Compositional variation in depositional sequences of the upper part of the Kazusa Group, a middle Pleistocene forearc basin fill in the Boso Peninsula, Japan

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Abstract Shelf to slope successions of the upper part of the Kazusa Group, as much as 700 m thick, constitute a northeasterly prograded clastic wedge of middle Pleistocene age and consist of six depositional sequences. Fundamental component building blocks of depositional sequences (i.e. lowstand, transgressive and highstand systems tract sediments) are characterized by unique combinations of depositional processes and petrofacies. Transgressive systems tract sediments are characterized by a high frequency of volcanic ash layers and volcanogenic petrofacies, indicating a substantial contribution of pyroclastic material to marine environments. This corresponds to the storage of coarse-grained, terrigenous clastic sediments at a river mouth area during transgressions. Highstand systems tract sediments are characterized by a lower frequency of volcanic ash layers and more quartzose petrofacies, and may have been affected by abrasion and destruction of unstable lithic fragments by nearshore dynamic processes, such as reworking by waves, tides and longshore currents, in response to active progradation of a coastal depositional system. In general, progradation of coastal depositional systems is associated with active supply of coarse-grained, terrigenous sediments by a fluvial system. The increase in supply of coarse-grained terrigenous sediments reduces the frequency of volcanic ash layers in the upper part of the highstand systems tract sediments. Lowstand systems tract sediments show more variable petrofacies than that of highstand systems tract sediments and may have been affected much more by the direct fluvial supply of coarse-grained, terrigenous sediments, which were not subjected to abrasion and destruction of unstable lithic fragments by nearshore dynamic processes. These results demonstrate that the development of the upper part of the Kazusa Group can best be interpreted in terms of the interaction between marine and fluvial processes, and fluctuation in the relative rate of coarse-grained, terrigenous sediment supply. The frequency of volcanic ash layers and the relative contents of unstable volcanic rock fragments in marine sediments may be a sensitive indicator of the dynamic processes and may provide an additional opportunity for characterizing systems tracts within a depositional sequence in a volcanic arc setting.