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Soft Sediment Structures and Intrusions in Boso Peninsula and Eiko Gakuin, Japan

Roberto Weinberg, Monash University, Australia

Ryo Anma-san, University of Tsukuba, Japan



Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL:


I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.


DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.


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 geological map



Thrust: Eiko Gakuin


This locality has a wide range of structures confined to particular layers within the sequence. These layers have a number of metric to decametric blocks within a coarse scoria matrix.


thrusts stack, duplex, soft sediment deformatin
Sequence of thrusts preserved in a decametric block within a flow breccia



Breccia Sill


Breccia Sill
B) Flow breccia containing silt blocks within coarse scoria matrix. This is a 50cm wide intrusive sill, fed from below. Note dykes cutting across lower silt layers (shown in C).
sedimentary dyke, Neptunian dyke
C) Detail of dykes in B) cutting silt layer
scoria dyke, Neptunian dyke
D) Dykelet of scoria mingling with siltstone blocks on the way up (click to see large figure)





Diapir, diapirism
A) Diapir of silt
Rayleigh-Taylor instability, diapirs
B) Rayleigh-Taylor instability
of a silt layer
C) Faults cut the layer of
diapirs shown in A) and B)



Local Gravity Instabilities at Contacts


In the area amost every contact is unstable leading to some spectacular decimetric structures.
A) Scoria-rich upper part of layer is buoyant and lower, silt-rich part is denser
B) Two scoria diapirs at the top of the silt-layer, detached from the underlying source
C) Unstable layer, where small diapirs formed at cm-scale and then the whole sequence underwent intralayering slumping and folding at m-scale





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.
folded soft sediments
A) Fold from Nishikawana which must be scanned
sheath folded soft sediments
B) Sheath fold of a metric layer embedded in scoria matrix and intruded by a dyke of scoria



Stems of diapirs, pipes


These are the stems which remove liquified, buoyant material from lower layers and allows their upward movement.


diapir stems
A) Stem at Jogashima
diapir stems
B) Stem at Nishikawana



Flow on sediments


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.


A) Asymmetry of siltstone blocks at the top of the scoria layer suggest sediment flow to the right (see B and C). Note also that the silt from the lower layer is rising into the scoria layer as matrix.

Same layer as in A). Note a cusp of silt being dragged up from the lower layer by flow (lower right-hand-side). The cusp indicates flow to the right. Note also a cusp detached from its source within the dark grey scoria-rich bed.

C) Detail showing a siltstone block indenting the lower silt layer, also indicating flow to the right.



Neptunian Dykes


Neptunian dykes intruding from above. From Eiko Gakuin.


Neptunian Dykes
Neptunian Dykes