The supratidal zone is above high tide and is flooded only at spring tide or during storms. The zone between mean high tide and mean low tide is the intertidal zone. It is exposed twice daily. The subtidal zone is below mean low tide and is rarely, if ever, exposed.

Characteristics of these environments reflect whether tidal flats were deposited under humid or arid conditions. We first discuss the "Humid-Tidal-Flat Facies Tract.”

Humid-Tidal-Flat Facies Tract

The humid-climate tidal flat is characterized by ponds and marshes that contain well-developed algal mat communities and numerous tidal channels that act as principal sedimentation agents on the flats. The aerial photograph and associated cross section are an excellent example of a modern humid-tidal flat.

James (1983) presented a model of a platform-margin reef that shows five distinct environments. A discussion of these environments is presented below.

Major environmental zones

Supratidal environment is where sediment is deposited above normal high tide by spring and storm tides. The sediment is exposed for long periods to subaerial conditions.

Diagnostic supratidal sedimentary structures include mud cracks; fine laminations; algal mats and domes; birds-eye or fenestrae structures; plant, animal, and insect burrows; intraclasts; and dolomite crusts.

Rock types include laminated mudstones to wackestones, algal boundstones (stromatolites), intraclasts conglomerates, and rare storm-deposited grainstones. These rock types are commonly dolomitized.

Intertidal environment is where the sediment is deposited between normal high tide and normal low tide. The sediment in this zone is generally exposed twice a day to subaerial conditions at low tide. Tidal channels are common in this zone.

Diagnostic intertidal sedimentary structures include fine laminations, algal mats and algal heads, crab and mollusk burrows, and thin rippled sands.

Rock types include burrowed to laminated mudstones to wackestones, algal boundstones with algal heads (stromatolites), thin crossbedded grainstones, and rare storm-deposited grainstones.

Subtidal environment occurs immediately seaward of the normal low-tide mark or in a tidal channel dissecting the flats and is rarely exposed to the subaerial environment.

The diagnostic subtidal sedimentary structure is bioturbation.

Rock types include bioturbated wackestones to mud-rich packstones. Peloids are a common grain type.