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

Abstract

The migration of ridge and runnel systems on microtidal coasts is a rather simple process starting with the formation of a ridge at the upper shoreface after storms and its migration landwards until it welds onto the berm. This results in an ideal sequence of structures which includes (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 range and higher wave activity on mesotidal coasts induce a more differentiated pattern. At early stages of accretion, the ridge is covered at high tide and exposed at low tide. Swash accumulates a secondary bar of coarse sediment during low water that is removed and incorporated in the slip face of the ridge by avalanching during rising tides. At high tide, large amounts of finer sediment are brought by waves and deposited on top of the ridge and in the slip face, generating fining-upward sequences. Erosional low-angle 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 accretion, the ridge is submerged mainly during spring high tide. After the runnel has been filled up, a vertical growth of the new berm occurs associated with fining-upward sequences due to decreasing energy and shoreward shifting of swash zones, during the transition from neap to spring tides. The resulting ideal sequence of sedimentary structures is larger and better differentiated than those described for microtidal and tideless coasts.