Schindlbeck, Julie , Kutterolf, Steffen , Freundt, Armin , Scudder, R. P., Pickering, K. T. and Murray, R. W. (2013) Emplacement processes of submarine volcaniclastic deposits (IODP Site C0011, Nankai Trough). Marine Geology, 343 . pp. 115-124. DOI 10.1016/j.margeo.2013.06.017.

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Abstract

During IODP NanTroSEIZE Expedition 322, four packages of tuffaceous sandstones (TST 1, 2, 3a, 3b) were recovered within a moderately lithified and bioturbated silty claystone succession in the Late Miocene (> 7.6 to ~ 9.1 Ma) upper part of the middle Shikoku Basin deposits. To assess the emplacement processes of the tuffaceous sandstones we investigate modal and geochemical compositions of 24 thin sections that reveal systematic vertical changes within each bed. TST 1, 2 and 3b are single beds whereas TST 3a is composed of at least two beds suggesting several rapidly succeeding sedimentation events. The beds are density-graded such that low-density pyroclasts including pumice lapilli are enriched at the top whereas dense lithic components and minerals are enriched at the bottom.

The volcanic glass particles (pumice and shards) that are the dominant modal constituents of each sandstone bed have homogeneous geochemical compositions in each bed. Moreover, TST 1, 2, and 3a glass compositions overlap completely but TST 3b glass has a different composition, as is analogously observed for the mineral compositions.

This unique multistage approach of sedimentological and geochemical methods facilitated the detailed investigation of distal volcano-derived, probably tsunamogenic, turbidites in order to contribute to the distinction between primary and secondary induced mass flows. We interpret that all tuffaceous sandstones were emplaced by turbidity currents that were formed during major explosive volcanic eruptions. However, while TST 1, 2, and 3a turbidity currents formed by the entry into the ocean of voluminous pyroclastic flows erupted at a volcano on mainland Japan, TST 3b was emplaced by a turbidity current formed by a shallow submarine or subaerial volcanic eruption at the Izu–Bonin arc where it collided with Japan. These results regarding distal turbidites encourage the revisiting of older marine deposits in the scope of hazard evaluation through past events, especially in regions near to volcanic sources.

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