Imbricate thrust stack model for the formation of glaciotectonic rafts: an example from the Middle Pleistocene of north Norfolk, UK

In this article, we present a glaciotectonic model for raft emplacement based on a study of large-scale and small-scale deformation structures associated with the accretion of chalk rafts at three Middle Pleistocene sites on the north Norfolk coast, eastern England. Detailed structural measurements taken from the three localities indicate an overall sense of ice movement and raft emplacement towards the south/southeast, suggesting a source area for the rafts located to the north of the present Norfolk coast in the offshore area of the North Sea. Provenancing of the chalk rafts, based on analysis of the foraminifera, also indicates a northern nearshore provenance for the chalk. Mechanisms for the detachment, transport and accretion of the rafts are explored, and it is concluded that pressurized pore water played an important role in all three phases. An imbricate thrust stack model of glaciotectonic raft generation is presented, with the structural history of raft emplacement explained by the geometric relationships between the large-scale basal shear planes and associated deformation structures within adjacent preglacial and glacial sediments.     

The relationship of porosity and angle of repose to mixture proportions in assemblages of different sized materials

Rotating drum experiments on the repose angles of mixtures of glass spheres have shown that φ_r (angle of shear) is strongly influenced by the proportions of the mixture. It was found that φ_r reached a peak value for the minimum porosity mixture; where the pore spaces between the large particles were just filled with small material; which was attributed to increased dilatation on the shear plane during avalanching. The geomorphic significance of these observations, in terms of slope development, is discussed. Secondly, the results of the experiments showed that, although more constant than φ_i (limiting angle of repose), φ_r was subject to some variation. Thus φ_r, as measured in a rotating drum, is not a true constant and can not be exactly analogous to φ'_cv (angle of internal sliding friction at constant volume) as measured in a shearbox test—as has been previously suggested. It is tentatively suggested that at least some of the variability in φ_r is attributable to the magnitude of the immediately preceding value of φ_i, in that an unusually high value of φ_i, favours a lower value of φ_r due to the greater amount of kinetic energy released on failure.