Definition
Sediment composition in estuaries relates to the different
types of sediments found along and across an estuarine
waterbody.
Introduction
Estuarine sediment composition is a complex topic requiring some background on estuarine classification. Estuarine classifications may be based on physiography
(Kinsman and Pritchard, 1965), tidal range (Hayes,
1975), evolution (Dalrymple et al., 1992), morphology
(Fairbridge, 1980), or morphogenesis (Perillo, 1995).
For a classification to be of use, it must be based on physical and geological parameters that are common to all estuaries but that are different from one estuary type to another
(Perillo, 1995). For example, tidal currents, winds, and
sediments are all common enough to all estuaries, but variations of these parameters within a single estuary can be
very large, so a classification based only on pattern distributions of these parameters would not be possible. Perillo
(1995) argued that all previous estuarine classifications
were too inclusive, so he introduced a genetic differentiation of estuaries which separated them into primary and
secondary estuaries. Primary estuaries have not been
changed significantly by marine processes, whereas secondary estuaries have evolved into different forms since
their genesis. Estuaries can then be classified into
a number of categories such as coastal plain estuaries, rias,
fjords, delta front estuaries, structural estuaries, coastal
lagoons, and others (see Perillo, 1995, for a more complete
category list).
Sediment composition in estuaries is strongly
influenced by the tidal range, wave heights (near the estuary mouth), sediment availability, and sediment transport
processes, as shown in the Figure 1 (Dalrymple, 1992;
Bianchi, 2013). Also, estuarine sediment composition is
heavily dependent on the dominant source of sediment,
which is either alluvial or marine. Independently of the
estuary type, sediment composition in an estuarine environment varies axially and laterally, as well as vertically
(Nichols and Biggs, 1985). Towards the head of the estuary, sediment composition includes silts, clays, plants,
and roots, grading down to sand, gravel, and cobbles.
Large clay and silt deposits may be found towards the
mixed region of the estuary, together with sandy lenses
and laminae. Nearer to the estuary mouth, the main sediments are marine sands with abundant cross-bedding,
and tidally driven sandbanks may be found here. These
sandbanks have a low-angle cross-bedding in fine sands
with silt laminae. Lateral variations also occur, with shorelines composed of sand, gravel, shells, muds, plant fragments, and basal peat; subtidal flats composed of
laminated muddy sands and sandy muds; and
mid-channel environments dominated by coarse marine
sands and massive cross-bedding. Sediment composition
will now be discussed for some of the estuary types cited
above.
Sediment composition in coastal plain estuaries
Most estuaries may be found in former river valleys on
subtropical and temperate regions or former glacial valleys in polar and subpolar regions (Bokuniewicz, 1995).
In pure coastal plain estuaries, sediment load provided
by the rivers is small compared with the forces
redistributing the material (e.g., current acceleration and
gravity). Low-relief, coastal plain coasts favor the development of salt marshes and mangroves, with the subsequent infill of biological matter. Ocean waves will drive
sand towards the mouth of the estuary, where complex
systems of sandbars, spits, or barrier beaches may form.
Marine sands may be transported landwards by
overwashing of barriers by waves, transport due to tidal
asymmetry, or transport by tides and estuarine circulation
(Officer, 1981). In the central parts of the estuaries, finegrained sediments may be found, consisting of submerged
muds with abundant plant debris, or possibly fluid muds.
The head of the estuary is characterized by fluvial
sediment deposits with abundant plant debris and some
brackish fauna. Some impression of the reversing tidal
action will be preserved in the sedimentary structures
(Goldring et al., 1978).
Sediment composition in delta front estuaries
Contrary to coastal plain estuaries, delta front estuaries
have small forces redistributing the sediment compared
to the amount of material available. This causes the formation of deltas, sediment protuberances of riversupplied sediment around the estuary mouth. A delta
front estuary is the portion of the delta that is affected
by tidal dynamics and water mixing (Hart, 1995). The
factors controlling delta form and size include the characteristics of the source basin, which affect sediment size
and supply, and the characteristics of the receiving basin,
which affect the redistribution of that sediment. These
factors are, in turn, controlled by climate, tectonics, and
their interactions (Elliot, 1986). Attempts have been
made to classify delta morphology in terms of fluvial,
tidal, and wave influences (Galloway, 1975), but the variability found between deltas can only be explained by
including other factors, such as the characteristics of the
alluvial feeder system, the sediment grain size, the water
depth of the receiving basin, and change in mean sea level
(Postma, 1990; Orton and Reading, 1993). The alluvial
feeder system supplies most of the water and sediment
to the delta. Sediment yield will determine whether
the delta will prograde, aggrade, or be transgressed.
Sediment grain-size characteristics affect channel morphology, with the ratio of suspended load to bed load
being a primary controlling factor (Schumm, 1977).
Mud-dominated deltas form at the mouth of most long
rivers; in humid tropical areas, alluvial systems supply
large volumes of fine sediments to the delta; fan deltas,
and braidplain deltas, are composed of coarse sediments
(see Hart, 1995, and references therein). Finally, tectonic
and climatic processes combine to redistribute the
286
ESTUARINE SEDIMENT COMPOSITION
Sediment composition in estuaries relates to the different
types of sediments found along and across an estuarine
waterbody.
