3.2 Tidal Flats
neously, shells of nonnal marine organisms can be
swept in.
In humid regions, the lagoons often accumulate
organic matter, which later may fonn coal seams, and
their bottom waters can easily become anoxic. Arid
to semi-arid lagoons tend to collect more carbonate,
and their tidal deltas may become oolitic. Marginal
portions of the lagoons can periodically dry out and
accumulate gypsum (cf. Sect. 6.4.2). Migration of
eolian sand over dry lagoons is eommon. Tidal flat
sediments bordering the lagoon may include plant
roots, peat, or mierobial mats and stromatolites, as
weIl as evaporites fonned in coastal sabkhas.
Response of Barrier Island-Lagoon Complexes
to Sea-Level Changes
BaITier-island complexes are dynamic systems responding to changing sediment input and sea-Ievel
changes. Rapid sea-Ievel fall (or emergence of the
baITier system by uplift) tends to destroy the sedimentary eomplex by erosion and is therefore not diseussed further here. With eonstant or rising sea level,
the island chain can migrate either land ward or seaward and thus bury and preserve some of its older
deposits (Fig. 3.l8b).
(1) If sediment input from the hinterland or alongshore is high, the island chain moves seaward
(prograding or regressive barrier). In this case, vertical seetions show a transition from shallow marine
and shorefaee facies into the foreshore, backshore,
and dune sands of the baITier island. Lagoonal deposits may follow on top of the foreshore and backshore
sands.
(2) Low sediment input favors the landward transgression of baITier systems. Then a complete vertical
section starts with continental or tidal flat sediments
and grades into lagoonal muds, flood-delta sands and
channel fills of the inlet system. Finally, washover
sands and eolian dunes may follow. Consequently,
vertieal sections clearly document the regressive or
transgressive history of the baITier. However, in
many cases the development of a transgressive barrier system is considerably modified during rising sea
level (or submerging coast).
(3) If the sea level rises relatively quickly, the preexisting baITier islands are drowned in-place and
fonn a kind of inner shelf shoal (Fig. 3.l7c) as described, for example, from the abandoned western
part of the Mississippi delta (Penland et al. 1988).
Then, a new baITier may be built up farther landward.
Consequently, the fonner lagoonal muds and flooddelta deposits are covered directly by shoreface sediments.
(4) A further alternative, as mentioned be fore
(Fig. 3 .18b), is a slow landward migration of the
123
total baITier system with moderately rising sea level.
Then, the transgression can be accompanied by significant shoreface erosion or shoreface retreat (Fig.
3.l7d). In this case, the sediment eroded from the
upper shoreface is redeposited either in deeper water
(transgressive lower shoreface) or swept through inlets or stonn-generated washovers into the back-barrier region.
Shoreface erosion ean affect not only baITier
sands, but mayaiso proceed into lagoonal and tidal
deposits. As a result, the transgressive beds of the
lower shoreface can overlie both back-baITier sands
and lagoonal muds (including flood-delta deposits),
only lagoonal muds, or directly older bedrock (Fig.
3.l7dl, d2, d3). The bedrock may have been exposed
to air prior to the transgression. Deep erosion into
pre-existing beds is favored by slow sea-Ievel rise
and low sediment supply. It often creates planar erosion faces and appears to be rather common in the
ancient sedimentary record, whereas the former barrier islands may be completely gone. Only the infillings of deep channels of the former inlet systems
have a good chance of being preserved under such
conditions.
These and other complications have been repeatedly described from modem and ancient examples (e.g.,
Numrnedal et al. 1987; Numrnedal and Swift 1987). Lagoons and lagoonal sediments are dealt with, e.g., by
Nicols (1989) and Ward and Ashley (1989); see also references listed above.
Remarks to Ancient Barrier-Island Sands and
Lagoonal Sediments
Ancient baITier-island sands are more difficult to
identify than tidal sediments, particularly when their
upper portions (foreshore, washover, and eolian
sands) are missing. The lower portions of these complexes may be easily confused with the normal
shoreface sands of shallow seas or with mouth bar
sands of marine deltas (Sects. 3.4.2 and 3.4.3). In
such cases it is neeessary to investigate the largerscale facies assoeiation of the sand complex under
consideration.
Even ancient lagoonal sediments are not always
readily recognized in the geologic record, because of
the variety of associated deposits. Lagoons are shallow features, and lagoonal sediments are volumetrically less significant than many other coastal and
shalIow-water deposits, particularly deltaic sequences. Modem lagoons frequently display sedimentation rates in the order of 1 to 2 mrnJa. They are
therefore filled up with sediments in a relatively short
time period, and their lamination is often relatively
weIl preserved. The biota of lagoonal sediments
strongly reflect the loeal climate and may change rapidly, both vertically and laterally.
neously, shells of nonnal marine organisms can be
swept in.
