The origins of glacially related soft-sediment deformation structures in Upper Ordovician glaciogenic rocks: implication for ice-sheet dynamics

In the upper Ordovician glaciogenic rocks of West Gondwana, many soft-sediment deformation structures are attributed to glaciotectonic processes. However, little is known about the subglacial and ice-marginal processes associated with this ice sheet. A systematic description and interpretation of these soft-sediment deformation structures is supplemented by a comparison with Late Quaternary and Modern glacial settings to provide qualitative constraints on the character of ancient subglacial thermal regimes. Soft-sediment striations were formed by discontinuous, shear-induced deformation beneath an ice sheet that oscillated between warm and cold-based thermal regimes. Beneath warm-based ice, coupling across the ice–bed interface was poor with sliding and loading occurring at this surface. Loading of soft, waterlogged sediments allowed the development of fluted surfaces, which are also associated with the formation of truncated folded zones. During times of cold thermal regimes, coupling across the ice–bed interface was enhanced by freezing, which caused changes in the position of the freezing front. These discontinuous changes, coupled with sustained subglacial shearing, resulted in the development of stacked, soft-sediment striated surfaces. In areas where the subglacial substrate had an irregular topography, complex shear zones formed that comprised stacked duplex systems interlayered with highly strained units preserving pervasive, folded lineations. At the ice front, large glaciotectonic deformation structures (comparable with push moraines) developed during sustained periods of (re)advance that created composite thrust and fold systems. Deformation was transmitted for up to 1 km from the ice front and reduced in intensity with distance. Other deformation structures include dome-like folds that consist of central anticlines (30 to 70 m in width) with tight rim synclines that formed by the load-induced, diapiric intrusion of muds. An ice-sheet surging mechanism is inferred for the development of composite thrust and fold systems and the coeval sediment diapirs. These surges potentially caused the widespread reorganisation of a glacial load resulting in the pro- to subglacial extrusion of soft sediments.