Campbell S. Nelson

The place of tubular concretions in hydrocarbon cold seep systems: Late Miocene Urenui Formation, Taranaki Basin, New Zealand

The late Miocene Urenui Formation cropping out on the eastern margin of Taranaki Basin, currently New Zealands only commercially exploited hydrocarbon basin, is dominated by siliciclastic slope mudstone (5% carbonate) that locally includes a variety of tubular concretions (50–85% carbonate). Dominated by commonly large pipe and bulbous varieties, as much as 0.5 to more than 10 m (1.6 to 33 ft) long and 5 to 70 cm (2 to 28 in.) across, these concretions formed from the precipitation of fine-grained calcite or dolomite cement within siliciclastic mud in the shallow subsurface (likely 100 m burial). Most support a central conduit that may be empty or filled with sediment or multiple generations of post-shallow-burial carbonate cements. Depleted 13C values of the concretionary calcite cements (40 to 25 PDB) indicate that their carbon was sourced from oxidized methane. In contrast, enriched 13C values of the concretionary dolomite cements (10 to +11 PDB) indicate that their carbon was derived from methanogenic CO2 and/or extensive methane oxidation. The range of 18O values for the concretionary cement types (2 to +5 PDB) supports precipitation from fluids both depleted and enriched with respect to New Zealand Miocene marine waters, which are suggested to reflect episodes of gas hydrate formation and dissociation, respectively. The tubular carbonate concretions are interpreted to mark the subsea-floor pathways of focused methane infused upward fluid escape, and as such, they represent a part of the shallow subsurface plumbing system of a cold seep system developed on the continental margin bounding the Taranaki Basin in the late Miocene. This margin has been strongly influenced by periodic movements along the major Taranaki fault thrust system, which coincidentally directly underlies the Urenui concretion occurrences and was probably the most likely avenue for active fluid migration within the hydrocarbon seep system. The tubular concretions experienced periodic intervals of exhumation and burial caused by margin failure because of greater slope instability from injected fluids and resulted in conglomeratic channel-fill deposits within the Urenui Formation that include mass-transported tubular concretions.