MIOCENE HYDROCARBON SEEP-CARBONATE SYSTEMS IN NORTH ISLAND, NEW ZEALAND

Previously enigmatic occurrences of discrete limestone bodies, and of small to large tubular carbonate concretions, occur in thick deep-water mudrocks of Miocene age in North Island, New Zealand. They are interpreted to represent sites of ancient hydrocarbon seep-carbonate systems, the limestones being paleo-seafloor deposits and the tubular concretions part of the sub-seafloor plumbing network. In all cases, Cretaceous to Paleogene oil- and gas-prone source rocks occur in the underlying stratigraphic succession. Seep-related features occur in both convergent and passive margin settings which, together with the varied sedimentological and structural settings of the deposits, has led to some significant spatial differences in lithologies (in situ, current reworked, slumped, debris flow), authigenic carbonate mineralogies (low to high-Mg calcite, dolomite and aragonite), stable carbon and oxygen isotope values of the carbonates (d¹³C from +10 to -52‰; d¹⁸O from +5 to -6‰), and faunal associations. The seep-limestones (to 10 m thick, 200 m long) archive methane signatures in their depleted carbon isotopes, and contain chemosynthesis-based paleocommunities (e.g. worm tubes, bathymodioline mussels, and vesicomyid, lucinid and thyasirid bivalves). Some special attributes of the New Zealand Miocene seep-limestones include: (1) their local buildup to a sufficient size to develop talus-debris piles on their flanks, which were populated by lucinid bivalves and terebratulid brachiopods; (2) the common existence of firmgrounds and hardgrounds, as evidenced by trace fossil assemblages or caryophyllid coral thickets atop some seep-limestones; (3) the development of digitate thrombolites of clotted microbial micrite encased in thick, isopachous horizons and botryoids of aragonite; and (4) the occurrence of seep plumbing features beneath the limestones, displaying probable gas-explosion breccias filled with aragonite, carbonate-cemented thin sandstone beds and burrows within otherwise impermeable mudstones, and different carbonate concretionary structures. In many localities, networks of morphologically variable tubular concretions, snaking their way through the exposed mudrock stratigraphy, are all that now testify to a former active methane seepage system.