Facies models for silicic subaqueous tephras in the Niigata region, central Japan are presented. These tephras are intercalated in the upper Miocene to lower Pleistocene of deep to shallow marine, lacustrine and fluvial environments. Lithofacies of these tephras are diverse, though mostly medial to distal facies, and possible origin of each lithotype is discussed in terms of subaqueous fallout, ash cloud, subaqueous flow and epiclastic ones. Origin of ash flow turbidites, some subaqueous tephra sequences and the possible origin of tephras by subaqueous eruption are also discussed. To establish facies models, tephras were firstly grouped into three grain-size populations; that is (A) silt to fine sand-sized, (B) fine sand-sized to granule-sized, (C) containing 5% or more pumice clasts larger than granule size. TypeA1 (massive or partly laminated), TypeA2 (normally graded, gradational to mud upwards) and TypeBl and TypeC1 (sorted pumice clasts making laminations in sediments) are variations of fallouts. TypeA3, TypeB4 (inverse-to-normally graded turbidites with crude laminations) and TypeC3 (inverse-to-normally graded pumiceous turbidite with pebbly base) are referred to ash flow turbidites possibly as well as TypeB3 (inverse-to-normally graded pumice clasts in ashes). TypeA5 (crudely laminated fine ashes with some pumice clasts) is representative of ash clouds as well as TypeB2 (scattered pumice clasts in ashes or sediments). TypeC2 (inversely graded pumiceous ash) is indicative of pumiceous debris flow. TypeA4 (organized to Bouma sequence) is mostly dilute ash flow turbidite. Some of TypeB5 (organized to Bouma sequence with accidental grains to some extent) are also referred to ash flow turbidites, but most of TypeB5 are epiclastic, as well as TypeC4 (normally graded pumiceous volcaniclastic sediments with rip-up clasts). It is noteworthy that most of TypeB4 as well as TypeA3 and TypeC3 are widely traceable (more than 50km) with small -ΔT/ΔD (differential aspect ratio) such as 1:20000 to 1:200000. They are turbidites directly transformed from subareal ash flows by mixing with water in subaqueous environments, or differentiated at the base of the dense ash clouds of subaqueous eruption column. The author referred them to ash flow turbidites. The model sequence inlcuding ash flow turbidite units is also presented, in which ash flow turbidite occupies the basal unit (Unit-I). In the case of subaqueous eruptions, fines and pumices are confined in water as suspension or dense ash cloud, then ash clouds of fines with some pumice clasts (TypeA5), dilute ash flow turbidites (TypeA4) and/or pumiceous debris flows (TypeC2) will be generated. Thus, the tephra sequence such as the piles of TypeA4, TypeA5 or TypeC2, and composite sequences of TypeC3-TypeA4 (double grading) implicate the possibility of subaqueous eruptions.
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