Tidal flats are built of marine sediments intersected by runnels and channels in a specific vertical sequence. Bidirectional current marks and subaerial features complete the environmental indicators.

Sediments in the intertidal zone lie between high- and low-water line across a vertical distance of 1-4 m or more, depending on the tidal range. The tidal range causes tidal currents which in turn form numerous gullies and channels. Currents of a high tidal range erode deeper channels than currents of low tidal ranges. The current velocity on the flats may reach 1 knot; therefore, on sandy bottoms small-scale current ripple marks are formed. The current velocity in gullies and channels is 3 knots or more, so that megaripples and underwater dunes are common in the channels. The tidal flats are sheltered by barrier islands or sand bars or they are in a sheltered bay. Though wave action is not too strong, it is nevertheless an important factor.

The whole wedge-shaped tidal flat body is elongate, parallel with the shoreline for many miles, but is intersected by channels or river estuaries.

The clastic sediments are mud (clay and silt), and sand, which is mostly fine grained. Gravel is scarce but clay pebbles and shells are common on channel bottoms. The mineral content of clay and silt is mainly clay minerals, quartz, iron minerals, garnet, mica, feldspar, some heavy minerals, dolomite, and carbonate. The mineral content of sand is mainly quartz, feldspar, mica, heavy minerals, fecal pellets, broken shells, and Foraminifera.

Near the high-water line (mud flats) the mud content is high, especially on wind- and wave-sheltered coastlines. The mud content decreases in the mixed flats and is low near the low-water line in the sand flats. The mud content increases near channels and below the low-water line, especially in the lateral channel deposits (except in the channel-bottom sediments). Even in the mudflats, the channel-bottom sediments are very muddy.

The tidal flats in The Netherlands are less muddy than the tidal flats in the German bay. Most tidal flats in Great Britain are very sandy. South of San Felipe, Gulf of California, the tidal flats are built of skin sand; north of San Felipe the mud content increases markedly toward the Colorado delta, whence the silt is derived.

Cross-bedding of megaripples is rare on the flats but common in the channels. On sand flats cross-beds of small-scale current ripples are very common. Locally the cross-beds shows herring-bone structures in sections normal to the ripple-crest axis and festoon bedding in sections parallel with the ripples. Laminated sand is not common. Climbing ripple structures are very rare. Flaser bedding, wavy bedding, lenticular bedding, interbedding, and interlamination of mud and sand are common bedding types in the mixed flats and in the lateral channel deposits. In mud flats there are thick mud layers with thin strips of sand. None of these bedding types is restricted to tidal flats. Salt-marsh deposits are characteristically interfused by roots and by uneven noduled lamination.

In the microstructure there is graded bedding in thin laminae, mostly less than 1 mm thick. Some beds are graded from coarse to fine, and some from fine to coarse. Small-scale erosional features are common.

The origin of these bedding types commonly is related to the alternation of tidal currents and tidal slack water. In a vertical column there are thicker sets changing in the bedding type from set to set, resulting from changes of wind and wave direction and force. Most of the layers are deposited in shallow morphological depressions as flat erosional patches (shallow runnels), but some are deposited laterally in channel deposits (point bars) and others on sheltered, gently inclined places. On the flats above the depressions and channels, overall (net) sedimentation is small because of alternating erosion and sedimentation.

Tidal flat faunas are plentiful, but only a few species are present. Most parts of the tidal-flat surface layers are strongly bioturbated by bottom-living animals. Where layers are deposited rapidly, bioturbation is not as common. This is especially true of the lateral channel deposits and the channel-bottom sediments.

In some places there are units with bottom-living invertebrates in living positions. In a few layers, fecal pellets are concentrated. Rolled algal mats develop on tidal flats in certain climates.

The most common surface-structure features on tidal flats are ripple marks, mostly of current ripples, but also symmetrical oscillation ripples. Subaerial marks are important; small runnels and erosional depressions are abundant. The flat depressions are commonly sculptured by oscillation ripples whereas the surrounding bottom is covered by current ripples. Tracks of birds and other land animals, raindrop and hail imprints, and desiccation cracks are on the surface. Groove casts also are common.

Transgressive sequence on tidal flats may develop as follows (from top to bottom): e^prime, sand (sand-flat deposits); d^prime, mixed sediment (mixed-flat deposits); c^prime, mud (mud-flat deposits); b^prime, brackish and freshwater clay; and a^prime, sphagnum peat. Regressive tidal-flat sequence, from top to bottom, consist of: f, peat; e, freshwater and brackish deposits; d, salt-marsh deposits; c, mud-flat deposits; b, mixed flat deposits; and a, sand flat deposits.

This sequence is common only if there is an abundant sediment supply. If the sediment supply is not plentiful, meandering channels rework the sediments and the thick channel sediments directly overlie the transgressive sequence. Even where channels are developed, a regressive sequence sand, mixed sediments, mud, and salt-marsh deposits can develop, though they are deeply dissected by runnels and channels. In many examples of fossil and recent tidal flats, the sequences given here may not be fully developed.

Seaward from tidal flats, and parallel with the coast, sandbars or barrier islands may develop. The landward side of the tidal flats is the line where land soils develop by older sediments are exposed.

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