Mass-emplaced sand-fingers at Mararoa construction site, southern New Zealand

During work on a dam construction site in South Island, New Zealand, outflowing ground water resulted in extensive local mass-transport of the sands being excavated. A series of small scale sand fans was built up, probably mainly by laminar mass-flow processes. The surfaces of the fans were made up of a series of complexly interdigitating sand-fingers that are inferred to have been emplaced by viscous plug-flow. Other sedimentary processes associated with the building and synsedimentary destruction of the fans included rapid grain-flow, liquefaction and progressive slumping. Although small in scale, the Mararoa mass-flow fans may be a close analogue for some of the many‘fluxoturbidite’ or‘proximal flysch’ facies described in the sedimentological literature, and their geologic implications are therefore briefly discussed.