of tide-aligned low-tidal to mid-tidal sand shoals, bars,
and lenses that have shoaled to the level of high tide and
that have been covered by floodplain sand deposits.
The sand-and-mud-dominated delta developed under
fluvial-dominated conditions is a wedge of sand overlain
by muddy sand and in turn overlain by mud prograded
into the estuary. The sand-and-mud-dominated delta
developed under wave-dominated conditions is a sheet
of mud and muddy sand deposited at levels below
the prevailing wave base and capping by a wedge of
sand of beach-to-beach ridges, stacked and prograded into
the estuary. The sand-and-mud-dominated delta developed under tide-dominated conditions is a sequence of
low-tidal to mid-tidal, tide-aligned sand shoals, bars, and
lenses that have shoaled to the level of high tide
progressing through lithologies of muddy sand and mud
and finally capped by floodplain mud deposits.
The mud-dominated delta developed under fluvialdominated conditions is a wedge of mud prograded into
the estuary. The mud-dominated delta developed under
(moderate) wave-dominated conditions (i.e., prevailing
waves hydrodynamically dominate over tides and river
flow) is generally a wedge of mud prograded into the
estuary, but with the wave action and intermittent storms,
there is local concentration of exogenic sand and biogenic
sand and gravel through winnowing. These coarser sediments find expression in sheets of muddy sand, lenses of
sand, and in cheniers. Mud accumulates below the
prevailing wave base. The mud-dominated delta developed under tide-dominated conditions is a sequence of
low-tidal to mid-tidal tide-aligned mud shoals that have
shoaled to the level of high tide to form mud sheets that
have been covered subsequently by floodplain mud
deposits.
Case studies of the gross stratigraphy of fluvialdominated,
wave-dominated,
or
tide-dominated
intra-estuarine deltas from southwestern Australia demonstrating generalized lithology in terms of sand, muddy
sand, and mud are illustrated in Figure 7. The fluvialdominated Harvey River delta shows a finger of sand
(the prograded fans of sand at the delta front) overlying
prodelta muddy sand and levee deposits of mud.
The fluvial-dominated Collie River delta shows a sheet
of riverine sand overlying prodelta mud and a capping of
finer sand that has developed by construction of
cheniers. The wave-dominated Deep River delta shows
a stratigraphy of delta front and prodelta subtidal muddy
sand overlain by the sand of beaches and beach ridges,
with muddy sand filling inter-beach ridge swales.
The wave-dominated Murray River delta shows
a stratigraphy of delta front and prodelta subtidal sand
overlain by sand of beaches and beach ridges and bars,
with muddy sand filling inter-beach ridge swales and
lagoons in a prograded bar-and-lagoon sequence. The
tide-dominated Preston River delta shows a stratigraphy
of subtidal sand that has shoaled through muddy sand with
a capping of low-tidal to high-tidal mud.
Discussion and conclusions
Intra-estuarine deltas (also termed “bayhead” deltas) are the
fluvial deposits that accumulate where one or more rivers
enter an estuary. As with deltas formed in open marine
coastal environments, these deltas can be classified as to
morphology based on the response of the riverine sedimentary deposits to hydrodynamic setting. As such, deltas
developed by fluvial-dominated, wave-dominated, or tidedominated conditions can be identified. However, unlike
the open marine coastal environment where the hydrodynamic conditions are regionally more uniform, deltas in
estuaries experience a diversity of hydrodynamic conditions in the one deltaic setting and across the estuary. This
is particularly the case where the estuary is large and complex in shape and where there is a strong component of
wind that directs surface currents and wind waves. The
main factors determining the morphology and landforms
of deltas within estuaries are (1) the seasonality and strength
of river flow which determines hydrodynamic conditions
and the supply of sediment; (2) the salinity of the
estuarine-receiving basin which determines the style of
interchange of the river water with estuarine water
(hypopycnal flow versus homopycnal flow versus
hyperpycnal flow), the style of sediment delivery into the
estuary, and, to some extent, the shape of the delta; (3) the
magnitude of tides and wind waves which, in concert with
the magnitude of river flow, will determine whether the
hydrodynamic conditions will be dominated by fluvial,
wave, or tidal processes; (4) the shape of the estuary; and
(5) where the river(s) is/are located. Relatively simple estuaries, e.g., narrow linear valley tracts with a single river
mouth, are subject to interactions of river flow, tidal flux,
and wind waves, with the delta morphology reflecting the
locally dominant hydrodynamic condition. Complex estuaries and estuaries with large fetch create conditions where
complicated hydrodynamics of waves and wind-induced
currents interact with the shores of the estuaries and act on
the deposits of the river or rivers entering the estuary, each
river potentially being subject to differing hydrodynamics.
The morphology and landforms of intra-estuarine
deltas respond to fluvial, wave, and tidal conditions.
Fluvial-dominated deltas can be lobate, fan-shaped, elongate, or digitate. Wave-dominated deltas can be a lobate
complex of prograded beach ridges, a series of beach
ridges and/or recurved spits with intervening swales
and/or linear lagoons, or a prograded series of bars
and linear, oval to circular lagoons. The delta front of
tide-dominated deltas usually is a crenulate to palmate
complex of prograded subaqueous to tidal shoals, and
the landward part of the delta is a floodplain. Complexity
of local hydrodynamics may result in an intra-estuarine
delta with different morphologies in different parts of the
delta or contrasting landforms reflective of hydrodynamic
conditions in a specific part of that delta.
