Conceptual model for early diagenetic chert and dolomite, Amuri Limestone Group, north‐eastern South Island, New Zealand
Authors: LAWRENCE MARK J. F. 
Published: June 2006

The Late Cretaceous to Early Eocene, dominantly micritic, Amuri Limestone Group (ALG) was deposited in an approximately NW trending trough, in eastern Marlborough, New Zealand. The ALG comprises: the Mead Hill Formation; the Teredo, Lower and Middle Limestone formations; and the Upper and Lower Marl formations. Chert and dolomite are concentrated in the Mead Hill Formation, which contains five of six recognized diagenetic zones: Zone I at the base of the ALG consists almost entirely of chert; Zone II consists solely of chert and dolomite; Zone III comprises chert and limestone; Zone IV is composed of chert plus dolomite; Zone V is a chertified mudstone; and the minor amounts of chert found in the Middle Limestone Formation comprise Zone VI. With the exception of Zones IV and V, chert decreases stratigraphically upwards and away from the basin centre. All the dolomites are composed of <1 mm diameter rhombohedra in discontinuous beds and lenses. Generally Ca‐rich, and non‐ to slightly ferroan, the dolomite contains approximately 500–900 ppm Mn and 200–400 ppm Sr. δ13C values average 1–2%PDB with δ18O ratios of about ‐4%PDB. Mass balance calculations indicate that the Mg2+ for dolomitization was derived from sea water. Sr, Fe and Mn concentrations are interpreted as indicating dolomite formation in the marine environment, with no influence from meteoric waters. The intimate association with pyrite implies dolomite formation in association with sulphate reduction, in the upper sediment column. δ18O data show that the bulk of the dolomite formed at temperatures below 50°C. All chert samples contain in excess of 90 wt% SiO2, about 1 wt% Al2O3 and 1 wt% from losses on ignition. Generally all other major elements total less than 2 wt% oxide. δ18O values range from 26·8 to 29·0%SMOW. Chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate‐bound components from the site of chertification, and the effective dilution by SiO2 of non‐carbonate‐bound insoluble residues. δ18O data indicate that chert formed in fluids of similar composition and temperature as the dolomite. The abundance of disseminated pyrite in cherts implies an association with sulphate reduction. Silica for chertification is thought to have initially come from dissolution of siliceous organisms. However, there is insufficient biogenic silica available to form the volumes of chert observed. It is suggested that the bulk of the silica came from SiO2‐rich pore waters generated by clay mineral reactions in the thick underlying mudstones. The ALG compacted down through these pore waters. Chert and dolomite nucleation are considered to have been penecontemporaneous. Dolomitization was initially probably the faster process, continuing as long as sulphate reduction prevailed and there was an adequate supply of Mg2+. The nucleation of chert, although initially slower (probably due to a relatively lower initial SiO2 supply), continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix. The amount of chertification decreased progressively as SiO2 supplies diminished, both stratigraphically upwards and away from the basin centre.