Development of the AMS method for unconsolidated sediments. Application to tsunami deposits

Abstract

AMS (Anisotropy of Magnetic Susceptibility) is a geophysical method, which uses the electromagnetic properties of samples as a proxy of their fabric. Commonly used on hard rocks and cohesive sediments, the method has never been developed for unconsolidated sediments before. In this contribution, we present the usability of the AMS method on unconsolidated rocks and the new insights that AMS brings for the study of tsunami deposits, and for geomorphology. We have been working on the deposits of the coast of North Sumatra (Indonesia) because a devastating tsunami has struck these coasts and has redesigned the coastal environment and sediments distribution the 26 December 2004. Hence, this geographical setting has provided us with the necessary material for the study. We have carried out the analysis from 6 samples, which evidenced: (i) a first layer deposited by an uprush oriented to the SSW; (ii) a decanting phase; (iii) a layer deposited by a backwash oriented to the North; (iv) two other uprushes that deposited two units oriented to the SSW and the SSE. This research has proven that the AMS could be used on unconsolidated deposits, and that the orientation and an approximation of the energy during deposition could be inferred as well. It leads to new developments in marine and or fluvial geomorphology, even for unconsolidated sediments.