Sedimentology and paleontology of a tsunami deposit accompanying the great Chilean earthquake of February 2010

At Pichilemu, in the northern third of the rupture area of the moment magnitude scale (M_w) 8.8 2010 Chile earthquake, deposits of the tsunami accompanying the earthquake consist of a lower layer of medium to fine sand (mean grain size of 200μm) containing rock clasts, overlain by a thin, silty, very fine sand (mean grain size of 125μm) layer. Based on a sedimentological model, most (90%) of the deposit is finer than 401-408μm suggesting tsunami flow velocities were between 7m/s and 13.5m/s. Ostracods were common in the upper layer along with a small number of broken benthic foraminifera and a single planktonic foraminifera. Diatoms were abundant throughout. Species assemblages represent a mixture of diatoms from differing environments, life forms and substrate preferences. We attribute the mixed assemblages to turbulence within the water column during tsunami inundation, with erosion of beaches and salt marshes followed by redeposition of sand and mud inland. Breakage of fragile diatom valves in the lower layer may also support transport by turbulent flow. A higher abundance of diatom species with mud substrate preferences in the upper layer implies a decrease in flow velocity from lower to upper layers.