Fundamental study on the mode of transportation and sedimentation of subaqueous tephras

点击翻译按钮获取中文摘要

Mode of transportation and sedimentation of subaqueous tephras was investigated especially on those intercalated in the upper Miocene to lower Pleistocene in the Niigata backarc basin, central Japan. They were classified in four categories; (1)subaqueous fallout, (2)ash cloud, (3)subaqueous flow and (4)their epiclastic reworks. Fourteen facies models of these tephras were established in respect to grain-size populations; TypeA1-A5 for silt to medium sand-sized tephras (Population A), TypeB1-B5 for medium sand to granule-sized tephras(Population B) and Type C1-C4 for those containing considerable amounts of pumice clasts larger than granule size(Population C).It is noteworthy that among these subaqueous tephras, subaqueous flows which were directly transformed from subareal ash flows or subaqueous dense ash clouds are not uncommon regardless of their grain-size population; fine-grained ones(TypeA3), medium-grained ones(TypeB4) and coarse- grained-ones(TypeC3). They show inverse-to-normal grading, containing mud balls or rip-ups, and flowed more than 50km with little decrease of thickness. In this research, they are named ash flow turbidites. The Joetsu Ash (Kurokawa and Hirata,1986) is a representative of the ash flow turbidites of TypeB3. It is cordierite-cummingtonite bearing ash of subareal ash flow origin, flowed into water.The basic sequence containing the ash flow turbidites was clarified; the ash flow turbidite unit occupies the base of the sequence (Unit I), overlaid by thin alternations of fine-grained ashes (Unit II) and pumice-rich ashes (Unit III). This sequence suggests the flow differentiation from subareal ash flow or subaqueous ash cloud.Besides, osumilite and magmatic andalusite were found from Pliocene subaqueous tephra (the Sug-a Ash) in the course of this study.