Coarse-Grained Tsunami Deposits in the Canary Islands: Evidence of the Tsunamigenic Potential of Volcanic Flank Failures

Coarse-grained, polymict deposits draping hillslopes at high elevations on ocean island volcanoes have been variously interpreted as sourced from sea-level high-stands and tsunamis, their origin is thus controversial. Here, we present a detailed facies analysis of coarse-grained, fossiliferous sediments located at Agaete, on the north-west coast of Gran Canaria. Previously interpreted as the result of a sea-level high-stand, these deposits have recently been re-interpreted as sourced from a tsunami resulting from a volcano flank collapse. The tsunami deposit occurs at several locations in the Agaete valley, up to 188 m a.s.l. and two kilometres inland from the coast. The sediments comprise two reverse graded units, separated by a soil horizon with calcrete. The units are up to 2 m in thickness and contain a diverse assemblage of volcanic clasts, large beachrock boulders and a marine fauna that includes planktic, shallow and deep-water benthic foraminifera. The base of the lower unit is erosional and truncates plant roots and contains large soil clasts up to 1.5m in diameter. The upper unit is finer grained and is more extensive. At one location the lower unit comprises prograding beds that are interpreted as alluvial. The alternative 'high-stand' interpretation of the coarse-grained deposit is here discounted because of the absence of supporting geomorphological features such as a marine terrace and/or a wave cut platform, the composition of the sediments and their altitude. A tsunami origin is therefore proposed. Gran Canaria is in its post-shield erosional stage of development and has experienced limited vertical uplift of ~40-50 m over the past 1.75 Ma. Thus uplift cannot account for the occurrence of the sediments at elevations of 188 m a.s.l. The Güimar lateral collapse on the neighbouring island of Tenerife (~800ka) has been cited as a possible source. U/Pb dating of the deposits is currently being undertaken.