A Multi-Scale and Multi-Proxy Study of Dramatic Marine Deepening in the Waitematā Basin (Early Miocene), Aotearoa New Zealand

Lower Miocene (late Aquitanian to middle Burdigalian) shelf to bathyal strata of the Waitematā Group in Aotearoa New Zealand record the influence of basement relief and tectonism on sedimentation and paleocommunity distributions in a rapidly subsiding marine basin. At Te Kohuroa/Mathesons Bay, basal Waitematā strata (Kawau Subgroup)—including conglomerate, lithic sandstone, and bioclastic limestone of the Cape Rodney Formation—onlap an irregular paleotopographic surface carved into the uplifted Mesozoic metagraywacke basement. These deposits include a ∼3-m-thick boulder breccia interpreted as a co-seismic mass-wasting deposit, along with in situ coralline algal (rhodolith) pavements and oyster biostromes, indicative of shallow shelfal conditions. The in situ shelfal succession is sharply overlain by a patchy, condensed (∼20 cm-thick) glauconitic sandstone bearing bathyal barnacle plates, and laterally equivalent sandy limestone containing fossil octocorals. These deposits mark the base of the Warkworth Subgroup and record a significant deepening from shelfal (< 200 m) to bathyal depths (∼500–1000 m). This deepening was accompanied by the localized exposure of basement hardgrounds during a period of sediment starvation. The succeeding deposits consist of macrofossil-poor bathyal turbidites and mudstones of the Pakiri Formation. A nodular micritic carbonate body within the mudstone is identified as a fossil methane seep, based on microfossil content, isotopic signatures, and stratigraphic characteristics. Carbon and oxygen isotopic analysis of carbonate components (e.g., shells and cements) across all sedimentary facies reveals multiple sources of carbonate-carbon, including seawater, methane seepage, and burial fluids. Together, these integrated sedimentological, paleontological, and geochemical data document rapid basin deepening, benthic ecosystem turnover, and early tectonic forcing during the onset of plate boundary propagation in Miocene Aotearoa New Zealand.