Wrinkle marks: An intertidal sedimentary structure due to aseismic soft-sediment loading

The structures have a wide distribution intertidally in the macrotidal Severn Estuary and inner Bristol Channel, occurring on the active sedimentary surface chiefly in association with a millimetre-scale sand-over-mud doublet. The sand surface is marked by depressions and ridges, of a wide variety of shapes and orientations relative to slope, that are matched antithetically by ridges and depressions on the mud-sand interface below. The sand layer consequently takes the form of a series of touching to detached pillow-shaped masses, whose centres lie 2–4 sand-layer thicknesses apart. Wrinkle marks originate soon after the emergence of the sedimentary surface into the atmosphere, and are considered to be soft-sediment deformation structures on account of their (1) occasional displacement and/or severance of early-emergence surface organic trails and feeding marks; (2) similarity in shape and internal structure to pseudo-nodules recorded from the field and produced experimentally; and (3) association with gravitationally unstable sediment layers in settings that favour the vigorous seepage of pore water through water-logged sediments. Owing to their origin at the sedimentary surface, wrinkle marks appear to be preserved mainly as surface markings on eroded and (discordantly) reburied mud-sand interfaces.