Heavy minerals in tsunami and storm deposits have been used to establish sediment sources and to infer the inundation and backwash phases (Morton et al., 2007). The abundance of these minerals is dependent on the hydrodynamic conditions that existed during transport and depositional stages. Overall, heavy mineral analysis allowed interpretations on sediment dynamics. Heavy mineral studies on tsunami deposits allowed the establishment of source-to-sink relationships thus, contributed to establish transport paths and inundation routes (Jagodzinski et al., 2012; Putra et al., 2013; Costa et al., 2015; Cascalho et al., 2016).
After the Tohoku-oki tsunami event, GeoSlicer were excavated and tsunami imprints were retrieved from the slices in Misawa coastal area (Japan). Heavy minerals from thirty-six samples were analyzed. Heavy minerals in the sediment fraction of 0.125-0.500 mm were separated by centrifugation in sodium polytungstate (2.90 kg/m3) and recovered by partial freezing with liquid nitrogen. An average of about 220 transparent heavy-mineral grains per sample were identified and counted under a petrographic microscope. Heavy minerals not mounted on glass slides were subjected to the ferromagnetic separation using a Frantz Isodynamic Magnetic apparatus to estimate the weight of magnetite in each sample.
Heavy-mineral weight in total sediment fraction presented a mean value of 31%, ranging between 18 and 59%. The magnetite weight percentage present in the heavy-mineral fraction has a mean of 26% ranging between 14 and 43%.
Considering the mean frequency of the transparent heavy minerals it was identified the presence of orthopyroxenes (67%), followed by clinopyroxenes (30%).
These results indicate that the main original source of heavy minerals are basic volcanic rocks. The wide ranges of variation of the total heavy mineral fraction and the magnetite present in that fraction provides useful information about the flow competence of the tsunami waves. The samples that reveal higher concentration in total heavy minerals tend to be richer in magnetite. These results could be used to pinpoint water flow conditions (velocity thresholds) promoting grain sorting leading to the formation of layers enriched in heavy minerals. Confirming previous cases, heavy mineral analysis in Misawa tsunami deposit seems to provide useful insights into tsunami-derived sediment dynamic.
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Putra, P.S., Nishimura, Y., Nakamura, Y. and Yulianto, E., 2013. Sources and transportation modes of the 2011 Tohoku-Oki tsunami deposits on the central east Japan coast. Sedimentary Geology, 294, pp.282-293.
The author would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and by FCT OnOff project PTDC/CTAGEO/28941/2017.