Bored pebbles and ravinement surface clusters in a transgressive systems tract, Sant Llorenç del Munt fan-delta complex, SE Ebro Basin, Spain

A 25-m thick transgressive systems tract in the Sant Llorenç del Munt, wave-influenced, fan-delta system (Eocene, SE Ebro Basin) has an internal framework consisting of a cluster of transgressive erosion surfaces, each of which has minor relief and which are collectively stacked vertically no more than two metres apart. Each erosion surface bounds a cycle containing a conglomeratic lag (up to 0.5-m thick) followed by a coarsening-upward sandstone to conglomeratic unit (the uppermost levels of which can be nonmarine). Individual cycles become entirely nonmarine landwards of the termination of the basal-bounding erosion surface, whereas they thin and eventually become entirely marine basinwards.The individual erosion surfaces within the transgressive tract, some 16 of them within a 20-m thick lithosome, are interpreted as wave-ravinement surfaces that repeatedly eroded into the conglomeratic shoreface during transgression. This interpretation, rather than one invoking nonmarine flooding or other marine erosion surface types, is consistent with the arrangement of bivalve and sponge borings on the top surfaces of clasts and with the associated lag pavements. Multiphase boring around the entire surface of clasts, as well as erosion of the clasts at some horizons, particularly in reaches of the tract where the ravinement trajectory is sub-horizontal, suggest repeated reworking of previously generated lag pavements in zones of minimal aggradation during transgression. Where the transgressive shoreline trajectory rises more steeply and there has been more rapid aggradation during transgression, the lag pavements show only single-phase borings, with the borings on the upper surface of the pebble-pavement only.The close spacing of erosion surfaces within the transgressive systems tract, together with estimates of time span in the tract, suggest that transgressive erosion occurred with a frequency of less than 500 years.