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.

新西兰北岛东角附近晚中新世波胡图组（Pohutu Formation）半深海泥岩中的潮间带滨岸平台，产出大量形态以甜甜圈状为主、直径可达6米的巨型白云质管道结核。其碳同位素值（δ¹³C为+6‰至+9‰，相对于佩迪贝箭石标准（PDB））表明，白云石沉淀所需的碳源自热成因甲烷的大规模厌氧氧化；而氧同位素值（δ¹⁸O为+2‰至+6‰，相对于PDB）则暗示碳酸盐沉淀时期发生了天然气水合物的分解。这些结核被推断为汇聚大陆边缘古冷泉系统中，向上运移的富烃流体在浅部海底以下的赋存位置标志。结核的分布与断层紧密相关，我们提出，甜甜圈状结核的生长与周期性活动的断层阀机制有关：该机制包含流体压力升高、断层破裂与流体泄出，随后流体压力降低、矿物沉淀并使断层封闭。