Stratigraphy of intra-estuarine deltas is variable from
delta to delta within the one estuary and variable between
deltas in different estuaries because of the sediment types
DELTAS
185
and lenses that have shoaled to the level of high tide and
that have been covered by floodplain sand deposits.
The sand-and-mud-dominated delta developed under
fluvial-dominated conditions is a wedge of sand overlain
by muddy sand and in turn overlain by mud prograded
into the estuary. The sand-and-mud-dominated delta
developed under wave-dominated conditions is a sheet
of mud and muddy sand deposited at levels below
the prevailing wave base and capping by a wedge of
sand of beach-to-beach ridges, stacked and prograded into
the estuary. The sand-and-mud-dominated delta developed under tide-dominated conditions is a sequence of
low-tidal to mid-tidal, tide-aligned sand shoals, bars, and
lenses that have shoaled to the level of high tide
progressing through lithologies of muddy sand and mud
and finally capped by floodplain mud deposits.
The mud-dominated delta developed under fluvialdominated conditions is a wedge of mud prograded into
the estuary. The mud-dominated delta developed under
(moderate) wave-dominated conditions (i.e., prevailing
waves hydrodynamically dominate over tides and river
flow) is generally a wedge of mud prograded into the
estuary, but with the wave action and intermittent storms,
there is local concentration of exogenic sand and biogenic
sand and gravel through winnowing. These coarser sediments find expression in sheets of muddy sand, lenses of
sand, and in cheniers. Mud accumulates below the
prevailing wave base. The mud-dominated delta developed under tide-dominated conditions is a sequence of
low-tidal to mid-tidal tide-aligned mud shoals that have
shoaled to the level of high tide to form mud sheets that
have been covered subsequently by floodplain mud
deposits.
Case studies of the gross stratigraphy of fluvialdominated,
wave-dominated,
or
tide-dominated
intra-estuarine deltas from southwestern Australia demonstrating generalized lithology in terms of sand, muddy
sand, and mud are illustrated in Figure 7. The fluvialdominated Harvey River delta shows a finger of sand
(the prograded fans of sand at the delta front) overlying
prodelta muddy sand and levee deposits of mud.
The fluvial-dominated Collie River delta shows a sheet
of riverine sand overlying prodelta mud and a capping of
finer sand that has developed by construction of
cheniers. The wave-dominated Deep River delta shows
a stratigraphy of delta front and prodelta subtidal muddy
sand overlain by the sand of beaches and beach ridges,
with muddy sand filling inter-beach ridge swales.
The wave-dominated Murray River delta shows
a stratigraphy of delta front and prodelta subtidal sand
overlain by sand of beaches and beach ridges and bars,
with muddy sand filling inter-beach ridge swales and
lagoons in a prograded bar-and-lagoon sequence. The
tide-dominated Preston River delta shows a stratigraphy
of subtidal sand that has shoaled through muddy sand with
a capping of low-tidal to high-tidal mud.
Discussion and conclusions
Intra-estuarine deltas (also termed “bayhead” deltas) are the
fluvial deposits that accumulate where one or more rivers
enter an estuary. As with deltas formed in open marine
coastal environments, these deltas can be classified as to
morphology based on the response of the riverine sedimentary deposits to hydrodynamic setting. As such, deltas
developed by fluvial-dominated, wave-dominated, or tidedominated conditions can be identified. However, unlike
the open marine coastal environment where the hydrodynamic conditions are regionally more uniform, deltas in
estuaries experience a diversity of hydrodynamic conditions in the one deltaic setting and across the estuary. This
is particularly the case where the estuary is large and complex in shape and where there is a strong component of
wind that directs surface currents and wind waves. The
main factors determining the morphology and landforms
of deltas within estuaries are (1) the seasonality and strength
of river flow which determines hydrodynamic conditions
and the supply of sediment; (2) the salinity of the
estuarine-receiving basin which determines the style of
interchange of the river water with estuarine water
(hypopycnal flow versus homopycnal flow versus
hyperpycnal flow), the style of sediment delivery into the
estuary, and, to some extent, the shape of the delta; (3) the
magnitude of tides and wind waves which, in concert with
the magnitude of river flow, will determine whether the
hydrodynamic conditions will be dominated by fluvial,
wave, or tidal processes; (4) the shape of the estuary; and
(5) where the river(s) is/are located. Relatively simple estuaries, e.g., narrow linear valley tracts with a single river
mouth, are subject to interactions of river flow, tidal flux,
and wind waves, with the delta morphology reflecting the
locally dominant hydrodynamic condition. Complex estuaries and estuaries with large fetch create conditions where
complicated hydrodynamics of waves and wind-induced
currents interact with the shores of the estuaries and act on
the deposits of the river or rivers entering the estuary, each
river potentially being subject to differing hydrodynamics.
The morphology and landforms of intra-estuarine
deltas respond to fluvial, wave, and tidal conditions.
Fluvial-dominated deltas can be lobate, fan-shaped, elongate, or digitate. Wave-dominated deltas can be a lobate
complex of prograded beach ridges, a series of beach
ridges and/or recurved spits with intervening swales
and/or linear lagoons, or a prograded series of bars
and linear, oval to circular lagoons. The delta front of
tide-dominated deltas usually is a crenulate to palmate
complex of prograded subaqueous to tidal shoals, and
the landward part of the delta is a floodplain. Complexity
of local hydrodynamics may result in an intra-estuarine
delta with different morphologies in different parts of the
delta or contrasting landforms reflective of hydrodynamic
conditions in a specific part of that delta.
Stratigraphy of intra-estuarine deltas is variable from
delta to delta within the one estuary and variable between
deltas in different estuaries because of the sediment types
DELTAS
185