Debris flow and glacioisostatic-induced soft-sediment deformation structures in a Pleistocene glaciolacustrine fan: The southern Baltic Sea coast, Poland

Abstract

Middle Pleistocene glaciolacustrine fan sediments are exposed along a cliff at the Baltic Sea coast, where plastic and brittle soft-sediment deformation structures (SSDS) commonly occur: (1) fold structures (various types of folds and flexures); (2) load structures (load casts, pseudonodules) and flame structures; (3) water-escape structures (WES); and (4) brittle deformation structures, such as faults (medium- and small-scale reverse and normal), and fragments of broken-up laminae. The origins of the SSDS can be linked to gravitational processes promoted by shear stresses in subaqueous debris flows on the glaciolacustrine fan and by the weight of overlying sediments; and/or to the glacial rebound of Earth's crust during deglaciation because in the Pleistocene the Baltic Sea and surrounding areas were covered by ice sheets many times. The question investigated here concerns recognition of the trigger mechanisms responsible for the development of SSDS, and the criteria needed to recognize those mechanisms based on the lithological and deformational features of the sediments involved. Some types of SSDS can look similar regardless of trigger mechanisms, but some occur more often as a result of a specific mechanism. Is it possible to distinguish SSDS of seismic origin in a glaciolacustrine fan succession affected by multiple debris flows and of SSDS that evolved as a consequence of slope processes? We suggest criteria to recognize SSDS triggered by the glacioisostatic rebound as well as those which can develop as a result of the rebound and slope processes.