DETERMINING THE ORIGINS OF KARST-RELATED DEPRESSIONS IN CASTLE HILL BASIN, NEW ZEALAND

Sinkholes are a geomorphic hazard created by the dissolution of bedrock in karst landscapes. Understanding the development of a karst landscape can be difficult, especially in highly deformed regions, but is important for informing future hazard and risk management. Here we use field mapping techniques and ground penetrating radar (GPR) to evaluate the origin of previously unstudied depressions in the deformed Castle Hill Basin, New Zealand. Three separate clusters (Sites A, B, C) of potential sinkholes are identified and their geomorphic characteristics compared with a well-established collection of sinkholes (Site D) at the nearby Cave Stream. The size, depth, and spatial patterns for depressions at Sites A, B, and C are consistent with those observed at Cave Stream, suggesting a similar karst origin, although depressions at Site A are located outside the limits of the mapped limestone unit. GPR reveals subsurface features that indicate potential karst development at the same time as ongoing river processes, and indicates the limestone may be present at shallow depth further to the north-northwest than previously thought. Though further research would be needed to confirm in more detail how the area developed, our study demonstrates some of the challenges in establishing a karst origin for geomorphic depressions. It also illustrates the importance of a multi-technique approach in the study of karst, particularly in structurally deformed regions where controls on karst development may vary even when forming in the same local geologic unit.