Load structures: gravity-driven sediment mobilization in the shallow subsurface

Abstract

Abstract Load structures are a type of soft-sediment deformation structure comprising synforms (load casts and pseudonodules) and antiforms (flame structures and diapirs) at an interface. They form in response to unstable density contrasts (density loading) or lateral variations in load (uneven loading) when sediment becomes liquidized or otherwise loses strength. They are here classified into five varieties: simple and pendulous load casts, in which the upper (denser) layer is laterally continuous; and attached pseudonodules, detached pseudonodules and ball-and-pillow structure, in which discrete masses of the upper layer are separated by matrix. Conceptual models demonstrate that there are several possible modes of formation for each type of load structure. One interpretation of the variation of load structure morphology is as a deformation series representing varying degrees of deformation, controlled by the magnitude of the driving force and/or the duration of its effective action. An interpretation of the commonly observed pattern of wide load casts and narrow flame structures is presented in terms of their differential growth. Fluidization has an important influence on the development of load structures and their relationship to other products of sediment mobilization.