Sedimentary structures generated on the foreshore by migrating ridge and runnel systems on microtidal and mesotidal coasts of S. Spain

Abstract

The migration oí ridge and runnel systems on mícrotidal coasts is a rather simple process starting with the formation oí a rídge at the uppcr shoreface after storms and its mígration landwards until it welds onto fue berm. This results in an ideal sequence of structures which ineludes (from bottom to top): cross~laminated sands (runnel facies); cross·bedded sands with local discontinuity surfaces (ridge facies); and even-laminated sands (berm facies). Larger tidal tange and higher wave activity on mesotidal coasts induce a mote differentiated pattern. At early stages of accretion~ the ridge is covered at high Ude and éxposed at low tide. Swash accumulates a secondary bar of coarse sedi~ ment dufÍng low water that is removed snd incorporated in the slip face of the ridge by avalanching during rising tídes. At high tide, large amounts of finer sedíment are brought by waves and deposíted on top of the rídge and in tr-e slip face} generating fining~upward sequences, Etosionallow~~mgle discontinuity surfaces develop during falling tides. Within the runnel~ longshore currents are strong enough to build up small megaripples with crests roughly normal to the shoreline and waves induce wave ripples with crests more or less parallel to the shoreline. At later stages of ~ccretion> the ridge is submerged tnainly during spring high tide, After the runnel has been filled up~ a vertical growth of the new berro occurs associated with rrníngRupward sequences due to decreusing \mergy and sho:reward shíftíng oí swash zonas, during the transition from neap to spring tides. The resulting ideal sequence oí sedimental'Y structures is larger and hetter diffel'entiated than those described for microtjdal and tideless coasts.

Evidence for catastrophism at the Famennian-Dinantian boundary in the Iberian Pyrite Belt

“…The upper part of the sand ridges, corresponding to the tabular thin layers described above, was continously reworked by fair-weather waves. Similar sandbodies are common throughout the geological record from the Precambrian up to the present day (Banks 1973;De Raaf et al 1977;Dabrio 1982;MacCarthy 1987;Driese et al 1991;Rine et al 1991). Sedimentary environments of the same type have been reported previously in the Carboniferous of the IPB (Moreno 1985(Moreno ,1993.…”

Late highstand patterns of shifting and stepping coastal barriers and washover-fans (late Messinian, Sorbas Basin, SE Spain)

“…Along tideless coasts, such as the present Mediterranean, the plunge step below the swashzone occurs between O m and -1 m below mean sea-Ievel (Dabrio, 1982). Therefore, the interfingering zone of low-angle seaward-dipping parallel lamination and seaward-oriented cross-bedding of the Sorbas Member is al so interpreted to indicate this palaeodepth (Fig.…”

Low-energy Morphodynamics of a Ridge and Runnel System

“…They thus form a part of the cycle of erosion of the upper beach during a storm and post-storm beach recovery and berm building due to the collective onshore migration of sand bars and welding to the beach face. This process has been described from many parts of the world, including: the Great Lakes ( DAVIS and FOX, 1972;DAVIS et al, 1972;STEWART and DAVIDSON-ARNOTT, 1988), western Europe ( VAN DEN BERG, 1977;DABRIO, 1982;AAGAARD et al, 1998), Australia (SHORT, 1978;WRIGHT AND SHORT, 1984), and the east and west coasts of North America (DAVIS et al, 1972;OWENS and FROEBEL, 1977). A number of studies have emphasized the role played by swash bores in transporting sediment across the bar crest and onto the landward dipping slip face when the water depth over the crest is small (DAVIS et al, 1972;VAN DEN BERG, 1977;DABRIO and POLO, 1981) and the bars are sometimes referred to as swash bars (AAGAARD et al, 1998).…”