Migration of Major Depositional Settings of Barrier Island Systems
Barrier Island systems are not unmoving island settings but a system which is constantly changing and moving. Close examination of barrier islands suggest there are three main types of island migration that include lateral, prograding or regressive and transgressive.
Lateral Migration: When longshore transport has a directional flow established along the beach front of a barrier island it transports sand and other material along the beach. A consequence of this transport (as seen in the previous section with tidal inlet migration) is that material that once was up current is now deposited down current. This allows for the island to build on down current and is eroded on the up current side. Below is a cartoon representing a time series of lateral migration of a barrier island.
Figure above shows the lateral migration of Fire Island. The net longshore transport is towards the southwest corner of the box and the Island migrates in that direction (from Prothero, 1990)
The lateral progradation of Barrier Island system in a seaward direction.
Prograding or Regressive Migration:
Regressive barrier island systems are those that prograde or build out into the ocean. Typically these systems are associated with a base level
fall. So either there is a eustatic
drop in sea level or a rise in continent level due to tectonic or sediment deposition. A cross-section of a prograding barrier island is shown below.
Above are depositional models for A) a prograding barrier island, B) a transgressive barrier island and C) a channel inlet migration (which will not be discussed in this section). (Prothero, 1990).
In the model A above notice that the seaward components of a barrier island system is preserved. This is due to the fact these components are buried by the regressive island and are not exposed to the longshore current.
Transgressive Migration: In a transgressive island system the island migrates back towards the land. These systems are typically associated with a eustatic rise in sea level or continental subsidence. This process of migration has become very important in recent years as barrier islands have become populated. As these barrier islands migrate landward (a process which is happening all along the eastern seaboard of the US) people are losing property and homes to the ocean. In model B above it can be seen that the strata goes from fine grained lagoon sediments and becomes more coarse upwards. This is a result of the barrier island migrating over the top of the lagoon as it migrates landward.