A delta is a distinctive body of sediment supplied by a river that accumulates along a coast line. Much of the delta is underwater. It is not surprising that in different parts of the delta different types of sediment are deposited. The more offshore and the deeper the site, the finer the sediment is one general rule. Often deltas have three parts, a topset with shallow water or emergent sediments, a foreset which is the front slope of the delta and which storms and floods will supply sediment to, and a bottomset, which is deep and quiet water deposition. This geometry is schematically depicted in the upper cross section. With time more and more sediment is supplied and the delta builds out (progrades). This is depicted in the lower cross section by the repeated brown colored lines representing the top, bottom and foresets. The result is three stacked geologic units - a lower mud rich unit, an upper sandstone unit and a mixed unit between. Important to note is that the three units are not of three different ages. They are different facies that are related to each other reflecting shifting depositional environments.
The diagrams below are from:
The Niger Delta Petroleum System: Niger Delta Province, Nigeria,
Cameroon, and Equatorial Guinea, Africa by Michele L. W. Tuttle,
Ronald R. Charpentier, and Michael E. Brownfield of the USGS
http://greenwood.cr.usgs.gov/energy/WorldEnergy/OF99-50H/ChapterA/fig9.html#TOP.
It nicely shows how the Niger delta coastline progrades
with time, but can also retreat due to sea level rise, as it has
done since the Pleistocene to now. Thus, sea level changes can
cause depositional environments to shift also.
Below is a portion of a seismic
cross section of an old buried delta up near the north shore of
Alaska. To create this image seismic waves were sent down into
the earth, and the return signals from reflections off rock boundaries
were recorded. The study this was taken from was investigating
the oil potential for these rocks. For our purposes the distinctive
observations to make is the topset, foreset and bottomset geometry
that can be seen. http://pubs.usgs.gov/of/2003/of03-039/captions.htm
-U.S. Geological Survey Open-File Report 03-039.
