Biomarkers in offshore Canterbury Basin sediments, New Zealand: Organic matter input and thermal maturity

IODP Expedition 317 drilled 4 sites on a landward-to-basinward transect in the Canterbury Basin off New Zealand. Site U1354 was drilled on the mid-shelf (48 km offshore), while site U1352 was on the continental slope (67 km offshore). Cored Holocene-Miocene sediments from the seafloor down to 1.8 km were analysed for their biomarker composition. Extractability of the sediments is low (<0.6 mg EOM/g sediment), consistent with the generally low TOC (<1.5%). n Alkanes are typically most abundant in the C23 -C33 range and are strongly dominated by the odd carbon numbered homologues, reflecting substantial higher plant input. Based on Pr/Ph, oxicity of the depositional environment was high on the shelf, but much less so on the slope. Biomarker distributions include immature hopanes, especially the C31 22R isomer, and sterane distributions dominated by C29 isomers. The presence of aromatic biomarkers such as cadalene is consistent with a strong terrestrial organic matter contribution, and it is notable how little marine organic matter is apparently preserved, as also shown by low hydrogen indices (<100 mg HC/g TOC). Thermal maturities for a sample from 1,554 m in U1352, based on calibrated aromatic hydrocarbon maturity ratios, indicate a calculated reflectance of 0.57%, and this is supported by the Tmax of 420-430 C. This thermal maturity just prior to the start of the oil window supports a high geothermal gradient of 45 C/km in this part of the Canterbury Basin.