The Ohta Tephra Bed in the Tokai Group Chita Peninsula, Japan is of different purported age from correlated tephra beds in the Kobiwako Group Mie Prefecture , Himi Group Toyama Prefecture , and Hamatsuda Formation Niigata Prefecture. We attempted to resolve the age of the Ohta Tephra Bed by establishing its correlation with a tephra bed in the Miura Group Boso Peninsula. The detailed petrography and chemical composition of the tephras show that the Ohta Tephra Bed is correlated with the An Tephra Bed in the Miura Group; further more, the An and An Tephra Beds of the Miura Group are correlated with the Kosugaya and Sakai Tephra Beds of the Tokai Group, respectively. Based on average sedimentation rate of the Miura Group as determind from magnetostratigraphy and biostratigraphy, the age of the Ohta Tephra Bed is estimated to be about. Ma. The Kosugaya and Sakai Tephra Beds are. Ma and. Ma in age by the same estimation. This study is an important step in establishing a comprehensive stratigraphy of the Lower Pliocene formations in central Japan.
This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.
Abstract:Studies of stratigraphy and sedimentary system for Cenozoic sequence in the Northern Fossa Magna have progressed in these two decades. Especially a regional stratigraphical correlation using volcanic ash markers has remarkably progressed, and contributed markedly in the geological studies of the Northern Fossa Magna. The Upper Miocene to Pliocene sequence has been so far correlated on the basis of zoning of benthonic foraminifera in the petroleum geology. However, the stratigraphic correlation on the basis of this paleontological zoning is not consistent with the stratigraphical correlation using the volocanic ash markers. The contradiction between these stratigraphic correlations was examined in this study. The geohistorical change of depositional system and tectonic evolution of the Northern Fossa Magna were discussed on the basis of sedimentary facies analysis and sedimentary petrography. Five geohistorical stages, namely pre-rift, rift, stable, conversion, and deformation stages, were consequently distinguished in the Cenozoic of the Northern Fossa Magna.
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