Identification of Quaternary alluvial-fan deposits (Rügen, SW Baltic Sea): Significance for recognition of syn-kinematic sedimentation in glacitectonic complexes

Abstract

We present a detailed study of an alluvial fan accumulated in a piggyback basin syn-kinematically to the emergence of a Weichselian glacitectonic complex (Jasmund peninsula, SW Baltic Sea). Although sediments formed contemporaneously with glacitectonic events have already been documented, a systematic approach is missing so far. Facies, architecture and distribution of alluvial-fan deposits are identified here as key features for analyzing glacitectonic complexes.The deposition of those syn-kinematic deposits was controlled by the progradation of thrust-faults and the resulting morphologic changes of both the thrust-bounded ridges and the adjacent piggyback basins. Another controlling factor is the episodic high runoff of meltwater discharge (glacial lake outburst flood, jökulhlaup) from lakes near the ice front. Sheetflood and debris-flow processes dominated the sedimentation and gave way to alluvial-fan, fan-delta and glacifluvial deposits. A stepwise rotation of the depositional plain within the piggyback basin is indicated by internal unconformities and the distinct converging stratal geometry. Based on three luminescence ages the development of the syn-kinematic alluvial-fan sequence and consequently of the glacitectonic complex took place between 21 and 19 ka during a late Weichselian readvance of the Scandinavian Ice Sheet. The overall results are summarized in a seven-stage genetic model, beginning with the initial glacitectonic phase and ending with the overfilled piggyback basin and subsequent overriding of the structure by the glacier front. With the presented criteria, the successive growth of the alluvial-fan deposits can be linked directly with the emergence of a glacitectonic complex formed during an ice advance.