Introduction
Estuarine sediment composition is a complex topic requiring some background on estuarine classification. Estuarine classifications may be based on physiography
(Kinsman and Pritchard, 1965), tidal range (Hayes,
1975), evolution (Dalrymple et al., 1992), morphology
(Fairbridge, 1980), or morphogenesis (Perillo, 1995).
For a classification to be of use, it must be based on physical and geological parameters that are common to all estuaries but that are different from one estuary type to another
(Perillo, 1995). For example, tidal currents, winds, and
sediments are all common enough to all estuaries, but variations of these parameters within a single estuary can be
very large, so a classification based only on pattern distributions of these parameters would not be possible. Perillo
(1995) argued that all previous estuarine classifications
were too inclusive, so he introduced a genetic differentiation of estuaries which separated them into primary and
secondary estuaries. Primary estuaries have not been
changed significantly by marine processes, whereas secondary estuaries have evolved into different forms since
their genesis. Estuaries can then be classified into
a number of categories such as coastal plain estuaries, rias,
fjords, delta front estuaries, structural estuaries, coastal
lagoons, and others (see Perillo, 1995, for a more complete
category list).
Sediment composition in estuaries is strongly
influenced by the tidal range, wave heights (near the estuary mouth), sediment availability, and sediment transport
processes, as shown in the Figure 1 (Dalrymple, 1992;
Bianchi, 2013). Also, estuarine sediment composition is
heavily dependent on the dominant source of sediment,
which is either alluvial or marine. Independently of the
estuary type, sediment composition in an estuarine environment varies axially and laterally, as well as vertically
(Nichols and Biggs, 1985). Towards the head of the estuary, sediment composition includes silts, clays, plants,
and roots, grading down to sand, gravel, and cobbles.
Large clay and silt deposits may be found towards the
mixed region of the estuary, together with sandy lenses
and laminae. Nearer to the estuary mouth, the main sediments are marine sands with abundant cross-bedding,
and tidally driven sandbanks may be found here. These
sandbanks have a low-angle cross-bedding in fine sands
with silt laminae. Lateral variations also occur, with shorelines composed of sand, gravel, shells, muds, plant fragments, and basal peat; subtidal flats composed of
laminated muddy sands and sandy muds; and
mid-channel environments dominated by coarse marine
sands and massive cross-bedding. Sediment composition
will now be discussed for some of the estuary types cited
above.
Sediment composition in coastal plain estuaries
Most estuaries may be found in former river valleys on
subtropical and temperate regions or former glacial valleys in polar and subpolar regions (Bokuniewicz, 1995).
In pure coastal plain estuaries, sediment load provided
by the rivers is small compared with the forces
redistributing the material (e.g., current acceleration and
gravity). Low-relief, coastal plain coasts favor the development of salt marshes and mangroves, with the subsequent infill of biological matter. Ocean waves will drive
sand towards the mouth of the estuary, where complex
systems of sandbars, spits, or barrier beaches may form.
Marine sands may be transported landwards by
overwashing of barriers by waves, transport due to tidal
asymmetry, or transport by tides and estuarine circulation
(Officer, 1981). In the central parts of the estuaries, finegrained sediments may be found, consisting of submerged
muds with abundant plant debris, or possibly fluid muds.
The head of the estuary is characterized by fluvial
sediment deposits with abundant plant debris and some
brackish fauna. Some impression of the reversing tidal
action will be preserved in the sedimentary structures
(Goldring et al., 1978).
Sediment composition in delta front estuaries
Contrary to coastal plain estuaries, delta front estuaries
have small forces redistributing the sediment compared
to the amount of material available. This causes the formation of deltas, sediment protuberances of riversupplied sediment around the estuary mouth. A delta
front estuary is the portion of the delta that is affected
by tidal dynamics and water mixing (Hart, 1995). The
factors controlling delta form and size include the characteristics of the source basin, which affect sediment size
and supply, and the characteristics of the receiving basin,
which affect the redistribution of that sediment. These
factors are, in turn, controlled by climate, tectonics, and
their interactions (Elliot, 1986). Attempts have been
made to classify delta morphology in terms of fluvial,
tidal, and wave influences (Galloway, 1975), but the variability found between deltas can only be explained by
including other factors, such as the characteristics of the
alluvial feeder system, the sediment grain size, the water
depth of the receiving basin, and change in mean sea level
(Postma, 1990; Orton and Reading, 1993). The alluvial
feeder system supplies most of the water and sediment
to the delta. Sediment yield will determine whether
the delta will prograde, aggrade, or be transgressed.
Sediment grain-size characteristics affect channel morphology, with the ratio of suspended load to bed load
being a primary controlling factor (Schumm, 1977).
Mud-dominated deltas form at the mouth of most long
rivers; in humid tropical areas, alluvial systems supply
large volumes of fine sediments to the delta; fan deltas,
and braidplain deltas, are composed of coarse sediments
(see Hart, 1995, and references therein). Finally, tectonic
and climatic processes combine to redistribute the
286
ESTUARINE SEDIMENT COMPOSITION