Major lateral sediment displacement in till-sand-peat associations of the Tjonger Valley fill (the NE Netherlands) - Holocene cryoturbation?

Abstract

Attention is drawn to an extraordinary type of large-scale cryoturbation in the Tjonger Valley fill (NE Netherlands). A large lateral displacement of sediments, involving sand and till intrusion, is the most conspicuous feature. Sand intrusion has been found especially in gyttja and peat, whereas till and some loam intruded into sand. In the former case elongated ridges, some 10's x 100's metres wide, exhibit dipping layers, partly with extensive folding and some thrustplanes. Till intruded from part of the valley margin into sand layers with an abundance of flow structures and some rare thrusts. Intrusion of the sand into gyttja and peat presents opportunities to date the event. The first indications point towards an early Holocene age. Major cryoturbation was previously only known from the Pleniglacial of the last ice age in the Netherlands. Density inversions over thawing permafrost, in a watersaturated environment, are thought to have induced gravity spreading in the vast, but discontinuous layers of various compositions. The type of cryoturbatic structure encountered within deposits not only strongly varies with composition, but also with the dimensions of layers of the same composition.