New Zealand limestone purity

Geographic coordinates and lithostratigraphic names have been assigned to 2255 limestone samples that were previously analysed for soluble carbonate content. This permits, for the first time, a comparative assessment of the purity of onland New Zealand limestones by location and age. The three purest limestone populations have median CaCO₃ contents of 93–97 wt%, and are from: (1) Cambrian to Early Cretaceous basement terranes, (2) Eocene–Pliocene limestones of the Chatham Islands and (3) the Eocene Ototara Limestone. These have CaCO₃ contents comparable with carbonates found in isolated submarine bank settings that lack appreciable siliciclastic detritus and biogenic silica. By contrast, New Zealand Oligocene–Early Miocene limestones (Whangarei, Te Kuiti, Mahurangi, Nile Group, Otekaike and Forest Hill stratigraphic units) have median CaCO₃ contents of 71–91 wt%, comparable with New Zealand Paleocene–Eocene and Middle Miocene–Pleistocene limestones. Petrographic data from a subset of 52 Otekaike and Forest Hill limestones confirm that the non-carbonate content is mostly siliciclastic sand, not glauconite. The limestone purity data support, but do not on their own prove, a hypothesis of a terrigenous source (emergent land) during the Late Oligocene-Early–Miocene maximum marine inundation of Zealandia.