Where wave action is dominant because the delta
resides in an estuary with a strong component of wind
and wind-generated waves, regardless of the mechanism
that delivers sediment to the front of the river mouth
(viz., hypopycnal, homopycnal, or hyperpycnal), the sediment deposited at the river mouth is subsequently
reworked shoreward into a series of beach ridges, or
recurved spits, or bars and their leeward lagoons, all built
by waves and wind to levels of the high tide and above.
Progradation of the delta thus is by beach ridge accretion,
recurved spit accretion, or as a series of bars and lagoons.
The delta usually is 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
beach ridges, recurved spits, and bars are underlain by
sand and are often accreted to above the level of high tide.
The swales and linear, oval to circular lagoons, depending
on the style of sedimentary filling, are underlain by sand,
muddy sand, mud, or peat. River floods, unable to reach
the height of beach ridges, are confined to the distributary
channels or flood into the beach ridge swales.
Where tides are the dominant hydrodynamic force,
again, regardless of the mechanism that delivers sediment
to the front of the river mouth, the sediment is subsequently
reworked by tidal currents into tidal-current-aligned
(usually shore-normal) subaqueous sand shoals that accrete
vertically to levels of the high tide. Progradation of the delta
thus is by tidal shoal vertical accretion to a level where the
deposits are finally capped by floodplain sediments.
The delta front (or delta slope) usually is a crenulate to
palmate complex of prograded subaqueous to tidal shoals,
and the landward part of the delta is a floodplain.
Locally in estuaries, where river gradients are relatively
steep, there may be development of Gilbert-type deltas.
These are a specific type of fluvial-dominated delta, usually fan-shaped and coarse-grained, with internal geometry of simple large cross-stratification corresponding to
the delta morphology of topset, foreset, and bottomset
(Postma, 1990).
While deltas can be classified as fluvial dominated, tide
dominated, or wave dominated depending on their hydrodynamic setting, often in estuaries, because of the complexity of the hydrodynamics, an intra-estuarine delta
may exhibit different morphology in different parts of the
delta or contrasting landforms reflective of hydrodynamic
conditions in a specific part of that delta. For instance, the
wave-dominated intra-estuarine delta of the Deep River
in southern Western Australia (the Walpole-Nornalup
Inlet Estuary; Semeniuk et al., 2011) comprises two distinct
geomorphic responses reflecting different degrees of
wave action and sediment transport/mobility. There are
prograded beach ridges on one half of the delta and
a prograded bar-and-lagoon complex on the more sheltered
other half. In the same estuary, another intra-estuarine delta
(the Frankland River) also reflects the variable hydrodynamic forces across the delta depositional environment. It
comprises a wave-dominated part in its northern third
(prograded beach ridges and inter-ridge swales), a fluvialdominated part in its central third (prograded and shoaled
digitate/palmate sedimentary accumulation), and a tidedominated part in its southern third (prograded sand platform). Within an intra-estuarine delta in another estuary
in southwestern Australia (the Leschenault Inlet Estuary;
Semeniuk, 2000), one side of a fluvial-dominated palmate
delta faces a 12 km fetch, and during intermittent winter
storms deriving from the north-west, waves break on the
shore to create a repetition of storm-wave-generated
cheniers across the floodplain.
In hydrodynamically and geomorphically complex
estuaries with multiple river entries, there may be
a range of intra-estuarine types within the same estuary.
For instance, the Peel-Harvey Estuary of southwestern
Australia, with three river entries (Semeniuk and
Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b), has
two wave-dominated deltas (composed of prograded
bar-and-lagoon complexes) because they face the
prevailing regional summer breezes that generate wind
waves on the estuarine water body and one fluvialdominated delta (composed of prograded fans of sand,
levee deposits, and floodplain deposits) that is not subject
to these wind waves. The Leschenault Inlet Estuary, with
two river entries (Semeniuk, 2000), has one fluvialdominated delta (a palmate delta) and another delta that
is, in part, tide dominated (composed of tidally-aligned
shoals) and, in part, fluvial dominated. The WalpoleNornalup Inlet Estuary of southern Australia, with three
river entries (Semeniuk et al., 2011), has two wavedominated estuaries (composed of prograded bar-andlagoon complexes, or of beach ridges) because they face
the prevailing regional summer breezes that generate wind
waves on the estuarine water body and one hydrodynamically complex delta that is one third wave dominated
(facing the wind waves generated by sea breezes), one
third fluvial dominated, and one third tide dominated
(the latter two not subject to wind waves).
Factors determining the stratigraphy
of deltas in estuaries
Deltas within estuaries exhibit a variety of stratigraphic
sequences, depending on the sediments available, and their
hydrodynamic setting. However, given that estuaries as
enclosed to semi-enclosed coastal bodies of water where
multiple interactions between marine and riverine environments may take place, there are other factors that result in
a richness and variety of stratigraphy in intra-estuarine
deltas. These differences not only occur between deltas
from different estuaries but even between deltas within
the same estuary. The main factors determining the stratigraphy within a delta are (1) the provenance of the contributing rivers and types of sediment entering the estuary,
(2) seasonality and strength of river flow, (3) nature of tides,
(4) contribution of wind and wind waves, and (5) climate.
