Barrier and lagoon systems

Barriers

Along some coastlines a barrier of sediment separates the open sea from a lagoon that lies between the barrier and the coastal plain. Beach barriers are composed of sand and/or gravel material and are largely built up by wave action. They may be partially attached to the land, forming a beach spit, or wholly attached as a welded barrier that completely encloses a lagoon, or can be isolated as a barrier island in front of a lagoon. In practice, the distinction between these three forms can be difficult to identify in ancient successions and their sedimentological characteristics are very similar. Barriers range in size from less than 100 m wide to several kilometres and their length ranges from a few hundred metres to many tens of kilometres. The largest tend to form along the open coasts of large oceans where the wave energy is high and the tidal range is small.
The seaward margin of a barrier island has a beach and commonly a beach dune ridge where aeolian processes rework the sand. Vegetation helps to stabilise the dunes. On the landward side of the island the layers of sand deposited during storms pinch out into the muddy marshes of the edge of the lagoon. During storm surges seawater may locally overtop the beach ridge and deposit washovers of sediment that has been reworked from the barrier and deposited in the lagoon. Washover deposits are low-angle cones of stratified sands dipping landwards from the barrier into the lagoon.
The conditions required for a barrier to form are as follows. First, an abundant supply of sand or gravelsized sediment is required and this must be sufficient to match or exceed any losses of sediment by erosion. The supply of the sediment is commonly by wavedriven longshore drift from the mouth of a river at some other point along the coast and there may also be some reworking of material from the sea bed further offshore. Second, the tidal range must be small. In macrotidal settings the exchange of water between a lagoon and the sea during each tidal cycle would prevent the formation of a barrier because a restricted inlet would not be able to let the water pass through at a high enough rate. Barrier systems are therefore best developed in microtidal and, to some extent, mesotidal settings. Third, barrier islands generally form under conditions of slow relative sea-level rise. If there is a well-developed beach ridge, the coastal plain behind it may be lower than the top of the ridge. With a small sea-level rise, the coastal plain can become partially flooded to form a lagoon, and the beach ridge will remain subaerial, forming a barrier. For the barrier to remain subaerial as sea level rises further, sediment must be added to the beach to build it up, that is, the first condition of high sediment supply must be satisfied.

Lagoons

Lagoons are coastal bodies of water that have very limited connection to the open ocean. Seawater reaches a lagoon directly through a channel to the sea or via seepage through a barrier; fresh water is supplied by rainfall or by surface run-off from the adjacent coastal plain. If a lagoon is fed by a river it would be considered to be part of an estuary system. They are typically very shallow, reaching only a few metres in depth. Lagoons generally develop along coasts where there is a wave-formed barrier and are largely protected from the power of open ocean waves. Waves are generated by wind blowing across the surface of the water, but the fetch of the waves will be limited by the dimensions of the lagoon. Ripples formed by waves therefore affect the sediments only in very shallow water. The wind may also drive weak currents across the lagoon. Tidal effects are generally small because the barrier– lagoon morphology is only well developed along coasts with a small tidal range. Fine-grained clastic sediment is supplied to lagoons as suspended material in seawater entering past the barrier and in overland flow from the adjacent coastal plain. Organic material may be abundant from vegetation which grows on the shores of the lagoon. In tropical climates, trees with aerial root systems (mangroves) colonise the shallow fringes of the lagoon. Mangroves cause the shoreline to prograde into the lagoon as they act as sites for accumulation of sediment and organic matter along the water’s edge. In more temperate climates, saline-tolerant grasses, shrubs and trees may play a similar role in trapping sediment. Coarser sediment may enter the lagoon when storms wash sediment over the barrier as washover deposits, which are thin layers of sand reworked by waves. Sand is also blown into the water by onshore winds picking up material from the dunes along the barrier. An important characteristic of lagoons is their water chemistry. Due to the limited connection to open ocean, it is common for lagoon water to have either higher or lower salinity than seawater. Low salinity, brackish water will be a feature of lagoons in areas of high rainfall, local run-off of fresh water from the coastal plain or small streams. Mixing of the lagoon water with the seawater is insufficient to maintain full salinity in these brackish lagoons. In more arid settings the evaporation from the surface of the lagoon may exceed the rate at which seawater exchanges with the lagoon water and the conditions become hypersaline, that is, with salinities higher than that of seawater. If salinities become very elevated, precipitation of evaporite minerals will occur. A lagoonal succession is typically mudstone, often organic-rich, with thin, wave-rippled sand beds. The deposits of lagoons can be difficult to distinguish from those of lakes with similar dimensions and in similar climatic settings. The processes are almost identical in the two settings because they are both standing bodies of water. Two lines of evidence can be used to identify lagoonal facies. First, the fossil assemblage may indicate a marine influence, and specifically a restricted fauna may provide evidence of brackish or hypersaline water. Second, the association with other facies is also important: lagoonal deposits occur above or below beach/barrier island sediments and fully marine shoreface deposits.
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