Late Ordovician glacial pavements revisited: a reappraisal of the origin of striated surfaces

The recognition of ice-related unconformities is of prime importance when dealing with the sequential architecture of pre-Pleistocene glacial successions. Late Ordovician striated surfaces strikingly preserved in North Gondwana were long considered as abrasion surfaces at the sole of a grounded glacier overriding unlithified sediments, or having resulted from the scouring action of drifting icebergs. Field observations from Mauritania and Libya show that these striated surfaces require an alternative interpretation, which can be applied to similar surfaces described elsewhere, such as in Permo-Carboniferous or Neoproterozoic glacial sequences. A new model is proposed that involves superimposed concurrent decollement planes within a subglacial brittle shear zone in unlithified sand beds. Shear zones in subglacial sediments are well known in Quaternary deposits but, to date, it has not been demonstrated that they may form striated surfaces occurring at continent scale with orientations consistent with palaeo ice-flow reconstructions based on larger-scale indicators.