The September 16, 2015 Illapel tsunami, Chile - Sedimentology of tsunami deposits at the beaches of La Serena and Coquimbo

On September 16,2015, a Mw 83 earthquake occurred off the coast of central Chile, 46 km west of the town of Illapel. The quake caused a tsunami impacting the coast mainly between the towns of Caldera (c. 27 degrees S) and Pichidangui (c. 32 degrees S). At Coquimbo and La Serena (c. 30 degrees S) the tsunami attained large wave heights on the order of 4.5 m leading to flooding and destruction of infrastructure including the coastal road. We report the results of a post-tsunami sedimentological field survey undertaken in October 2015 focussing on the beaches of Coquimbo and La Serena. At Playa Changa, a beach at Coquimbo affected by the highest waves, the tsunami destroyed large parts of the coastal road embankments on both the seaward and landward sides. On the landward side the tsunami distributed triangular concrete construction elements of the embankment across the coastal plain. On the seaward side granite boulders of up to 2 t were ripped out of the coastal road embankment and transported onto the beach. Sand used in construction of the embankment was redistributed across the beach as an outflow deposit. At La Serena we observed a laminated package of tsunami deposits along a 20 m long trench perpendicular to the coast. The sediments have erosive basal surfaces with an amplitude of up to 50 cm. The deposit thickness varies between 10 and 50 cm. The sedimentary package consists of 6 mostly lenticular layers of variable thickness and spatial extent, ranging between dark laminae a few millimeters thick and rich in heavy minerals, and lighter colored sand layers up to 15 cm thick. Grain size distributions are moderately well to well sorted and unimodal with modes between 13 and 2.0 Phi (medium sand). Cross-beds in the lower four layers indicate deposition from tsunami inflow, cross bedding in the second layer from the top layer records outflow. The abundance of cross beds and laminae suggests that deposition of sand from bed load traction currents was the prevailing mode of deposition. Water escape structures occur as small sand diapirs and volcanoes. Water escape through small volcanoes appears to have been coeval to formation of the overlying layer by traction deposition. This synchroneity is indicated by sand sourced from the lower layer which has been preserved as a thin plume deformed in the downcurrent, i.e. landward direction in the newly forming upper layer. Other sediment layers show sand diapirs intruding up to 15 cm into the overlying tsunami deposit. The assemblage of sedimentary structures indicates rapid deposition, from one or several waves associated with the tsunami event. (C) 2017 Elsevier B.V. All rights reserved.