Multiproxy analysis of tsunami deposits-The Tirúa example, central Chile

Abstract

The study of prehistoric tsunami deposits enables us to appraise the recurrence frequency and improve risk assessments of coastlines. We present a study of tsunami deposits (A.D. 500 to present) exposed at Tirúa, central Chile. Analyses of various proxies such as grain size, petrography, geochemistry, and diatoms were applied to the tsunami deposits intercalated in the uppermost 2 m of the Tirúa River floodplain (1.2-2 km inland) in order to distinguish these tsunami deposits from the surrounding river marsh sediments. The tsunami sand layers are characterized by erosional bases and landward thinning and fining, and they consist of well-sorted, unimodal sand commonly > 3Φ. The fluvial marsh sediments, in contrast, exhibit polymodal grain-size distributions with a distinct fraction commonly < 3Φ. However, some older tsunami sand layers (sand layers 4 and 5) exhibit similar grain-size characteristics as the surrounding river marsh sediments, implying environmental changes. The diatom data also indicate environmental changes caused by neotectonic movement, at least for the lowermost sand layers of the profile (sand layers 4, 5, and 6). However, all six sand deposits are enriched in Ca, Si, Sr, Ti, and Fe and depleted in Al, K, and Rb, and they yield low loss on ignition (LOI) values, resulting from heavy mineral accumulations in these layers. Based on the bulk of these findings, in combination with an optically stimulated luminescence (OSL) age inversion in sand layer 5, all six sand layers in the floodplain succession at Tirúa are interpreted as tsunami deposits. They represent six different events connected to vertical neotectonic movements that occurred at the central Chilean margin during the last 1500 yr, extending the historical tsunami record in central Chile over 1000 yr.