Delta facies and progradation.

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. Click here for a PowerPoint animation of this process.

Usually it is difficult to clearly see the prograding forsets of a delta in the geologic record because they are large enough, and at a low-angle to the enclosing strata. However, in this image of Triassic strata in a cliff side from Edgeoya, Svalbard, a careful inspection will find layers at an angle sandwiched inbetween the other horizontal layers about two thirds the way up on the cliff side. The vertical distance between the top and bottom of these foresets is about 20-30 m and gives an indication of the minimum water depth at the time. Here the delta built out from right to left (towards the north), the opposite of the cartoon depiction above.

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 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 to create this cross section image. It is a bit like an ultrasound. 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. -U.S. Geological Survey Open-File Report 03-039.