Multi-proxy analysis of tsunami deposits – the Tirúa, Chile, example

Abstract:

Tsunami and paleotsunami deposits are the basis of useful tools to assess both magnitude and frequency of tsunami hazards. Our multi-method study of recent and paleotsunami deposits at the coastal locality Tirúa, Central Chile, demonstrates the necessity of combining different proxies when reconstructing the tsunami record of a locality with a complex tsunami history. Analysis of grain size, petrography and geochemical characteristics, and diatoms, in combination with OSL dating and inverse modelling using TSUFLIND were applied to inferred tsunami and river marsh sediments in the uppermost 2 m of a flood plain in distances of 1.2 to 2 km from the shoreline. The river marsh profiles comprise a complex record of several tsunamigenic sand intercalations showing erosional bases, landward thinning and minor decreases of grain size along the river floodplain.

Some of the tsunamigenic intercalations consist of unimodal sand with most fractions coarser than 3Φ, whereas the river marsh sediments have polymodal grain size distributions with fractions finer than 3Φ. Some layers interpreted to be tsunamigenic yield mixed grain size distributions similar to the river marsh sediments. Geochemical signatures further discriminate between the different sediments. The tsunami sand intercalations are enriched in Ca, Si, Sr, Ti, Fe and depleted in Al, K, Rb, with low LOI. The enrichment in Fe and Ti, often related to terrigenous input, implies an accumulation of heavy minerals in these layers. The presence of marine diatom assemblages, OSL age inversions in some tsunami layers combined with results of inverse modelling of maximum deposition distance help to identify non-distinct tsunamigenic layers in the river marsh.

Only the combination of these different and independent methods allowed for the recognition of six different tsunami events of varying magnitude in the last 1,500 years.