Miocene tubular concretions in East Coast Basin, New Zealand: Analogue for the subsurface plumbing of cold seeps

The uplifted accretionary prism of East Coast Basin, in Hikurangi Margin, North Island, New Zealand, exposes late Miocene slope mudrocks (Whangaehu Mudstone, < 10% carbonate) in coastal cliffs north of Cape Turnagain that contain conspicuous tubular carbonate concretions (50–85% carbonate) supporting near-central conduits. Pipe and bulbous morphologies dominate, ranging in exposed length up to 5 m and up to 1 m in diameter. The concretions were formed by the precipitation of micritic dolomite (and calcite) cement within the host mudstone at shallow burial depths (< 100 m). δ13C values of the cement range from − 22 to + 13‰ PDB and are interpreted to reflect carbonate precipitation from either the extensive anaerobic oxidation of methane (AOM) and/or mixing of microbial methane and methanogenic CO2. AOM is confirmed by lipid biomarker evidence indicating that methane oxidation occurred in the sediments at the time of carbonate precipitation. The mixed dolomite/calcite mineralogies and the trend of δ13C in the tubular concretions from strongly negative to strongly positive values are interpreted to reflect methane oxidation from the onset of ascent through to the end of a migration event. Depleted and enriched δ18O values suggest an evolved fluid source influenced by the dissociation of gas hydrates. Collectively, our results indicate that the tubular concretions within the upper slope mudstones delineate parts of the subsurface plumbing network of a cold seep system on the late Miocene paleo-Hikurangi Margin in which the fluids were sourced from ascending methane. The intermediate location of the Whangaehu concretions between older (early Miocene) seep carbonates to the west and modern ones offshore to the east indicates a progressive eastwards shift with time of a long-lived, if only periodically active, seep system. The concretionary plumbing features at Whangaehu provide a conceptual model for subsurface fluid pathways and seep-related processes beneath the modern Hikurangi Margin seabed, and possibly also for other modern and ancient cold seep carbonate systems.