Sinusoidally deformed veins ("Sigmoidalkluftung") in the Lower Muschelkalk(Triassic, Anisian) of Central Germany: sheet injection structures deformed within the shallow subsurface

Sinusoidally deformed veins (SDVs) are a distinct soft-sediment deformation al structure of the Lower Muschelkalk of the Germanic Basin. The origin of these structures is confined to marry limestone/marlstone alternations. The environment of formation is that of the shallow subsurface, several decime tres of burial, situated between the range of bioturbation and burial zones of continuing vertical compaction, late diagenetic brittle deformation and pressure solution. The development of SDVs is twofold: At first instantane ous shrinkage of marry lime muds coupled with sediment injection leads to a rectilinear sheet injection structure with passively filled vertical shrin kage cracks (dykelets) and actively propagated subhorizontal sills and apop hyses displacing host sediment. The injected material consists of clayey mu d and quartz silt, exhibits layering subparallel to dykelet walls, and is m ost probably derived from mud layers directly below and above. Subsequently , the sheet injection structure is deformed either unidirectionally or bidi rectionally. Contemporaneous deformation of Rhizocorallium burrows in SDV b eds strongly suggest lateral shear stress rather than vertical compaction t o cause sinusoidal deformation of the sheet injection structure. Occasional ly, liquefaction and homogenisation occurs within the upper parts of the af fected bed. Bidirectional geometries suggest an earthquake-related trigger mechanism. Unidirectional geometries may reflect gravitational creeping tow ards depocentres, possibly due to short-term lateral mass disequilibria. Th e relative rare occurrence of SDVs seems best explained by a correspondingl y rare shallow burial rheological attribute defined by a combination of per meability, particle size, clay content, pore water pressure, and amount and type of organic matter. Interestingly, sinusoidally deformed veins are als o important in some Meso-/Neoproterozoic argillaceous lime mudstones, where they form part of so-called molar tooth structures.