Physical criteria’s to distinguish a Tsunami deposit form a Storm deposit.. By, Frango C.Johnson Research Scholar Department of Civil Engineering IIT Kanpur, U.P (India)

TSUNAMI DEPOSITS STORM DEPOSITS  Different characteristics found in tsunami and storm deposits, described in the literature EVIDENCE Morphological Wash-over fans behind breached barriers Stratigraphical Fines inland Erosional basal contact Large inland extent Thins inland Fines inland Erosional basal contact Relative smaller inland extent Thins inland and becomes discontinuous Wash-over fans behind breached barriers

EVIDENCE Sedimentological Intraclasts from underlying material Not found Loading structures at base Not found Bi-directional imbrications Unidirectional imbrications Poorly sorted (particle size ranging from mud to boulders) Relatively better sorted Sedimentary structures very seldom found Sedimentary structures more common

EVIDENCE Geochemical Palaeontological Plant fragments Shell rich units Rafting light material Buried plants at base Plant fragments Shell rich units Not found Buried plants at base Increase in geochemical elements indicating marine origin No information found, but similar signature is expected because of marine origin Marine fossils: Mixture of marine and fresh water fossils Relative well/poorly preserved fossils Marine fossils: Increased diversity (mixture marine and brackish fossils) Relative well/poorly preserved fossils Mineralogical Significant Heavy mineral concentration Not found

Twig orientation Aerial photograph of tsunami deposit distribution and flow directions at Arop transect, Papua New Guinea. From Gelfenbaum and Jaffe (2003)

Tsunami deposit characteristics By : Morton et al. (2007)

Typical Characteristics of tsunami deposits : Tsunami deposits produced as a result of onshore run- up and backwash can be very distinctive (Nanayama et al., 2000; Dawson and Shi, 2000). Tsunamis comprised both landward- and seaward- directed layers Typhoon-related storm deposits showing fore set bedding Landward of these continuous tapering sediment wedges there may be discontinuous lenses of tsunami deposited sediment (Dawson, 1994).

Large coral-reef fragments deposited on Pakarang Beach, near Khao Lak, Thailand, by the 26 December 2004 tsunami. The high energies needed to remove these boulders may ensure they have a higher preservation potential than other tsunami-related onshore sediments.

Tsunami sediment-Macro- and micro fauna. The macro-faunal content can range from fish remains to a wide range of shell debris. Presence of benthic foraminifera ought to be an important criterion that can be used to identify past tsunami deposits. Nanayama and Shigeno (2006) report deep water marine benthic foraminifera within the 1993 Hokkaido tsunamis, indicating that the tsunami was picking up foraminifera tests on the seabed depths of between 45 and 90 m (and implying seabed current velocities of 0.2–0.5 m/s).

Sandy storm deposits Sandy storm deposits tend to be >30 cm thick, generally extend <300 m from the beach They typically are composed of numerous sub- horizontal planar laminas Multiple lamina sets that are normally or inversely graded. They do not contain internal mud laminas and rarely contain mud intra- clasts.

Proximal overwash deposits of Hurricane Carla on (A) Bolivar Peninsula, Texas and (B) Matagorda Peninsula, Texas. Deposits were approximately 130 and 60 cm thick, respectively, and consisted of poorly sorted sand and shell. Abrupt basal contacts overlie well-sorted sand of older barrier-island deposits. By : Morton et al. (2007)

Hurricane deposits and sediment texture exposed in the ocean side By : Morton et al. (2007)

Conclusion The above mentioned physical criteria’s are the preliminary clues; and it can be very useful for the determination between tsunami deposits from the storm deposits. Application of these distinguishing characteristics depends on their preservation potential and any deposit modifications that accompany burial.

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