Marginal Marine Environments:

Transitional Environments: Depositional environments that are influenced both by sea water, fresh, water, and often subaerial exposure. Effectively includes everything terrestrial along a coast and the shallowest portion of the marine shelf. These intergrade, but for simplicity we recognize four major transitional environments:

Some of their primary characteristics are indicated by the table below.


Peritidal environments


Lagoon of Assateague Island, VA.
What if a stream met the ocean without transporting in copious sediment. This yields a tide dominated environment of tidal estuaries and wetlands. The key features of this environment are:

Tidal Flats: If a peritidal environment experiences enough tidal current flow that vegetation can't become established, the result is a tidal flat. (E.G. the Turnagain Arm of the Cook Inlet, AK.) Diagnostic features include:

Shoreface/Beach:

A deposit of unconsolidated sediment extending from low tide to a topographic break such as a line of dunes or cliffs.



Waves strike Rex Beach, QLD, obliquely, creating longshore current.

Barrier complexes:

Barrier: elongate sandy islands or peninsulas that run parallel to shoreline. Their formation requires:

The seaward side of barriers have typical shoreface environments


Tombolo from Wikipedia
  • Tombolo. Sometimes, a sea stack or island may form a wave shadow. When that happens, sediment can accumulate behind it, eventually connecting it to the mainland.
  • Spits: Imagine that a beach ends at an embayment, and that a longshore current is carring sediment into the embayment. Voila! A spit! A projection of a beach into a body of water.


  • Baymouth bar: If the spit continues to develop, it may completely enclose the embayment, forming a baymouth bar.
  • Barrier islands: Of course, a baymouth bar does form a barrier between two bodies of water. If this is broken, tidal inlets or passes can form. The separated segments become barrier islands.

    Lagoon Body of quiet water isolated between the barrier and mainland.

    A final note: Whereas fresh water is acidic, sea water is just slightly alkaline. Thus, calcareous cements (usually aragonite) can cement beach rock in situ.