Sand intraclasts as evidence of subglacial deformation of Middle Pleistocene permafrost, North Norfolk, UK

Sand intraclasts are common within the Bacton Green Till Member, a glacitectonic mélange subjected to polyphase deformation during the Middle Pleistocene in North Norfolk, UK. The intraclasts range from a few tens of centimetres to >10 m in length and have sharp contacts with the surrounding till. Sand within the intraclasts is unconsolidated and contains well-preserved primary stratification. The wrapping of glacitectonic foliation around the intraclasts and the development of folds relating to mechanical instabilities indicate that the intraclasts acted as competent masses within a more easily deformable fine-grained till that accommodated the majority of the strain. Sharp contacts and distinctive heavy-mineral assemblages indicate little intermixing between the sand and till.Five hypotheses about the entrainment and evolution of the intraclasts are tested against sedimentological, structural and mineralogical observations. The most reasonable hypothesis attributes the intraclasts to glacitectonic deformation of “warm” permafrost. Initial ice advance caused large-scale thrusting of proglacial permafrost that led to the stacking of pre-glacial and ice-marginal sediments that were subsequently deformed sub-marginally to generate the intraclasts. Preservation of primary stratification within the intraclasts is attributed to deformation at temperatures slightly below the pressure-melting point, when pore ice cemented the intraclasts as rigid bodies. At the same time deformation was concentrated into the surrounding finer-grained till because of its significant liquid water content and ductile rheology. It is concluded that the intraclasts provide a criterion to identify past glacier–permafrost interactions and a potential means of differentiating between subglacial deformation under unfrozen and partially-frozen conditions.

► We examine the origin and significance of sand intraclasts within tills.
► Occur as competent masses within deformed tills in north Norfolk.
►  Cohesion associated with the development of an ice cement.
► Rheological contrast within deformed tills consistent with sub-freezing temperatures.
► Intraclasts are potentially diagnostic of glacier–permafrost interactions.