Abstract

Thin chaotic layers containing abundant intraformational mud clasts are found in the offshore laminated mudstone of the Triassic Osawa Formation in northeast Japan. The geometry and internal structures of the layers are somewhat similar to those of slump/slide or debris-flow deposits, however, the layers have both upper and lower erosional contacts. Projections of host sediments and mud clasts with planed surfaces are identified beneath the upper contact of the layers. Closely packed sheet-form or ellipsoidal mud clasts are embedded in a poorly sorted fine- to coarse-grained sandy matrix. Some mud clasts are tightly folded, or show imbricated stacking. Internal layering structures are also well developed in the layers.

The above features strongly suggest soft-sediment deformation as horizontal displacement of sand and mud under buried conditions. Here, we term these layers "sediment flow and deformation (SFD) layers." Deformation layers with similar attributes have been identified by some prior authors, as in the case of flow of liquefied and/or fluidized host sand and clastic sill intrusion. However, the SFD layers described herein have more complex structure indicating advanced deformation.

Occurrence of the SFD layers is stratigraphically limited to the uppermost part of the Osawa Formation, which is overlain by sand-rich submarine-fan deposits (the Fukkoshi Formation). It suggests that the stress field caused by differential loading of overlying deposits facilitated intrastratal flow and sill intrusions of sand. The vertical confinement was also controlled by the anisotropy in physical properties, such as tensile strength of laminated mud and/or conditions of gentle slope.

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