Genesis of a giant gravity-induced depression (gravifossum) in the Enköping esker, S. Sweden

A structure in the Enköping esker in southern Sweden appears to result from a series of deformations that followed, essentially, loading because of an unstable density gradient (diamicton on top of water-saturated sands). The more-or-less symmetrical structure, which is in fact a structure within a graben structure, is some 12 m wide at its top, 2.5 m wide at its visible base, and some 13 m high. It shows a variety of both synthetic and antithetic faults. In addition, numerous folds are present so that a complex structure results. Both the structural characteristics and the shape resemble those of the (much smaller) gravifossums that have been described from subrecent lagoonal silty deposits in The Netherlands. It is the first time that a gravifossum has been described from glacigenic deposits and, consequently, also the first time that such a structure has been found in esker deposits.