Beach and Barrier Systems

Beach & Barrier Systems

Photo by W. W. Little

Simplified Models

Beaches are typically separated into shore-parallel environments based on water depth and relationship wave processes.

Reineck & Singh, 1980

Common Shoreline Facies

Prograding shorelines are typified by a coarsening-upward succession of deeper water to shallower water to above water facies all dominated by sand. Facies are distinguished primarily by grain size and sedimentary structures.


Laminated Mud

Laminated mud forms offshore below storm wave-base just beyond the beach system and in back-barrier lagoons through suspension-settling of clay and silt.

Hummocky Cross-bedded Sand

Hummocky cross-bedded sand is produced mostly during storms by a combination of current and oscillatory flow and is found primarily on the lower shoreface and in the transition zone.




Trough Cross-bedded Sand

Small-scale trough cross-stratified sand is common in the surf zone of the upper shoreface.


Horizontally-stratified Sand

Deposition in the swash zone produces relatively continuous laminae of sand that dip slightly basinward. Small cavities formed from trapped air and water escape structures are common.



Shell Beds

Mollusc shells commonly accumulate during storm events, as the shells are piled upon one another and finer sediment is winnowed away.


Edgewise Conglomerate

Edgewise conglomerate is an intraclastic conglomerate produced during storm events as partially lithified sediment is “ripped up” and redeposited.


Thrombolites

Thrombolites are biological structures similar to stromatolites but without internal structure.



Ripple Bedding

Ripple bedding commonly forms as part of the backwash; however, because of the higher-energy swash part of the couplet, they have a low preservation potential.


A: End of swash B: Beginning of backwash

C: End of backwash D: Rhomboidal ripples Photos by W. W. Little

Rhomboidal Ripples

Current Ripples



Coal

Coal forms through the accumulation of peat in back beach and back barrier regions.

Coastal Depositional Systems

A variety of depositional systems are found along a shoreline. Their distribution, geometry, and extent are determined by proximity to a sediment source, sediment abundance, energy conditions, and fluctuations in base-level.

This lecture focuses on the beach and barrier island systems.

Progradational Beach Associations

Beaches are typically separated into shore-parallel environments based on water depth and relationship to wave processes.

Beaches are typically separated into shore-parallel environments based on water depth and relationship wave processes.

Retrogradational Beach Associations

Idealized Vertical Profiles

Vertical profiles for shoreline successions vary depending, in part, on whether they are produced under transgressive or regressive conditions. Progradational successions are more common, but transgressive successions tend to be more complex.

BeachBeach Barrier IslandBarrier Island Tidal inletTidal inlet

Progradational ProfileProgradational Profile Retrogradational ProfilesRetrogradational Profiles

Barrier Islands & Lagoons

Barrier islands form mostly during transgressive conditions and are, in essence, beaches that are separated from the mainland by a lagoon. Modern barriers are commonly associated with estuaries.

Tidal Inlets

Tidal inlets are breaches in a barrier island that allow sea water to move into and out of a lagoon. As the inlet migrates in the direction of long-shore transport, a typical vertical profile would consist of fining-upward channel deposits overlain by spit sediments.

Washover Fans

Storm surges will often erode sand from the basinward side of a bar and redeposit it on the lagoonal side, producing a washover fan.

Tidal Deltas

Barrier islands often become breached by large storms. Tidal currents then flow through the opening producing small “deltas.” Ebb deltas are much more common than flow deltas because of the shallower basin in the ebb side of the bar.

Back-barrier (bay mouth) Tidal Flat

Bay mouths behind barrier bars can become filled with fine-grained sediment deposited by estuarine fluvial systems, producing tidal flats.


Large-scale Architecture(progradational beach)

Progradational beach successions tend to be cyclical, in which each cycle, as well as the entire succession coarsen upward. Each small-scale cycle is referred to as a parasequence. Boundaries between parasequences are flooding surfaces and represent relatively abrupt rises in base-level.



Flooding surface/parasequence boundary


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Flooding surfaces


Offshore Deposits

Above the flooding surface are deeper-water shelf mudstones and poorly-developed limestones, both representing deposition below storm wave base.



Transition Zone

Transition zone deposits are composed of interbedded offshore mudstone and hummocky cross-bedded sand deposited during storms. This zone represents sedimentation below normal and above storm wave-base. Relative abundance and thickness of hummocky beds increases upward.




Lower Shoreface

Shoreface deposits consist primarily of stacked hummocky cross-stratified sand beds deposited between normal wave base and the surf zone.




Upper Shoreface

Trough and low-angle trough cross-bedded sand transported by surf comprises the upper shoreface.


Foreshore

Swash deposition in the foreshore zone produces horizontally-laminated sand.




Backshore Deposits

Backshore deposits can take a variety of forms, including eolian dunes, lagoonal mud, tidal sand or mud, and peat swamps.




Date post:	27-Nov-2014
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Beach and Barrier Systems

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