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

A 25-m thick transgressive systems tract in the Sant Llorenc: del Munt, wav e-influenced, fan-delta system (Eocene, SE Ebro Basin) has an internal fram ework consisting of a cluster of transgressive erosion surfaces, each of wh ich has minor relief and which are collectively stacked vertically no more than two metres apart. Each erosion surface bounds a cycle containing a con glomeratic lag (up to 0.5-m thick) followed by a coarsening-upward sandston e to conglomeratic unit (the uppermost levels of which can be nonmarine). I ndividual 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 surf aces that repeatedly eroded into the conglomeratic shoreface during transgr ession. 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 assoc iated 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 subhorizontal, suggest rep eated reworking of previously generated lag pavements in zones of minimal a ggradation during transgression. Where the transgressive shoreline trajecto ry rises more steeply and there has been more rapid aggradation during tran sgression, the lag pavements show only single-phase borings, with the borin gs on the upper surface of the pebble-pavement only. The close spacing of erosion surfaces within the transgressive systems trac t, together with estimates of time span in the tract, suggest that transgre ssive erosion occurred with a frequency of less than 500 years. (C) 2000 El sevier Science B.V. All rights reserved.