Styles of early diagenesis and the preservation potential of onshore tsunami deposits – a resurvey of Isla Mocha, Central Chile, 2 years after the February 27, 2010 Maule tsunami

The style of early diagenesis and preservation of onshore tsunami deposits are poorly constrained. Only tsunami surveys and subsequent re-surveys can fill this information gap. Here we present the results of a first re-survey in 2012 on Isla Mocha following the original survey in 2010 in the wake of the February 27, 2010, Maule earthquake and tsunami in central Chile. As a result of this tsunami, a large number of boulders consisting of clay-rich fine sandstones representing the Miocene age bedrock of the island had been transferred from the tidal to shallow subtidal zone onto the coastal plain. Coarse elastic sediment mixtures of pebbles, granules, and sand entrained at coastal plain terraces and transported up to the maximum runup position c. 600 m from the coast by the inflow had been left behind as extensive backflow blankets on the lower coastal plain. In 2012, vegetation had covered the 2010 tsunami deposits. Sediment beyond 200 m from the coast had been removed by a combination of surface processes and grazing cattle. Grain-size distributions of the preserved sediment show an increase of the sand fraction at the expense of the coarser grain sizes. Boulders show patterns resembling mud cracks on the surface and evidence of disintegration into smaller fragments and sand. Veneers of dried algae documenting the derivation of the boulders from the tidal zones had flaked off partly or completely from most rock surfaces. At the northern, wind-facing coast of the island, a c. 130 m long and 1.2 m high beach ridge had accumulated, most likely from reworked tsunami sediment. Boulders deposited by tsunamis are commonly assigned a high preservation potential. We demonstrate for the first time that such boulders may in fact disintegrate rapidly and disappear from the record over short geological time scales, given a lithology susceptible to weathering. The degree of modification to the Isla Mocha tsunami boulders and deposits strongly questions the applicability of inverse models using paleotsunami deposit thickness and grain-size distributions to infer parameters and magnitudes of the causative tsunamis. Inverse models would consequently underestimate tsunami flow parameters and may result in the inference of erroneous transport modes and event magnitudes. Our study gives strong evidence in support of re-survey studies following the deposition of tsunami sediments by tracing and quantifying the changes affecting them.