Sediment Flow and Deformation (SFD) Layers: Evidence for Intrastratal Flow in Laminated Muddy Sediments of the Triassic Osawa Formation, Northeast Japan

Thin chaotic layers containing abundant intraformational mud clasts are found in the offshore laminated mudstone of the Tri-assic 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 struc-tures 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 identi-fied 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 de-formation. Occurrence of the SFD layers is stratigraphically limited to the up-permost 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 facili-tated intrastratal flow and sill intrusions of sand. The vertical confine-ment was also controlled by the anisotropy in physical properties, such as tensile strength of laminated mud and/or conditions of gentle slope.