Micromorphological analysis of liquefaction features in alluvial and coastal environments of Christchurch, New Zealand

Abstract

This study presents a micromorphological analysis of liquefaction features (dykes, sills and sand blows) associated with the 2010 to 2011 Canterbury Earthquakes Sequence, dykes of an earlier generation of liquefaction (palaeoliquefaction) and hosting soil. The aim of this research was to determine whether diagnostic features existed that would allow robust identification of liquefaction features and discrimination of different generations of liquefaction features in thin sections. This study serves to enhance the tools available to support palaeoliquefaction studies and earthquake hazard assessment. Similarities were found in the sedimentary fabric between all forms and ages of liquefaction features distinct from hosting soil. Palaeoliquefaction features could also be distinguished on the basis of microscopic pedogenic characteristics. Liquefaction and palaeoliquefaction were investigated in two distinctive sedimentary settings: the floodplain of the Halswell River, and coastal sand dunes near or in Christchurch. Analysis was carried out on thin sections prepared from resin-impregnated blocks of soil sediment hosting the liquefaction features. The fabric of modern liquefaction features in the alluvial and coastal system is similar, both showing a single grain micro-structure, with moderate to well-sorted grains with many simple packing voids and planar voids. In both environments it was possible to see evidence of fluidization such as water escape structures, cutans and silt caps. The palaeoliquefaction fabric can be distinguished from the modern liquefaction fabric by the presence of textural pedogenic features in the form of silt coating of grains, infilling of planar voids, coating and hypocoating of voids and excrement pedofeatures. The results presented in this manuscript demonstrated how thin section analysis can be a valuable tool to corroborate field identification of liquefaction features, and to discriminate between features of different relative ages where stratigraphic evidence is equivocal.