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

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.

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

“…disharmonic folding, tight to closed fold limbs, flame-like bed contacts; cf. Kelling & Walton, 1957;Owen, 1996Owen, , 2003 and glacitectonic 'pushing' (e.g. open folding exhibiting axial-planar cleavage, parasitic folding and hinge-faulting; cf.…”

Sedimentary cycles in a Mesoproterozoic aeolian erg-margin succession: Mangabeira Formation, Espinhaço Supergroup, Brazil

“…The minor occurrence of mudstone drapes indicates settling of suspended sediments in a low-energy environment, probably in the aftermath of ephemeral floods (Miall, 2006). The simple load-cast structures were likely generated in response to fluidization in unconsolidated sediments, whereby water-saturated layers lose strength (Owen, 2003).…”

Famennian glaciation in the eastern side of Parnaíba Basin, Brazil: evidence of advance and retreat of glacier in Cabeças Formation

“…Climbing ripple cross-lamination (facies Sts) in the distal portions of the sigmoidal lobes is in agreement with the interpretation (Bhattacharya 2010). The presence of a waterscape structure is the evidence of partial liquefaction of unconsolidated, water-saturated sediments (Lowe 1975, Owen 2003. The facies Sts and Fm are associated with flow deceleration along the sigmoidal lobes, which are defined by combined bedload and suspended load processes in the proximal reaches (facies Sts) that progressively pass into clay fall out (facies Fm) in the distal reaches (Potter et al 2005, Olariu & Bhattacharya 2006.…”

“…The presence of tabular beds (facies Sh) interbedded with the sigmoidal lobes suggests sand migration through unidirectional flow, under transitional to upper flow regimes with high rates of suspended load (Røe 1987, Mutti et al 2003, Olariu et al 2010). Facies Sm is also an indication of high rates of sand fall out and water saturated sediments, resulting in total or partial liquefaction, which produced massive bedding or waterscape structures, respectively (Lowe 1975, Owen 2003. Coarse sand grains along the foresets of sigmoidal cross-bedding were produced through coarse grain segregation during the migration of small-scale bedforms in the stoss side of the sigmoidal lobes and deposited in the lee side (Slingerland 1984, Reesink & Bridge 2007.…”