Depositional Setting of the Early Miocene of Waitemata Group Sequence at Mathesons Bay, Leigh, New Zealand

Abstract

The Waitemata Basin reveals a complex history of rapid subsidence associated with the onset of Hikurangi margin subduction in eastern North Island, New Zealand. This study focuses on identifying the depositional setting of the basal to upper Waitemata strata (Early Miocene) at Mathesons Bay-Leigh area north of Auckland, New Zealand, by testing the hypothesis whether sedimentation occurred under transgressive conditions, with deposition in shallow water, or if shallow sediments were deposited at bathyal depths, taking into consideration the depositional facies of the different clastic and carbonate rocks, including a methane-seep limestone deposit. The Mathesons Bay costal area comprises irregular coastal topography where basement greywacke of the Waipapa Group (Triassic-Jurassic) is overlain unconformably by the Waitemata Group sediments. The study area included three discrete cliff and shore platform sections, each with lateral and vertical differences in sequence variations. Based on sedimentologic and stratigraphic analysis, nine lithofacies were identified and mapped in detail for the first time across the three locations. The basal strata of the Kawau Subgroup (Cape Rodney Formation) consist of four lithofaciesfacies, including basal conglomerate, sandstone-conglomerate, coarse breccia, pebbly sandy conglomerate to shelly limestone, and glauconitic sandstone. These units are abruptly overlain by the finer grained basal Warkworth Subgroup. The Warkworth Subgroup contains three lithofacies in the study area: sandstone, sandstone and mudstone with bathyal microfossils, and hydrocarbon seep carbonate. The major sediments of Mathesons Bay observed are mixed siliciclastics and carbonates. The majority of the sediments are composed of quartz, with minor feldspar, glauconite and pyrite, greywacke clasts, and rare detrital fragments that are volcanic in origin. The bioclasts are mainly bryozoans, echinods, corals, foraminifera and coralline algae. Bivalves and gastropods were rare in all samples. Based on the estimation of the peak heights of MgCO3, XRD analysis showed that the Mg-calcite content in the more carbonate dominated rocks of Warkworth sediments comprises low Mg-calcite (< 4 mol %). Based on petrographic analysis, eight microfacies types (MF) were identified and described, and grouped together into the following carbonate classification categories: grainstone, coral-bearing bioclastic rudstone, bioclastic bryozoan packstone, bioclastic rhodolith-foraminiferal packstone, calcareous mudstone, and nodular to brecciated micrite facies. The siliciclastics are represented by barnacle rich-calcareous sandstone, sandstone, and calcareous mudstone facies from the interbedded sandstone-mudstone lithofacies. The measured isotopic values of 25 samples are widely scattered in δ13C, from 0.03 ‰ to −50.43‰ PDB, and more tightly grouped in δ18O, from −0.34‰ to 0.85‰ PDB. Within this stable isotopic variability, the isotope signatures of the Mathesons Bay calcareous samples fall into four main categories, Types 1-4. Type 1 (Kawau Subgroup) and 2 (Warkworth Subgroup) correspond closely and have similar carbon but display a spread in oxygen isotope values. In contrast, the seep carbonate (Type 3) revealed strongly depleted δ13C values of -24.31 to -50.43‰ PDB. The presence of pyrite and textures in the micritic carbonate support formation by anaerobic oxidation of the methane (AOM). These values ranges of depleted carbonate carbon may come from a thermogenically-sourced methane and mixture of thermogenic or biogenic methane and dissolved inorganic carbon in sea-water. Paragenesis of the Mathesons Bay carbonate-bearing deposits involved eight diagenetic steps/events recognised in most of the Kawau samples. Within the seep carbonate, micrite containing scatterd thin-shelled bivalves and a hydrocarbon related foraminifera (Amphimorphinella butonensis) predates all other diagenetic features. Corrosion occurred in places to produce irregular nodular macro-texture, and light yellowish to pinkish calcite cement subsequently filled porosity. The pervasive light cement filled reflects early lithification at or the seafloor. During late diagenesis neomorphism caused patchy replacement of micrite (grey) to microspar (pink), and veins are filled with white blocky calcite occurred after seafloor seepage had ceased, and during burial of the seep-carbonates. This study confirmed that the Mathesons Bay carbonate mound is no doubt similar to modern seep carbonates methane-enriched pore waters reached the seafloor due to pore fluid maturation of organics and fluid expulsion in the Early Miocene. The concretionary bodies are strongly depleted in δ13C (-24 to 50.3‰ PDB). The erosion of the Mesozoic greywacke landscapes in the Early Miocene provided most of the different sized clasts that accumulated along this ancient coastline. The transition between shallow to deep marine sediments was largely controlled by climate, relative sea-level fluctuations and tectonic setting, influencing erosion and production of carbonate. In addition, the carbonate skeletal assemblages of the lower Waitemata strata at Mathesons bay indicates these rocks accumulated as temperate cool-water carbonates. The Mathesons Bay rhodolithic unit is the equivalent of the type A onlap shellbed sensu Nalin et al. (2008), which is interpreted as a distinctive facies deposit within a transgressive systems tract, and formed under conditions of low net sedimentation and relatively high-energy conditions. The macrofossils, microfossils, rhodolith pavements and foraminifera support the hypothesis of sea level rise during the basin subsidence.