Sediment distribution on the Ombrone River delta seafloor and related dispersal processes

This paper aims to define seafloor sediment distribution and dispersal processes on the Ombrone River delta (Tuscany), a clearly wave-dominated delta type. A total of 108 grab samples were collected and submitted to high-resolution grain size analysis by a laser instrument. The data set was treated by multivariate analyses, for grouping samples and size fractions. Contour maps based on these statistics were used for interpretation. The seafloor sediment distribution shows a land-sea graded pattern, with sand on the delta front and mud on prodelta areas. Moreover, individual evidences are present on the central delta portion, in response to wave-energy concentration (shallow seafloor) and river sediment overabundance (deeper seafloor). The most significant process is fine river-particles bypassing the delta front and sinking on the prodelta. It is accomplished by protruding buoyancy outflows (single-event dispersion) and by waves expelling river-particles from the delta front (long-term dispersion). In the receiving area of this bypassing (prodelta), particles settle progressively during seaward transport, determining a down current facies differentiation. This dispersion particularly affects the central section of the prodelta, and it is slightly deflected to the north by the Tyrrhenian current flowing parallel to the continent. Along the littoral, wave refraction energy-gradients induce a confined sand drift from the apex (area of multiple sources) to the delta wings. Delta front dynamics place the Ombrone River delta in the class of wave-dominated deltas.