migrates and mixes more easily. I think its a distinct possibility the bottom of that cross bedded layer could have consisted of the lighter dredging sands and anything that traveled with it road along for the ride ... like bundles of Cooper money?
At a later date that mix of cross bedded sand would appear as a semi-uniform mass when it fact it consisted of a number of different sands laid down and worked at different times over a longer span of time. The upper active layer is without a doubt a briefer span of time compared to the cross bedded layers below it ... that would also account for the chemical contamination of the bills from the dredging spoils if the USGS chemist is right.
Its just a hypothesis.
PS: The Fazios had to wait several days for the spoils to settle and firm up before they could do any spreading. And depending on how far their blade dug down, they would have been spreading lighter more mobile material?