Ash-flow deposits of the central king country, New Zealand

Publisher Website
Google Scholar
Abstract
Late Cenozoic ash-flow deposits in the central King Country, New Zealand, are divided on the basis of field observations, photogeology, and mineralogy into the following successively younger formations: Rangitoto Ignimbrite, Ongatiti Ignimbrite, Waipari Ignimbrite, Ahuroa Ignimbrite, Rocky Hill Ignimbrite, Whakamaru Ignimbrite. Fourteen member sheets are identified and seven unconformities are recognised or inferred. The Rangitoto Ignimbrite (new formation) consists of at least two distinct cooling units separated by an interval of faulting and erosion. The Ongatiti Ignimbrite, the most widespread and most porphyritic, consists of a single sheet which is possibly part of a compound cooling unit. The Waipari (new formation), Ahuroa, and Rocky Hill Ignimbrites are multiple sheets which together with the Ongatiti Ignimbrite form a sequence showing intervals of lacustrine sedimentation and progressive angular discordance. The Whakamaru Ignimbrite consists of at least two modally distinct cooling units, and a major unconformity separates it from all older formations. Comparisons of phenocryst modes determined by the thick-slice method suggest consanguineous sources of the parent ash flows, with an enrichment of potash and water components in the later stages of the eruptive history. The association of quartz-rich modes with more crystalline sheets, and .of quartz-free modes with more vitric, lenticulitic sheets is possibly related to magma temperature at the time of eruption. Compound cooling units consisting of a basal tuff-breccia or pumice-breccia sheet overlain by a highly welded sheet of the same modal composition may reflect the opening of a new vent. Source vents for many deposits were probably located in or east of the present Mangakino Basin. The Whakamaru eruptions were closely associated with collapse of the Mangakino-Taupo area, and may have originated in the vicinity of Lake Taupo. The formation of a fault-angle depression between the tilted Rangitoto and Hauhungaroa greywacke blocks allowed access of successive ash flows to terrain wess of the ranges. Additional tilting and subsidiary structural features developed as the volcanic cycle progressed.

This publication has 5 references indexed in Scilit: