Influence of faulting on the distribution and development of cold seep-related dolomitic conduit concretions at East Cape, New Zealand

An intertidal shore platform in bathyal mudstones of the Late Miocene Pohutu Formation near East Cape, North Island, New Zealand, hosts many impressive dolomitic conduit concretions having predominantly doughnut morphologies up to 6 m across. Carbon isotope values (δ13C + 6 to + 9‰ PDB) are interpreted to show that the carbon for dolomite precipitation was derived from extensive anaerobic oxidation of thermogenic methane, while oxygen isotope values (δ18O + 2 to + 6‰ PDB) implicate the dissociation of gas hydrates at the time of carbonate precipitation. The concretions are inferred to mark the shallow sub-seafloor locations of upward migrating hydrocarbon-enriched fluids in a fossil cold seep system on a convergent margin. The distribution of concretions and faults are intimately associated. We propose that doughnut concretion growth was related to a periodically active fault-valve mechanism involving fluid pressure increase, fault rupture and fluid discharge, followed by fluid pressure decrease, mineral precipitation and fault sealing.