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Soft Sediment Structures and Intrusions in Boso Peninsula and Eiko Gakuin, JapanRoberto Weinberg, Monash University, Australia Ryo Anma-san, University of Tsukuba, Japan |
Soft Sediment Structures
Soft sediments in the accretionary prism in the ...Peninsulas provide amazing opportunities to understand the processes of dewatering and compaction in volcaniclastic sediments. These rocks are particularly interesting because the clasts have large variation in density, from very low density basaltic scoria with 50-60% porosity to small, dense basalt clasts with very small porosity. This variation, the rapid deposition of sediments during or soon after volcanic eruptions, and the high seismicity of the region, creates ideal conditions for the development of the variety of soft sediment structures described in this page. |
Geological map of Tokyo
Bay ![]() |
![]() Sequence of thrusts preserved in a decametric block within a flow breccia |
![]() A) Diapir of silt |
![]() B) Rayleigh-Taylor instability of a silt layer |
![]() C) Faults cut the layer of diapirs shown in A) and B) |
In the area amost every contact is unstable leading to some spectacular decimetric structures.
Blocks of folded soft sediments, deformed in the process of sediment flow, disruption and brecciation of the silt-rich layer by flow of the scoria matrix.
![]() A) Fold from Nishikawana which must be scanned |
![]() B) Sheath fold of a metric layer embedded in scoria matrix and intruded by a dyke of scoria |
These are the stems which remove liquified, buoyant material from lower layers and allows their upward movement.
![]() A) Stem at Jogashima |
![]() B) Stem at Nishikawana |
This is a layer in Eiko Gakuin, where a top silt layer which has been disrupted by flow, and the lower silt layer is only partly disrupted.
Neptunian dykes intruding from above. From Eiko Gakuin.
![]() A) |
![]() B) |