The continental shelf of eastern Spain (extending from the Ebro River southward through the Gulf of Valencia) is accumulating a mixture of carbonate and siliciclastic sediment. Twenty-eight cores were analyzed to characterize sediment size and composition and to measure mixing and accumulation rates. Sandy mud consisting of about 40% carbonate and 60% siliciclastic sediment is accumulating over a transgressive sand and gravel layer that is exposed landward of the 20-m isobath. Profiles of Pb-210 and Cs-137 indicate that much of the mud deposit has an apparent accumulation rate of about 1 mm/yr. The observed accumulation is slow compared to dispersal systems associated with larger fluvial sources of siliciclastic sediment. The slow accumulation rates combined with moderate sediment mixing rates (measured from Th-234 profiles) predict preservation of homogeneous strata, which is observed in x-radiographs. Although the siliciclastic accumulation is relatively slow, the carbonate accumulation rates are similar to values for low-latitude carbonate environments. The slow accumulation of siliciclastic sediment combined with apparently high productivity of carbonate sediment results in the mixed carbonate and siliciclastic deposit. The special characteristics of the study area are: (a) siliciclastic sediment flux to the shelf is small but significant; (b) physical-oceanographic regime is quiescent enough to allow much of the fine sediment to accumulate on the shelf; and (c) shelf waters are warm (for mid-latitudes) allowing rapid production of carbonate sediment. /P>

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