| Summary: | Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated. In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements. As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation. The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.
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