In humid regions, the lagoons often accumulate
organic matter, which later may fonn coal seams, and
their bottom waters can easily become anoxic. Arid
to semi-arid lagoons tend to collect more carbonate,
and their tidal deltas may become oolitic. Marginal
portions of the lagoons can periodically dry out and
accumulate gypsum (cf. Sect. 6.4.2). Migration of
eolian sand over dry lagoons is eommon. Tidal flat
sediments bordering the lagoon may include plant
roots, peat, or mierobial mats and stromatolites, as
weIl as evaporites fonned in coastal sabkhas.
Response of Barrier Island-Lagoon Complexes
to Sea-Level Changes
BaITier-island complexes are dynamic systems responding to changing sediment input and sea-Ievel
changes. Rapid sea-Ievel fall (or emergence of the
baITier system by uplift) tends to destroy the sedimentary eomplex by erosion and is therefore not diseussed further here. With eonstant or rising sea level,
the island chain can migrate either land ward or seaward and thus bury and preserve some of its older
deposits (Fig. 3.l8b).
(1) If sediment input from the hinterland or alongshore is high, the island chain moves seaward
(prograding or regressive barrier). In this case, vertical seetions show a transition from shallow marine
and shorefaee facies into the foreshore, backshore,
and dune sands of the baITier island. Lagoonal deposits may follow on top of the foreshore and backshore
sands.
(2) Low sediment input favors the landward transgression of baITier systems. Then a complete vertical
section starts with continental or tidal flat sediments
and grades into lagoonal muds, flood-delta sands and
channel fills of the inlet system. Finally, washover
sands and eolian dunes may follow. Consequently,
vertieal sections clearly document the regressive or
transgressive history of the baITier. However, in
many cases the development of a transgressive barrier system is considerably modified during rising sea
level (or submerging coast).
(3) If the sea level rises relatively quickly, the preexisting baITier islands are drowned in-place and
fonn a kind of inner shelf shoal (Fig. 3.l7c) as described, for example, from the abandoned western
part of the Mississippi delta (Penland et al. 1988).
Then, a new baITier may be built up farther landward.
Consequently, the fonner lagoonal muds and flooddelta deposits are covered directly by shoreface sediments.
(4) A further alternative, as mentioned be fore
(Fig. 3 .18b), is a slow landward migration of the
123
total baITier system with moderately rising sea level.
Then, the transgression can be accompanied by significant shoreface erosion or shoreface retreat (Fig.
3.l7d). In this case, the sediment eroded from the
upper shoreface is redeposited either in deeper water
(transgressive lower shoreface) or swept through inlets or stonn-generated washovers into the back-barrier region.
Shoreface erosion ean affect not only baITier
sands, but mayaiso proceed into lagoonal and tidal
deposits. As a result, the transgressive beds of the
lower shoreface can overlie both back-baITier sands
and lagoonal muds (including flood-delta deposits),
only lagoonal muds, or directly older bedrock (Fig.
3.l7dl, d2, d3). The bedrock may have been exposed
to air prior to the transgression. Deep erosion into
pre-existing beds is favored by slow sea-Ievel rise
and low sediment supply. It often creates planar erosion faces and appears to be rather common in the
ancient sedimentary record, whereas the former barrier islands may be completely gone. Only the infillings of deep channels of the former inlet systems
have a good chance of being preserved under such
conditions.
These and other complications have been repeatedly described from modem and ancient examples (e.g.,
Numrnedal et al. 1987; Numrnedal and Swift 1987). Lagoons and lagoonal sediments are dealt with, e.g., by
Nicols (1989) and Ward and Ashley (1989); see also references listed above.
Remarks to Ancient Barrier-Island Sands and
Lagoonal Sediments
Ancient baITier-island sands are more difficult to
identify than tidal sediments, particularly when their
upper portions (foreshore, washover, and eolian
sands) are missing. The lower portions of these complexes may be easily confused with the normal
shoreface sands of shallow seas or with mouth bar
sands of marine deltas (Sects. 3.4.2 and 3.4.3). In
such cases it is neeessary to investigate the largerscale facies assoeiation of the sand complex under
consideration.
Even ancient lagoonal sediments are not always
readily recognized in the geologic record, because of
the variety of associated deposits. Lagoons are shallow features, and lagoonal sediments are volumetrically less significant than many other coastal and
shalIow-water deposits, particularly deltaic sequences. Modem lagoons frequently display sedimentation rates in the order of 1 to 2 mrnJa. They are
therefore filled up with sediments in a relatively short
time period, and their lamination is often relatively
weIl preserved. The biota of lagoonal sediments
strongly reflect the loeal climate and may change rapidly, both vertically and laterally.