Volcanology of tuff rings at Kellyville, Onewhero and Bombay, South Auckland Volcanic Field

Abstract

The South Auckland volcanic field hosts 82 volcanic centres over an area of approximately 300 km2 in the Pukekohe, Bombay, Tuakau, Pukekawa and Onewhero regions. The intraplate, monogenetic basaltic volcanic field was active between 1.59 and 0.51 Ma, and produced scoria cones, basaltic lava flows, tuff rings and maars. Three volcanic centres have been studied for the purpose of this thesis: the Kellyville volcanic complex, the Onewhero tuff ring and the Bombay volcanic complex. Each centre hosts a tuff ring and varying levels of associated magmatic activity, and has been studied through stratigraphic logs, facies analysis and componentry studies to illustrate the styles of eruptions and their controlling factors. The Kellyville volcanic complex hosts a breached tuff ring and two intra-tuff ring scoria cones. The tuff ring facies identified include a lithic-rich block and bomb facies with a massive fine lapilli to block and bomb facies, and a cross bedded coarse and fine ash facies with a laminated alternating coarse and fine ash facies, dominant in the early and late stages of the eruption, respectively. The aquifer for the tuff ring eruption was the Mercer Sandstone of the Waitemata Group. Both fall and surge processes occurred, with surges becoming dominant towards the end of the tuff ring eruption. Grainsize decreased through the eruption due to an increase in water/magma ratio and a decrease in magma ascent rate and eruption energy. The Onewhero tuff ring is the largest in the South Auckland volcanic field, and hosts a tuff ring with a separate lava flow on its outer flanks. The tuff ring facies identified include a well sorted, cross bedded alternating fine and coarse ash facies dominant in both the early and late stages of the eruption, and a poorly to very well sorted, fine ash to block and bomb facies present in discrete pulses. The aquifer for the tuff ring eruption was the ancestral Waipa/Waikato River and its alluvial sediments. Both fallout and surge processes occurred, with surges becoming dominant towards end of the tuff ring eruption. Grainsize was constant through the eruption due to a steady water/magma ratio and a stable interaction between magma and water. The Bombay volcanic complex hosts at least one tuff ring, a tuff cone, numerous scoria and spatter cones and at least two large deposits of ponded basalt lava. The tuff ring facies identified include a well sorted coarse ash to fine lapilli facies dominant in both the early and late stages of the eruption, and a poorly sorted coarse lapilli facies which occurs intermittently throughout the eruption. The aquifer for the tuff ring eruption was the Mercer Sandstone of the Waitemata Group. Fallout from steady eruption column was dominant in the tuff ring eruption. Grainsize decreased through the eruption due to a highly efficient water/magma ratio and an increase in ascent rate and eruption fragmentation. The eruption style of volcanism in the South Auckland volcanic field is largely controlled by the distribution of faults, as well as the interaction of differing magma supplies and ascent rates with water-bearing sedimentary rocks and surface water of the ancestral Waipa/Waikato River.