Coastal lagoons
Lagoons comprise the open-water areas between the barrier proper and the mainland beach. The occurrence of
overwash builds out the barrier platform, reducing the
tidal prism and altering circulation within the lagoon
(Cooper, 1994). Howard and Frey (1985) characterized
lagoons as salt marsh estuaries, driven by tidal circulation,
as opposed to riverine estuaries, which have a freshwater
river source at their head. They noted that the sedimentary
characteristics of the two environments were similar and
therefore difficult to distinguish in the rock record. Coastal
lagoon sediments are composed of silt and clay and are
extensively bioturbated (Oertel (1985). Lagoons can be
characterized as open-water lagoons or expandable
lagoons. Open-water lagoons have a relatively constant
water surface area. The surface area of expandable
lagoons may vary by as much as 50 % between spring
low and high tides. The latter can evolve into the former,
provided that the rate of submergence due to sea-level rise
exceeds the rate of sediment accretion (Oertel, 1985).
Flood-tidal deltas
Flood-tidal deltas are formed by tidal sediments deposited
landward of an inlet mouth. As inlet channels fill and
inlets migrate, flood-tidal deltas become inactive and
eventually become part of the barrier (Carrasco et al.,
2008). This process is one of the principal means by which
the backbarrier environment builds outward (Godfrey and
Godfrey, 1974). One of the most common backbarrier sedimentary sequences fines upward from coarser inlet
deposits to fine-grained flood-tidal delta sands to salt
marsh. These sequences comprise a major part of the barrier facies and account for up to half of the Holocene barrier sediment (Moslow and Tye, 1985).
Washover fans
Washover fans are the accumulated product of short-term
depositional events during storms that breach the barrier
front. Overwash, which affects both the width and height
of the barrier platform, is a major control on backbarrier
development. When a storm event causes marine water
to reach the lagoon, lenticular washover fans are deposited
on the backbarrier margin (Carter, 1988). The washover
sediments are the result of erosion of barrier dune and
beach environments and overlie former salt marsh
(Schwartz, 1981). The importance of overwash as part of
the barrier lithesome depends on the bathymetry of the
foreshore (Ritchie and Penland, 1988), wave conditions
(Fisher et al., 1974), and elevation of backbarrier beaches
(Morton and Sallenger, 2003). Overwash can have either
a positive or negative effect on backbarrier evolution,
depending on the frequency and intensity of overwash
events (Godfrey and Godfrey, 1974).
Sedgwick and Davis (2003) described the characteristics of washover facies. Washover beds are typically
landward-dipping plane beds of well-sorted sand. Shell
beds and heavy mineral laminae are often interbedded
with sand layers. Bioturbation and reworking by later
events can overprint the record. Washover deposits are
often difficult to distinguish from flood-tidal delta sediments. Washover deposits in the stratigraphic record are
characterized by (1) landward thinning, (2) occurrence of
clean sand deposits within the find-grained backbarrier
sediments, and (3) presence of shoreface and backbarrier
mollusk shells (Sedgwick and Davis, 2003).
Intertidal flats
Intertidal flats lie at elevations between mean high and
mean low tide. They may be thought of as salt marshes
lacking in vegetation and provide the substrate upon
which salt marshes build. The sediments of intertidal flats
consist of interbedded mud and sand, representing cyclic
changes in tidal current velocities (Howard and Frey,
1985). Bedding varies from flaser to wavy to lenticular,
depending on the relative proportion of sand and mud
(Reineck and Wunderlich, 1968).
Intertidal marshes
The backbarrier marsh environment includes grass beds
and tidal channels lying within the range of mean tides.
Backbarrier marshes generally evolve on tidal flats situated between the tidal channels of an abandoned inlet system (Kraft et al., 1979). Tidal current velocities flowing
over tidal marsh surfaces are typically an order of magnitude lower than those observed in tidal channels (Howard
and Frey, 1985). Bartholdy et al. (2010) reported that the
backbarrier marsh is highly sensitive to the rate of
sea-level rise. Continued deposition in the marsh environment requires a positive and constant rate of sea-level
change. Sea-level stasis, or an increase in the long-term
rate of rise, leads to loss of the marsh. Godfrey and
Godfrey (1974) noted that excessive overwash can overcome the ability of the marshes to recover and lead to
destruction of the marsh environment. The higher elevations in the salt marsh, however, are dependent on
overwash events to supply sediment for accretion
(French and Spencer, 1993).
Carrasco et al. (2008) developed an evolutionary
model for the backbarrier environment, based on the
linear extent of salt marsh development along the
backbarrier shoreline versus the length of non-vegetated
backbarrier beach. The ratio of salt marsh to beach was
found to be related to changes in local hydrodynamic conditions. A decrease in hydrodynamic intensity results in
a higher ratio of marsh length to beach length. An
increase in hydrodynamic intensity, such as the creation
of new overwash pathways, results in a lower ratio. The
model can be employed to project future changes in the
backbarrier environment.
