|dc.description.abstract||This study focuses on the five sites of Early Miocene aged cold seep carbonate deposits (Wanstead, Wilder Road, Haunui, Ugly Hill, and Ngawaka) in the southern East Coast Basin, North Island, New Zealand, which archive paleo-hydrocarbon seafloor seepage in a convergent margin setting. Seep deposits are hosted within calcareous slope mudstones of the Ihungia Fm and provide an important analogue to modern seep sites in the East Coast Basin. Individual fossilised seep sites have revealed, through mapping and logging, differences in outcrop extent, morphology, and lithology, while petrology has uncovered a complex array of textures, fabrics, and mineralogies within the methane derived authigenic carbonates (MDAC). Fabrics are diverse and often comprise chaotic multi-phase assemblages of various mineralogies in original and/or altered states. Enclosed within the carbonates are the fossilised remnants of chemosynthesis-based communities, particularly macro-invertebrates, which includes lucinid and vesicomyid clams, modiolid mussels, corals, gastropods, and worm tubes. These fossil types, and their distribution throughout the deposit, not only reflect the variations in fluid conditions but also the maturity of the seep system. Veining and (micro)brecciation are common at a variety of scales with multiple generations of cements forming thick fill sequences representing cyclic episodes of fracturing and sealing through carbonate formation.
Detailed analysis of MDAC deposits has led to the identification of thirteen mineral fabrics, four fracturing and/or corrosion phases, and two alteration phases. Relationships between the various fabrics and phases has led to the elucidation of a generalised paragenetic sequence while appreciating that some local variation occurs. The paragenetic sequence is divided into two diagenetic stages: an early stage that includes all mineralisation and events that occurred in the (near) seafloor environment, and a late stage that includes all mineralisation and events that occurred in the burial realm.
Early diagenesis (8 events) involved precipitation of mainly aragonite phases, including clotted and peloidal microcrystalline carbonate (micarb), fibrous and radiating crystal forms (often hydrocarbon inclusion-bearing) occasionally encase thrombolites between successive isopachous generations, and anhedral crystal masses and layers. Early destructive episodes of fracturing, (auto)brecciation, and dissolution via corrosive fluids significantly modified the deposits, ultimately increasing surface area available for further mineral growth. Late diagenesis (9 events) occurred within the burial realm and resulted in the addition of late stage pore and fracture occluding low magnesium calcite (LMC) and some minor dolomite and silica forms. Some of the early metastable aragonite forms remained pristine and unaltered, while others underwent varying degrees of fabric retentive to destructive neomorphic change generally resulting in stable LMC mineralogies. Later uplift and exhumation brought the carbonate deposits into their present-day positions.
Distinctly depleted stable δ13C isotope signatures indicate carbonate carbon was largely sourced from thermogenic methane, with some of the migrating fluids likely affected by methane pool oxidation, and mixing with marine water and residual CO2. Stable δ18O isotope signatures suggest that the ascending hydrocarbon-rich fluids experienced slight temperature fluctuations, possibly due to contact with warmer advecting fluids, and were also influenced by local gas hydrate formation and dissociation events. Preliminary lipid results importantly confirm the presence of microbes which carry out the anaerobic oxidation of methane (e.g. PMI and biphytanic diacid) and sulphate reduction (e.g. iso-and anteiso fatty acids).
The complexity revealed within these cold seep carbonates, and more particularly their cement stratigraphy, reflects the temporally and spatially dynamic nature of the fluids reaching the paleo sea floor of the East Coast Basin. Many of the features observed within these southern East Coast Basin cold seep deposits have a striking similarity with the Miocene deposits of the Italian Apennines, while others, particularly the occurrence of microbial mats (thrombolites), are an interesting feature more atypical of worldwide occurrences.||en_NZ