Lake-filled depressions resulting from cold gas discharge in the Ngawha Geothermal Field, New Zealand

Within the high temperature Ngawha geothermal field, there are four roughly circular, lake-filled depressions that are associated with cold gas discharge, thermal springs, peat deposits and lake sediments. Lake Waiparaheka is the largest (> 100 m in diameter) and Waitotara Pond is the smallest (∼50 m in diameter), and both are sites of vigorous gas bubbling. Two 14C dates on woody material from surficial peats on the edge of Lake Waiparaheka give an approximate age of 5500 to 6100 yr BP. Lake Waiparaheka and the other nearby lakes have been previously interpreted as sites of hydrothermal eruption because of their association with zones of surface thermal activity and the morphology of the depressions which they fill. Evidence of hydrothermal eruption deposits (i.e., apron deposits of mixed clasts of various altered rock), however, is lacking. Detailed field mapping around Lake Waiparaheka reveals only peat deposits and clastic sediments consisting of clay, silt, fine-sand-sized grains of quartz. pose an alternative explanation for the formation of the Ngawha lakes, namely that disaggregation of the Ngatuturi Claystone, which occupies the shallow stratigraphy and underlies the peat deposits, have been elutriated and suspended into the water column by vigorous CO2 bubbling through sub-lacustrine vents, excavating funnel-shaped depressions. Such a process can be observed today in Lake Waiparaheka and Waitotara Pond, but it may have started when a large shallow lake filled the whole of the Ngawha basin. Subsequent drainage left isolated, remnant water-filled depressions that form the modern lakes. Exceptional aspects of the Ngawha geothermal field that contribute to such lake formation are the 500 m thickness of fine-grained sediments, which are relatively impermeable to deep geothermal liquid rising to the surface, but not to deep geothermal CO2, and the occurrence of claystone and siltstone in the shallow subsurface, which is easily disaggregated and elutriated in the vicinity of sub-lacustrine gas vents.