Summary
Backbarrier sediments are a complex of various
interfingering subenvironments. Facies models of the several subenvironments can be useful in identifying barrier
BACKBARRIER
45
Lagoons comprise the open-water areas between the barrier proper and the mainland beach. The occurrence of
overwash builds out the barrier platform, reducing the
tidal prism and altering circulation within the lagoon
(Cooper, 1994). Howard and Frey (1985) characterized
lagoons as salt marsh estuaries, driven by tidal circulation,
as opposed to riverine estuaries, which have a freshwater
river source at their head. They noted that the sedimentary
characteristics of the two environments were similar and
therefore difficult to distinguish in the rock record. Coastal
lagoon sediments are composed of silt and clay and are
extensively bioturbated (Oertel (1985). Lagoons can be
characterized as open-water lagoons or expandable
lagoons. Open-water lagoons have a relatively constant
water surface area. The surface area of expandable
lagoons may vary by as much as 50 % between spring
low and high tides. The latter can evolve into the former,
provided that the rate of submergence due to sea-level rise
exceeds the rate of sediment accretion (Oertel, 1985).
Flood-tidal deltas
Flood-tidal deltas are formed by tidal sediments deposited
landward of an inlet mouth. As inlet channels fill and
inlets migrate, flood-tidal deltas become inactive and
eventually become part of the barrier (Carrasco et al.,
2008). This process is one of the principal means by which
the backbarrier environment builds outward (Godfrey and
Godfrey, 1974). One of the most common backbarrier sedimentary sequences fines upward from coarser inlet
deposits to fine-grained flood-tidal delta sands to salt
marsh. These sequences comprise a major part of the barrier facies and account for up to half of the Holocene barrier sediment (Moslow and Tye, 1985).
Washover fans
Washover fans are the accumulated product of short-term
depositional events during storms that breach the barrier
front. Overwash, which affects both the width and height
of the barrier platform, is a major control on backbarrier
development. When a storm event causes marine water
to reach the lagoon, lenticular washover fans are deposited
on the backbarrier margin (Carter, 1988). The washover
sediments are the result of erosion of barrier dune and
beach environments and overlie former salt marsh
(Schwartz, 1981). The importance of overwash as part of
the barrier lithesome depends on the bathymetry of the
foreshore (Ritchie and Penland, 1988), wave conditions
(Fisher et al., 1974), and elevation of backbarrier beaches
(Morton and Sallenger, 2003). Overwash can have either
a positive or negative effect on backbarrier evolution,
depending on the frequency and intensity of overwash
events (Godfrey and Godfrey, 1974).
Sedgwick and Davis (2003) described the characteristics of washover facies. Washover beds are typically
landward-dipping plane beds of well-sorted sand. Shell
beds and heavy mineral laminae are often interbedded
with sand layers. Bioturbation and reworking by later
events can overprint the record. Washover deposits are
often difficult to distinguish from flood-tidal delta sediments. Washover deposits in the stratigraphic record are
characterized by (1) landward thinning, (2) occurrence of
clean sand deposits within the find-grained backbarrier
sediments, and (3) presence of shoreface and backbarrier
mollusk shells (Sedgwick and Davis, 2003).
Intertidal flats
Intertidal flats lie at elevations between mean high and
mean low tide. They may be thought of as salt marshes
lacking in vegetation and provide the substrate upon
which salt marshes build. The sediments of intertidal flats
consist of interbedded mud and sand, representing cyclic
changes in tidal current velocities (Howard and Frey,
1985). Bedding varies from flaser to wavy to lenticular,
depending on the relative proportion of sand and mud
(Reineck and Wunderlich, 1968).
Intertidal marshes
The backbarrier marsh environment includes grass beds
and tidal channels lying within the range of mean tides.
Backbarrier marshes generally evolve on tidal flats situated between the tidal channels of an abandoned inlet system (Kraft et al., 1979). Tidal current velocities flowing
over tidal marsh surfaces are typically an order of magnitude lower than those observed in tidal channels (Howard
and Frey, 1985). Bartholdy et al. (2010) reported that the
backbarrier marsh is highly sensitive to the rate of
sea-level rise. Continued deposition in the marsh environment requires a positive and constant rate of sea-level
change. Sea-level stasis, or an increase in the long-term
rate of rise, leads to loss of the marsh. Godfrey and
Godfrey (1974) noted that excessive overwash can overcome the ability of the marshes to recover and lead to
destruction of the marsh environment. The higher elevations in the salt marsh, however, are dependent on
overwash events to supply sediment for accretion
(French and Spencer, 1993).
Carrasco et al. (2008) developed an evolutionary
model for the backbarrier environment, based on the
linear extent of salt marsh development along the
backbarrier shoreline versus the length of non-vegetated
backbarrier beach. The ratio of salt marsh to beach was
found to be related to changes in local hydrodynamic conditions. A decrease in hydrodynamic intensity results in
a higher ratio of marsh length to beach length. An
increase in hydrodynamic intensity, such as the creation
of new overwash pathways, results in a lower ratio. The
model can be employed to project future changes in the
backbarrier environment.
Summary
Backbarrier sediments are a complex of various
interfingering subenvironments. Facies models of the several subenvironments can be useful in identifying barrier
BACKBARRIER
45