River Delta - Sedimentary Structure

Sedimentary Structure

The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomset.

• The bottomset beds are created from the suspended sediment that settles out of the water as the river flows into the body of water and loses energy. The suspended load is carried out the furthest into the body of water than all other types of sediment creating a turbidite. These beds are laid down in horizontal layers and consist of smaller grains.
• The foreset beds in turn build over the bottomset beds as the main delta form advances. The foreset beds consist of the bed load that the river is moving along which consists of larger sediments that roll along the main channel. When it reaches the edge of the form, the bed load rolls over the edge, and builds up in steeply angled layers over the top of the bottomset beds. The angle of the outermost edge of the delta is created by the sediments angle of repose. As the foresets build outward (which make up the majority of the delta) they pile up and miniature landslides occur. This slope is created in this fashion as the bedload continues to be deposited and the delta moves outward. In cross section, one would see the foresets lying in angled, parallel bands, showing each stage of the creation of the delta.
• The topset beds in turn overlay the foresets, and are horizontal layers of smaller sediment size that form as the main channel of the river shifts elsewhere and the larger particles of the bed load no longer are deposited. As the channels move across the top of the delta, the suspended load settles out in horizontal beds over the top. The topset bed is subdivided into two regions: the upper delta plain and the lower delta plain. The upper delta plain is unaffected by the tide, while the boundary with the lower delta plain is defined by the upper limit of tidal influence.