FLANK MARGIN CAVE DEVELOPMENT IN NEW ZEALAND AS A QUATERNARY SEA LEVEL MEASURE

Coastal carbonate rock outcrops, of various levels of diagenetic maturity from eogenetic to telogenetic, were examined in New Zealand. On North Island, sites were studied at Raglan Harbour, Kawhia Harbour, Napier, and Waipu; and on South Island at Pohara, Paterau, Punakaiki, Kakanui, and Kaikoura, to determine if flank margin caves, produced by mixing dissolution, were present. In coastal settings, caves in carbonate rock can be the outcome of pseudokarst processes, primarily wave erosion, as well as karst products not associated with fresh and sea-water mixing such as epikarst features and conduit-flow stream caves. Flank margin caves were successfully differentiated from other cave types by the following criteria: phreatic dissolutional morphologies at the wall rock and chamber scales, absence of turbulent flow wall sculpture and sediment deposits, and lack of integration of adjacent caves into a continuous flow path. Relict flank margin caves at Te Mata, inland of Napier at 375 m elevation, indicate early subaerial exposure of the carbonates to establish a fresh-water lens, followed by rapid uplift. Denudation expressed as slope retreat has been sufficient to expose, but not completely remove, the cave remnants. The active tectonics of New Zealand creates a variable sea-level situation. The relatively short time of sea-level stability limits the size of the New Zealand flank margin caves compared to tectonically-stable environments, such as the Bahamas, where glacioeustasy alone controls sea-level position. Uplift events can be identified as slow and steady when the flank margin caves are uniformly elongated in the vertical direction, and episodic when the flank margin caves show widening and tube development at discrete horizons that cut across rock structure. Rock characteristics also control flank margin cave size, with highly fractured rocks forming the largest caves at Kaikoura by offering more mixing pathways between fresh water and sea water. New Zealand flank margin caves contain information on uplift duration and rates independent of other commonly used measures, and therefore can provide a calibration to other methods.