Soft-sediment deformation structures in the Upper Miocene deltaic sequence of the Lake Pannon (Vienna Basin)

The Upper Miocene fine-grained, lacustrine-deltaic deposits have been investigated in the Vienna Basin. An occurrence of the soft-sediment deformation structures is a noticeable sign of these deltaic multilayer heterolithic deposits of sand, silt, and clay. Distribution and frequency of the deformation structures are variable in vertical succession and depend on lithology, depositional processes, as well as on occurrence of the ichnofossils. With a respect to processes that generate a formation of such deformations, two categories of the soft-sediment deformation structures were distinguished. The first category comprises various structures (small upwarping cusps or siphons, pipes, concave-upward dish-and-pillar structures, conduits, and sedimentary intrusions), of which origin can be explicitly attributed to inorganic processes as a result of liquefaction–fluidization. We suppose that besides the process of spontaneous dewatering of sediments deposited at quick rates, overloading or seismic activities are main triggering mechanisms of deformation. The second category includes conical downwarped V-shaped structures that origin was conditioned by animal activity, and they are attributed to ichnotaxa. This soft-sediment deformation structures can be attributed to the collapse of sediment into cavities created by animals.