3.2 Tidal Flats
- The subtidal to sub aerial barrier-beach complex.
- The subtidal-intertidal ebb-delta associated with
shoreface sediments.
- The subtidal-intertidal flood-delta associated with
lagoonal and tidal flat sediments (back-barrier
zone).
The (laterally rnigrating) subtidal-intertidal inletchannel complex.
Barrier-island systems frequently form long, extended chains along straight coasts (cf. Fig. 3.2).
Therefore, their deposits usually represent elongate
bodies which parallel the strandline. Three of the
above-mentioned subenvironments are characterized
by sandy deposits, whereas in the back-barrier zone
both sand and muds accumulate. The sediment
source may be mainly terrigenous and/or biogenic
(carbonate) and thus magnify the variability of the
barrier-island complex as a depositional system. As
pointed out above, tidal sediments and biota in the
lagoon strongly reflect the climatic conditions of the
neighboring landmass.
Subenvironments of Barrier Island-Lagoon
Complexes
(1) A barrier-beach complex consists of, from bottom
to top:
- Shoreface sands down to a depth of 10 to 20 m
below mean sea level, where the normal wave action
ceases. These sands generally exhibit decreasing
grain size and increasing bioturbation with growing
water depth (Fig. 3.18). While the upper shoreface
deposits may contain gravelly sands and display
multi-directional sedimentary structures (trough
cross-beds and low-angle planar cross-beds), the
lower ones consist of fine to very fine-grained sand
or muddy sand. Here, the primary sedimentary structures, consisting mainly of planar larninated and
small-scale cross-bedded material, are frequently
obliterated by bioturbation.
- Beach-foreshore sands displaying parallel to lowangle, seaward-dipping larninations and usually little
bioturbation. They are overlain by
- Backshore eolian dunes starting with rather smallscale, multi-directional trough and planar cross-beds.
If storm surges overrun parts of the islands, they create
- Washover sands which extend into the lagoon.
These show either subhorizontal planar lamination
or, if they reach the lagoon, delta foresets. The
washover deposits rnay make up a significant portion
of the total barrier island complex. They reflect the
tendency of the islands to rnigrate landward. They
also feed the
121
- Back-barrier sand flats (Fig. 3.18). The sedimentary structures of both eolian dunes and washover
sands can be obliterated or modified by plant growth
and soil formation.
(2) Ebb-tidal sands accumulate seaward of the barrier and are associated with shoreface sands. In contrast to flood-tidal sands, they are also affected by
wind-generated waves and longshore currents. Therefore their sedimentary structures can reflect several
processes and current directions. Their faunal content
is mainly of open-marine origin, although ebb currents mayaiso transport lagoonal biogenic particles
seaward into the ebb-tidal delta.
(3) Flood-tidal sands alternate with lagoonal muds
and tidal flat sediments of the back-barrier zone.
These delta sands show planar and trough cross-beds
which are mainly generated by flood-oriented
megaripples and sandwaves, but a seaward orientation of cross-beds also occurs.
(4) Tidal-channel deposits between the ebb and
flood-tidal delta cancover relatively large areas, because the channels and tidal inlets between the
barrier-islands frequently rnigrate parallel to the
shoreline. Shifting of inlets involves erosion on one
side of the barrier and accretion of sand on the other
(spit accretion). Channel fills usually begin at the
erosional base with relatively coarse lag deposits.
These are overlain by large-scale bidirectional planar
and trough cross-beds. On top of this sequence
bidirectional small to medium-scale trough or planar
cross-beds, and parallel and ripple lamination are
observed.
Controls by Climate and Hydraulic Regime
Depending on the climate, lagoonal and tidal flat sediments can vary considerably. If the barrier-island
chain is interrupted by many wide inlets (mesotidal
hydraulic regime), the lagoonal waters are sufficiently exchanged with the open sea water and therefore are normal saline. Under these conditions, the
lagoonal fauna can develop a highly diverse assemblage, and the fine-grained, often laminated lagoonal
muds tend to become thoroughly bioturbated and
structureless.
However, if just a few narrow inlets are present
(microtidal regime), the lagoonal waters become either brackish or hypersaline, and their fauna is abnormal and of low diversity. Oyster beds, for exampie, indicate freshwater influence, specific gastropods and seagrass typify hypersaline conditions. Due
to storms which produce washovers, the lagoonal
environment may change episodically. Simulta-
- The subtidal to sub aerial barrier-beach complex.
