Deltas and Estuaries

Carboniferous delta channels of

Western Ireland, cliffs of Moher

Architecture is governed by tides, wave activity and the degree of clastic input from rivers. In microtidal environments with low wave energy “birdsfoot deltas” such as the Mississippi can build up due to local sedimentation and lack of reworking. In these settings sedimentation is concentrated close to the distributaries, although these switch around on 10s of yr. scale as sediment builds up in one area, making other basins more advantageous. Between the levee-defined channels sedimentation in marshy bays is quiet and fine grained. Break-out of levees during flood stage leads to infill of these bays so producing fining-upward sequences that are topped by muddy soil horizons.

In area of low tide but high wave activity, e.g., Indus and Niger a rounded delta front is built up. Many modern deltas are in a drowned state today because post-glacial sealevel rise. High tidal activity results in deep embayments in the delta front. The deeper water area in front of a delta is marked by slumping and soft sediment deformation. Muddy layers can be reactivated as diapirs around which later sedimentation is diverted. Ancient analogue is the Carboniferous (Pennsylvanian) deltas of British Isles.

Deltas in continental margin stratigraphy

Delta classification scheme

Delta types

Indus Delta - drowned wave dominated

Yukon Delta

Mississippi birds foot delta, river dominated

Mississippi Delta - block model

Mass wasting on delta fronts

Mississippi bay fill model and example

Mississippi bay fill stratigraphic cross section

Bay filling deltas - stratigraphic model

Levee breach and bay fill, Mississippi

Breached levee results in infilling of inter-distributary bays

Mississippi River - sea-level response

Switching of Mississippi delta in recent geologic past

Cross bedded channel sandstones in Jurassic delta sandstone, UK

Overbank sands, Jurassic UK

Root bed and paleosol in interdistributary overbank sediments

Jurassic delta channel of NE England

Flame structures, water escape

Fossil wood in channel sandstone

Load casts in channel sandstones

Channeled and climbing rippled sands, Jurassic UK

Irish Carboniferous, bottom feeding traces

Estuary model

Yangtze Estuary

Wave dominated river mouths

Estuary tidal bars - stratigraphic model

Pre-flood estuary

River in flood

Coastal bar starts to reform

Coastal bar reforms

Vegetation reforms, system stabilizes