4 research outputs found
Geological controls on focused fluid flow through the gas hydrate stability zone on the southern Hikurangi Margin of New Zealand, evidenced from multi-channel seismic data
Highly concentrated gas hydrate deposits are likely to be associated with geological features that
promote increased fluid flux through the gas hydrate stability zone (GHSZ). We conduct conventional
seismic processing techniques and full-waveform inversion methods on a multi-channel seismic line that
was acquired over a 125 km transect of the southern Hikurangi Margin off the eastern coast of New
Zealandâs North Island. Initial processing, employed with an emphasis on preservation of true amplitude
information, was used to identify three sites where structures and stratal fabrics likely encourage focused
fluid flow into and through the GHSZ. At two of the sites, Western Porangahau Trough and Eastern
Porangahau Ridge, sub-vertical blanking zones occur in regions of intensely deformed sedimentary
layering. It is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate
upwards and away from the deformed zone along layers that trend towards the seafloor. At Eastern
Porangahau Ridge we also observe a coherent bottom simulating reflection (BSR) that increases markedly
in intensity with proximity to the centre of the anticlinal ridge. 1D full-waveform inversions conducted at
eight points along the BSR reveal much more pronounced low-velocity zones near the centre of the ridge,
indicating a local increase in the flux of gas-charged fluids into the anticline. At another anticline,
Western Porangahau Ridge, a dipping high-amplitude feature extends from the BSR upwards towards the
seafloor within the regional GHSZ. 1D full-waveform inversions at this site reveal that the dipping feature
is characterised by a high-velocity zone overlying a low-velocity zone, which we interpret as gas hydrates
overlying free gas. These results support a previous interpretation that this high-amplitude feature
represents a local âup-warpingâ of the base of hydrate stability in response to advective heat flow from
upward migrating fluids. These three sites provide examples of geological frameworks that encourage
prolific localised fluid flow into the hydrate system where it is likely that gas-charged fluids are converting
to highly concentrated hydrate deposits
Origin of blocky aragonite cement in Cenozoic glaciomarine sediments, McMurdo Sound, Antarctica
Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physioâchemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROSâ1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a poreâfilling blocky fabric throughout the core. Oxygen isotope compositions (δ18O = â30¡0 to â8¡6â° Vienna PeeâDee Belemnite) and clumped isotope temperatures (TÎ47 = 13¡1 to 31¡5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ18Ow) of the parent fluid, which mostly range from â10¡8 to â7¡2â° Vienna Standard Mean Ocean Water. The fluid δ18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby ANDâ2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROSâ1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROSâ1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought