Sedimentology and diagenesis of the Pukenui limestone formation, Wairarapa, New Zealand

Abstract

The Pukenui Limestone Formation is a subantarctic/temperate, shallow marine deposit of Early Marahauan age. It outcrops along an arcuate belt within central and southern Wairarapa, New Zealand and comprises both carbonate and terrigenous sediments. Eight lithofacies are recognised within the formation. These are related to environments of deposition that range from offshore, shallow shelf to backshore, beach. Sediments presently accumulating on the sea bed of Foveaux Strait provide a close geographic, physiographic, ecological and sedimentational analogue to those of the Pukenui Limestone. Syndepositional tectonism has occasionally influenced sedimentation, as indicated by concentrations of shallow water facies and formation thinning near the crests of anticlines with deeper water facies and thickening along syncline axes. Intrabed slumping on the flanks of growing anticlines and a predominance of terrigenous material in areas where active faults dissect the greywacke basement rocks also suggest this. Despite its relatively young age, the Pukenui Limestone displays a wide range of carbonate diagenetic features. These include boring and micritization of allochems, neomorphic replacement of both carbonate mud and aragonitic shell fragments, submarine and subaerial cementation, and the development of syntaxial overgowths, tectonic and compaction fractures, and geopetal structures. Subaerial diagenetic effects are most pronounced due to the formation's history of prolonged subaerial exposure with minimal overburden protection. The carbonate mineralogy is dominated by low-Mg calcite, although metastable aragonite is preserved where finegrained, impermeable matrix has restricted the circulation of meteoric water most suitable for its dissolution. The susceptibility of skeletal grains to micritization appears to be controlled by internal microstructure; shell fragments with granular or fibrous microstructure are rarely micritized as compared to other grains. At several localities the Pukenui Limestone consists of alternating bands of well cemented and poorly cemented carbonate. The inhibited development of carbonate cement usually reflects an increase in terrigenous content. However, it also results where relatively small terrigenous fractions are predominantly clay sized. This fine-grained material provided isolatory coatings to surfaces that would otherwise have acted as host substrate for heterogeneous nucleation of calcite cement.