GEOMORPHIC MAPPING AND INTERPRETATION OF PARAGLACIAL FAN MORPHOLOGIES IN CASS, NEW ZEALAND

Although fans form in a variety of geologic settings, their development in canyons and well-established river valleys immediately following deglaciation is unique. During and immediately following glacial retreat, paraglacial processes drastically alter the landscape, and the rapid formation of fans directly impacts the river sediment budget by substantially increasing sediment influx. In this study we conduct geomorphic mapping and characterize fan morphologies in Cass (NZ), where deglaciation commenced ~17.5 ka. We used a combination of field-based mapping, rudimentary surveying techniques, sediment sampling, desktop mapping, and morphometric analysis to understand the conditions and relative timing of alluvial fan development, and their contribution to the Waimakariri sediment budget. 63.2% of the fans in our study area were interpreted as debris flow fans. We conclude that paraglacial processes have directly contributed to the prevalence of debris flow fans through over-steepening of slopes, de-vegetation, a release of tensile stresses in rock faces related to the presence of glaciers, and possibly permafrost degradation. Our findings confirm that fans in the region would have formed rapidly following deglaciation, and early in the postglacial succession of landforms. Furthermore, fans in the region have undergone a significant reduction in activity from the paraglacial period to the present. This study has implications for a refined understanding of the sediment budget of the Waimakariri River in relation to hazard mitigation, paleo-seismic studies, and other research and applied topics.