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SEDIMENTARY ENVIRONMENTS/Shoreline and Shoreface Deposits 575 (HCS) sheets of sandstone, typically less than 0.5 m thick; thin layers of coarser material (up to granule grade) may also be deposited if such material is available on the beach These sheets have erosive bases whereas the top surfaces commonly exhibit wave ripples formed during the waning part of the storm Bioturbation of the storm bed by opportunistic organisms is common Between storm events, mud is deposited from suspension The shoreface is the interval that lies landward of the point at which the fair weather wave base interacts with the seafloor Within the lower part of the shoreface, sediment is deposited as sheets of HCS sand during storms, and fair weather wave action prevents the deposition of mud between the storm events, resulting in a sand-rich depositional system The point at which the fair weather waves actively move sediment in a landward direction marks the base of the upper shoreface The upper shoreface may be a simple seaward-dipping surface or it may contain one or even several large bar forms Simple, ‘non-barred’ shorelines are generally steeply dipping and coarse grained They are covered with small dune forms that migrate in a landward and longshore direction; plunging waves generally produce seawarddipping planar laminations on a relatively simple beach (Figure 7) Non-barred shorelines prograde through the steady addition of sediment to the beach and shoreface Modern examples include the Pacific coast of North America The Book Cliffs of Utah display superb Cretaceous examples of non-barred shoreline deposits (Figure 7D, E, H, and J) Barred shorelines are more common than nonbarred systems and contain large (up to 50 m wide and hundreds of metres long) shore-parallel barforms separated by narrow rip-channels that run perpendicular to the shoreline Water and sediment are carried in an onshore direction by incoming waves that break over the bars and in an offshore direction by currents running through the rip-channels The combination of these processes generates circulatory cells (termed ‘longshore cells’) within the shoreface Barred shorelines are extremely dynamic and change markedly on a seasonal basis Bars are generally built during the winter months as storms move sediment offshore from the beach Bars can be symmetrical or asymmetrical and contain a series of seaward-dipping planer-beds deposited by storms on the seaward side and trough cross-bedding and landward-dipping planar-beds deposited by fair weather wave reworking Rip-channels are either erosional or contain seaward-directed trough cross-bedding During the summer months, the bars migrate landward and may emerge above the low tidemark, where they become part of the beach Within barred shorelines, the beach is more complex, compared to the simple seaward-dipping planar surface that exists in non-barred systems Beaches within barred coastlines comprise a series of subzones, not all of which are always present The lower intertidal portion includes a series of low, shore-parallel barforms, or ‘ridges’, separated from one another by hollows, or ‘runnels’ (Figure 6) Sedimentary features within this ridge and runnel system include low-angle dipping planar laminations on the ridges and shoreparallel current ripples formed by the draining of the runnels as the tide recedes These ridge and runnel systems generally develop during the winter months when storms occur most frequently, but may also be related to the emergence of landward-migrating bars during the summer period The main part of the intertidal zone is termed the ‘foreshore’ and comprises a series of seaward-dipping planar surfaces formed by the breaking waves The mean grain size of the sediment will strongly control the dip of the foreshore If the grain size is coarse, the water brought onshore by the breaking waves will rapidly percolate into the sediment, reducing the backwash and producing a steep beach Conversely, if the sediment is fine and less permeable, the beach will be gently dipping The top of the foreshore is marked by a ridge called a ‘berm’ The berm builds during the summer months as the fair weather waves push sediment onto the beach In the absence of large storms, beaches may develop a series of parallel berms Beyond the berm lies the backshore This is the area above the mean high tidemark, where sediment is introduced only during extremely high tides or by storms If a lot of coarse (cobble to pebble grade) material is available, a storm ridge may be formed This is typically composed of well-rounded, wellsorted, discoid clasts with a seaward imbrication that are thrown up during storms and are coarse enough to resist fair weather reworking In the absence of a storm ridge, the backshore may comprise partially vegetated sand flats in which aeolian processes rework the sediment that is infrequently washed in Vertical Succession through a Wave-Dominated Shoreface Succession The vertical succession deposited during the progradation of a wave-dominated shoreline succession reflects the lateral facies relationships as described The onshore coarsening of grain size and increasing bedform asymmetry are recorded vertically in the rock record (Figure 6) A typical vertical succession from base to top will exhibit mudstones and siltstones deposited in the offshore setting at the base Moving upward, the appearance of the first significant
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