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574 SEDIMENTARY ENVIRONMENTS/Shoreline and Shoreface Deposits rate of space creation is greater than the rate of sediment supply, then the shoreline will move landward (i.e., will be transgressive) Transgressive shorelines may be further subdivided into low- and high-relief forms Low-relief transgressive shorelines generally occur when relative sea-level rise floods a former delta or coastal plain In such cases, systems may contribute to the geological rock record and their deposits will either be barrier islands in systems with a moderate to low tidal range, or tidal flats in systems with a higher tidal range High-relief transgressive shorelines occur where the sea-level rise is flooding bedrock In such cases, the systems are erosional High-relief transgressive shorelines not contribute to the geological rock record, although they are an important part of modern systems (see later) Progradational Wave-Dominated Shoreline Systems Progradational shoreline systems, which contribute significantly to the geological rock record, are classified by the dominant depositional mechanism: wave, fluvial (see Sedimentary Processes: Fluvial Geomorphology), or tidal Progradational wavedominated systems include shorefaces and wavedominated deltas In a shoreface, the sediment is introduced by longshore drift; in a wave-dominated delta, the sediment is originally introduce by a fluvial system and is then reworked by wave processes In reality, especially in the rock record, it can be very difficult to distinguish the two, and because the depositional processes are the same, it is useful to consider them as largely synonymous In both cases, the system includes a beach that lies between the low tidemark and the next significant feature in a landward direction, which may be dunes, a coastal forest, a mire, a cliff, or in many modern systems a seawall Between the low tidemark and the FWWB is the shoreface, which includes the sediment that is being reworked by fair weather wave action Seaward of the FWWB is the offshore transition zone where, for the majority of time, deposition consists of very fine-grained suspended sediment, with brief influxes of stormderived coarser deposits Seaward of the SWB is the offshore shelf where waves no longer play a role in the introduction or deposition of sediment (Figure 6) Within a wave-dominated system, sediment is introduced by longshore drift, either from the reworking of a fluvial system (wave-dominated delta) or from wave erosion of the bedrock or older shoreline deposits In plan view, a variety of features may exist within the depositional system Offshore, seaward of the point at which the storm wave base interacts with the seafloor, deposition is restricted to mud deposited from suspension or to coarser sediments introduced by sediment gravity flows In the zone between the SWB and FWWB, storms deposit hummocky cross-stratified Figure Facies model for a progradational wave dominated shoreline system View shows both barred and non barred coastlines Sediment is introduced by longshore drift No scale is implied because the depths to the fair weather wave base and the storm wave base (SWB) are controlled by the size of the basin and the local climate
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