THE EBRO CONTINENTAL-SHELF - QUATERNARY SEISMIC STRATIGRAPHY AND GROWTH-PATTERNS

Single-channel seismic-reflection profiles on the Ebro shelf and upper slope between Cap Salou and the Columbretes Islands have been interpreted to outline the Quaternary shelf evolution in this deltaic area. Three types of unconformity surfaces can be differentiated in the seismic records, type 1 and 2 unconformities, and toplap surfaces. These surfaces are the stratigraphic expression of the difference between rate of eustatic sea-level fall and subsidence on the outer shelf: a type 1 unconformity forms if the rate of eustatic sea-level fall is the more rapid, a type 2 unconformity forms if subsidence is faster, and toplap forms if subsidence and eustatic sea-level fall are equal. Thirteen stratigraphic units can be differentiated on the basis of unconformities. Based on their relative location on the shelf, these units are subdivided into inner and outer units. The inner units occur on the inner and middle shelf and are characterized by an underlying transgressive surface, while the outer units extend onto the middle to outer shelf and upper slope and show unconformities at their bases. The inner units are interpreted to be stacked transgressive and highstand systems tracts bounded by a maximum flooding surface. The outer units correspond to shelf deltas and shelf margin deltas that prograded southward and seaward over an unconformity in relatively stable water depths when sea-level fall was compensated for by subsidence. The progradation of these units ends with an episode of generalized erosion of the shelf and canyon cutting across the new shelf edge. The growth pattern of the northern Ebro shelf is characterized by important hiatuses, condensed stratigraphic sections, and units which are older than those on the southern shelf. The depositional processes were mainly controlled by eustatic sea-level oscillations, subsidence and changes in sediment supply. The subsidence during the Late Pleistocene was sufficient to compensate for the sea-level falls and allow the development of shelf deltas and shelf-margin deltas on the middle and outer shelf during low sea-level stands.