Diagenesis of polymineralic temperate limestones in a cyclothemic sedimentary succession, eastern North Island, New Zealand

Abstract

Temperate carbonate petrofacies (calcarenite and coquina) in the Pliocene‐Pleistocene Petane Group of eastern North Island, New Zealand, are dominated by aragonite faunas consisting primarily of bivalves and gastropods. Unlike calcite‐dominated temperate limestones, these polymineralic carbonates have undergone extensive early diagenetic alteration including extensive calcite cementation induced by aragonite dissolution. Marine cementation (type 1: pore‐lining, bladed calcite) was isolated to biogenic pores. It predated glauconite and may have been precipitated as low‐magnesium calcite, possibly in marine phreatic environments during sea‐level transgressions. Four phases of calcite cement with varying but definitive degrees of meteoric influence occur in the Petane Group. Type 2 (ferroan scalenohedral calcite) was the initial pore‐filling cement and precipitated from reduced pore fluids in a phreatic environment, possibly during or soon after the transition from marine to meteoric diagenesis. Type 3 (moderately ferroan drusy) calcite and type 4 (non‐ferroan drusy) calcite were sequentially precipitated during meteoric conditions from pore waters that changed from reducing to oxidising. Type 5 (sinter) cements comprise several forms precipitated during vadose meteoric diagenesis, the final meteoric phase of alteration in the Petane Group. Ferroan calcite cementation of silt matrix in coquina limestones overlain by terrigenous silt (type 6: matrix cement) probably occurred simultaneously with type 2/3 pore‐filling phases. A similar ferroan to moderately ferroan to non‐ferroan suite of drusy calcite cements also lithified concretions in non‐carbonate (siliciclastic sand) facies in the Petane Group, but only after the onset of compaction. Extensive skeletal diagenesis (stabilisation of magnesium calcite allochems, dissolution/recrystallisation of aragonite) occurred during type 3 and 4 cementation phases. Diagenesis in the Petane Group was stratigraphically influenced and ultimately controlled by uplift. Alternating sequences of porous and non‐porous formations developed a stacked sequence of confined aquifers that forced carbonate diagenesis to operate laterally.