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 leveedefined 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