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Glaciotectonically deformed glaciofluvial sediments with ruptured pebbles (the Koczery study site, E Poland)

Significant quantities of ruptured pebbles are found in glaciotectonically deformed glaciofluvial sediments of the Saalian glaciation (MIS 6) at the Koczery site (E Poland). To identify the responsible mechanisms for the pebble-rupture activity, structural, petrographic, roundness and shape analyses were done. Additionally, till fabric of overlying glacial diamicton was analysed and compared to the other outcomes. The origin of fractures in ruptured pebbles of glaciofluvial sediments is directly linked to compressive stress caused by glaciotectonic processes because of 1) ruptured pebbles occur mainly in glaciotectonically deformed sediments (a quarter of all pebbles is fractured); 2) ruptured pebbles almost always occur one-by-one primarily in gravelly lithofacies; 3) fractures occur in pebbles derived from all petrographic groups; 4) fracture occurrence is independent of pebbles size, shape and roundness; 5) fractures mostly occur parallel to each other (along long ‘a’ or short ‘c’ axis of pebbles) and parallel to the bedding of lithofacies; and 6) in most cases broken fragments of ruptured pebbles survived in the host sediment indicating that the observed damage occurred in situ. This novel study of ruptured pebbles found in glacigenic environments sheds new light on the dynamics of glaciotectonic processes, and may be useful in the characterization of palaeostresses that occur during glaciotectonic deformations.