Covering sands, cryoturbations, and fractures in the dunes of the Plock Basin

Abstract

Article in French.

ORIGINAL TITLE: Les sables de couverture, les cryoturbations et les fractures dans les dunes du Bassin de Plock

Investigations on the dunes in the Płock Basin revealed secondary deformation in stratified sands and the interruption of layers as well as their complete destruction. These phenomena are classified in two groups: (a) covering sands and cryoturbation structures originated under the periglacial climatic conditions, and (b) changes of the dips of layers and the fault-like structures due to the melting of dead ice blocks under the dunes or in their close vicinity.

The covering sands are represented by the structureless sands lying in the top of all dunes and by marble-like sands occurring at their bottoms. In comparison with the stratified sands, they are much finer, less rounded, and richer in strongly resistant minerals in their heavy-mineral content. The covering sands originated in the result of the activity of frost weathering in periglacial climate and later soil processes.

Among the cryoturbation structures there were distinguished the fissure and solifluxion forms. These arose due to the local existence of an active layer on the dunes in some periods of the year when the periglacial conditions temporarily occurred. The local cooling of the climate might result from the presence of shallow buried dead ice, which preserved lakes and dead-ice hollows.

The melting of dead ice caused some disturbances in the parallel stratification of sands and even small faults observed on the slopes of the dunes occurring in the neighbourhood of the dead-ice hollows. The range and position of the disturbance show that the eolian process preceded the melting of dead ice or that both these processes sometimes overlapped.