- The subtidal-intertidal ebb-delta associated with
shoreface sediments.
- The subtidal-intertidal flood-delta associated with
lagoonal and tidal flat sediments (back-barrier
zone).
The (laterally rnigrating) subtidal-intertidal inletchannel complex.
Barrier-island systems frequently form long, extended chains along straight coasts (cf. Fig. 3.2).
Therefore, their deposits usually represent elongate
bodies which parallel the strandline. Three of the
above-mentioned subenvironments are characterized
by sandy deposits, whereas in the back-barrier zone
both sand and muds accumulate. The sediment
source may be mainly terrigenous and/or biogenic
(carbonate) and thus magnify the variability of the
barrier-island complex as a depositional system. As
pointed out above, tidal sediments and biota in the
lagoon strongly reflect the climatic conditions of the
neighboring landmass.
Subenvironments of Barrier Island-Lagoon
Complexes
(1) A barrier-beach complex consists of, from bottom
to top:
- Shoreface sands down to a depth of 10 to 20 m
below mean sea level, where the normal wave action
ceases. These sands generally exhibit decreasing
grain size and increasing bioturbation with growing
water depth (Fig. 3.18). While the upper shoreface
deposits may contain gravelly sands and display
multi-directional sedimentary structures (trough
cross-beds and low-angle planar cross-beds), the
lower ones consist of fine to very fine-grained sand
or muddy sand. Here, the primary sedimentary structures, consisting mainly of planar larninated and
small-scale cross-bedded material, are frequently
obliterated by bioturbation.
- Beach-foreshore sands displaying parallel to lowangle, seaward-dipping larninations and usually little
bioturbation. They are overlain by
- Backshore eolian dunes starting with rather smallscale, multi-directional trough and planar cross-beds.
If storm surges overrun parts of the islands, they create
- Washover sands which extend into the lagoon.
These show either subhorizontal planar lamination
or, if they reach the lagoon, delta foresets. The
washover deposits rnay make up a significant portion
of the total barrier island complex. They reflect the
tendency of the islands to rnigrate landward. They
also feed the
121
- Back-barrier sand flats (Fig. 3.18). The sedimentary structures of both eolian dunes and washover
sands can be obliterated or modified by plant growth
and soil formation.
(2) Ebb-tidal sands accumulate seaward of the barrier and are associated with shoreface sands. In contrast to flood-tidal sands, they are also affected by
wind-generated waves and longshore currents. Therefore their sedimentary structures can reflect several
processes and current directions. Their faunal content
is mainly of open-marine origin, although ebb currents mayaiso transport lagoonal biogenic particles
seaward into the ebb-tidal delta.
(3) Flood-tidal sands alternate with lagoonal muds
and tidal flat sediments of the back-barrier zone.
These delta sands show planar and trough cross-beds
which are mainly generated by flood-oriented
megaripples and sandwaves, but a seaward orientation of cross-beds also occurs.
(4) Tidal-channel deposits between the ebb and
flood-tidal delta cancover relatively large areas, because the channels and tidal inlets between the
barrier-islands frequently rnigrate parallel to the
shoreline. Shifting of inlets involves erosion on one
side of the barrier and accretion of sand on the other
(spit accretion). Channel fills usually begin at the
erosional base with relatively coarse lag deposits.
These are overlain by large-scale bidirectional planar
and trough cross-beds. On top of this sequence
bidirectional small to medium-scale trough or planar
cross-beds, and parallel and ripple lamination are
observed.
Controls by Climate and Hydraulic Regime
Depending on the climate, lagoonal and tidal flat sediments can vary considerably. If the barrier-island
chain is interrupted by many wide inlets (mesotidal
hydraulic regime), the lagoonal waters are sufficiently exchanged with the open sea water and therefore are normal saline. Under these conditions, the
lagoonal fauna can develop a highly diverse assemblage, and the fine-grained, often laminated lagoonal
muds tend to become thoroughly bioturbated and
structureless.
However, if just a few narrow inlets are present
(microtidal regime), the lagoonal waters become either brackish or hypersaline, and their fauna is abnormal and of low diversity. Oyster beds, for exampie, indicate freshwater influence, specific gastropods and seagrass typify hypersaline conditions. Due
to storms which produce washovers, the lagoonal
environment may change episodically. Simulta-