The provenance of the contributing rivers entering the
estuary, that is, the geology of the drainage basins,
182
DELTAS
resides in an estuary with a strong component of wind
and wind-generated waves, regardless of the mechanism
that delivers sediment to the front of the river mouth
(viz., hypopycnal, homopycnal, or hyperpycnal), the sediment deposited at the river mouth is subsequently
reworked shoreward into a series of beach ridges, or
recurved spits, or bars and their leeward lagoons, all built
by waves and wind to levels of the high tide and above.
Progradation of the delta thus is by beach ridge accretion,
recurved spit accretion, or as a series of bars and lagoons.
The delta usually is 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
beach ridges, recurved spits, and bars are underlain by
sand and are often accreted to above the level of high tide.
The swales and linear, oval to circular lagoons, depending
on the style of sedimentary filling, are underlain by sand,
muddy sand, mud, or peat. River floods, unable to reach
the height of beach ridges, are confined to the distributary
channels or flood into the beach ridge swales.
Where tides are the dominant hydrodynamic force,
again, regardless of the mechanism that delivers sediment
to the front of the river mouth, the sediment is subsequently
reworked by tidal currents into tidal-current-aligned
(usually shore-normal) subaqueous sand shoals that accrete
vertically to levels of the high tide. Progradation of the delta
thus is by tidal shoal vertical accretion to a level where the
deposits are finally capped by floodplain sediments.
The delta front (or delta slope) usually is a crenulate to
palmate complex of prograded subaqueous to tidal shoals,
and the landward part of the delta is a floodplain.
Locally in estuaries, where river gradients are relatively
steep, there may be development of Gilbert-type deltas.
These are a specific type of fluvial-dominated delta, usually fan-shaped and coarse-grained, with internal geometry of simple large cross-stratification corresponding to
the delta morphology of topset, foreset, and bottomset
(Postma, 1990).
While deltas can be classified as fluvial dominated, tide
dominated, or wave dominated depending on their hydrodynamic setting, often in estuaries, because of the complexity of the hydrodynamics, an intra-estuarine delta
may exhibit different morphology in different parts of the
delta or contrasting landforms reflective of hydrodynamic
conditions in a specific part of that delta. For instance, the
wave-dominated intra-estuarine delta of the Deep River
in southern Western Australia (the Walpole-Nornalup
Inlet Estuary; Semeniuk et al., 2011) comprises two distinct
geomorphic responses reflecting different degrees of
wave action and sediment transport/mobility. There are
prograded beach ridges on one half of the delta and
a prograded bar-and-lagoon complex on the more sheltered
other half. In the same estuary, another intra-estuarine delta
(the Frankland River) also reflects the variable hydrodynamic forces across the delta depositional environment. It
comprises a wave-dominated part in its northern third
(prograded beach ridges and inter-ridge swales), a fluvialdominated part in its central third (prograded and shoaled
digitate/palmate sedimentary accumulation), and a tidedominated part in its southern third (prograded sand platform). Within an intra-estuarine delta in another estuary
in southwestern Australia (the Leschenault Inlet Estuary;
Semeniuk, 2000), one side of a fluvial-dominated palmate
delta faces a 12 km fetch, and during intermittent winter
storms deriving from the north-west, waves break on the
shore to create a repetition of storm-wave-generated
cheniers across the floodplain.
In hydrodynamically and geomorphically complex
estuaries with multiple river entries, there may be
a range of intra-estuarine types within the same estuary.
For instance, the Peel-Harvey Estuary of southwestern
Australia, with three river entries (Semeniuk and
Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b), has
two wave-dominated deltas (composed of prograded
bar-and-lagoon complexes) because they face the
prevailing regional summer breezes that generate wind
waves on the estuarine water body and one fluvialdominated delta (composed of prograded fans of sand,
levee deposits, and floodplain deposits) that is not subject
to these wind waves. The Leschenault Inlet Estuary, with
two river entries (Semeniuk, 2000), has one fluvialdominated delta (a palmate delta) and another delta that
is, in part, tide dominated (composed of tidally-aligned
shoals) and, in part, fluvial dominated. The WalpoleNornalup Inlet Estuary of southern Australia, with three
river entries (Semeniuk et al., 2011), has two wavedominated estuaries (composed of prograded bar-andlagoon complexes, or of beach ridges) because they face
the prevailing regional summer breezes that generate wind
waves on the estuarine water body and one hydrodynamically complex delta that is one third wave dominated
(facing the wind waves generated by sea breezes), one
third fluvial dominated, and one third tide dominated
(the latter two not subject to wind waves).
Factors determining the stratigraphy
of deltas in estuaries
Deltas within estuaries exhibit a variety of stratigraphic
sequences, depending on the sediments available, and their
hydrodynamic setting. However, given that estuaries as
enclosed to semi-enclosed coastal bodies of water where
multiple interactions between marine and riverine environments may take place, there are other factors that result in
a richness and variety of stratigraphy in intra-estuarine
deltas. These differences not only occur between deltas
from different estuaries but even between deltas within
the same estuary. The main factors determining the stratigraphy within a delta are (1) the provenance of the contributing rivers and types of sediment entering the estuary,
(2) seasonality and strength of river flow, (3) nature of tides,
(4) contribution of wind and wind waves, and (5) climate.
The provenance of the contributing rivers entering the
estuary, that is, the geology of the drainage basins,
182
DELTAS