Textural procedures for the environmental discrimination of late Neogene coastal sand deposits, southwest Auckland, New Zealand

Abstract

The coastal region of much of the northwestern North Island, New Zealand consists of late Neogene, poorly consolidated, coastal sand and tephra deposits of the Kaihu Group. Determination of the environments of formation of these deposits using standard grain size analytical approaches has met with limited success, both in this and previous studies. However, here we demonstrate that samples from known beach, dune and tephra facies can be differentiated in at least 90% of cases using stepwise and canonical discriminant analyses of the textural data. Classification models derived from the canonical analyses are used to distinguish the depositional facies of homogenised, subsurface drillhole samples collected at Taharoa, the site of a large, currently mined ironsand deposit in southwest Auckland. A SEM, process-based, interpretation of the quartz grain surface features in the sand deposits shows general agreement with the environments determined from the discriminant function analyses despite a considerable degree of post-depositional alteration.