Evolution of geology and groundwater-geothermal systems in the Okataina caldera groundwater catchment

Abstract

The Okataina groundwater catchment (OGC), with the Okataina caldera complex and its large caldera structures and rhyolite domes, 11 lakes, numerous shallow, cold, groundwater systems and 15 geothermal fields, is a spectacular part of the Taupō Volcanic Zone (TVZ) and an example of the many meteoric-dominated hydrothermal systems, ancient and modern, that bound the Pacific Ocean. This paper describes Quaternary OGC evolution in four phases with 4D models of geology and groundwater-geothermal systems.The early-TVZ Proto-KWG phase had development of the KWG graben, the Proto-Tarawera River and catchment, and ended with deposition of Whakamaru Group ignimbrites, sourced south of the OGC. Then, Matahina phase volcanism resulted in the Matahina caldera with Matahina Formation ignimbrite deposited over a wide area within, and beyond, the OGC. Groundwater flowed into Lake Matahina which was the location of multiple geothermal fields.Thirdly, the Penultimate phase produced the Rotoiti caldera, Lake Haroharo, and two OGC ignimbrites, i.e., Rotoiti Formation and Earthquake Flat Formation. Lake Haroharo recieved inflow from groundwater-geothermal systems. Lastly, the Infill phase resulted in today's major OGC landforms, e.g., the Mt. Tarawera and Haroharo domes, multiple lakes, their catchments, groundwater systems and geothermal fields.Deep drilling is recommended to address key science gaps, i.e., the locations and properties of subsurface caldera-lake sediments and the processes (chemical and physical) of fluid flow from deep geothermal reservoirs to surface hydrothermal areas.