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Sandy Mixed Terrigeous/Calcareous Drift Deposits Off New Zealand as a Model for Petroleum Exploration


Sandy contourite drifts have been promoted as potential petroleum reservoir facies but their compositional variation, particularly in the terrigenous fraction, remains understudied. Scientific ocean drilling on the Canterbury Margin of eastern South Island, New Zealand is yielding new insights. Here the drift history extends back into the Miocene, but we have focused our work on the ∼300 m-thick mid-Pliocene to Pleistocene (<3 my) section that was continuously cored at ODP (Ocean Drilling Program) Site 1119. Sand detrital modes of very fine and fine sand fractions from 19 unconsolidated core samples (38 thin sections) were determined based on counts of 400 points using the Gazzi-Dickinson method and a detailed bioclast classification scheme. Very fine sand fractions contain an average of 25% bioclastic debris, with bioclasts making up 51% of the fine sand fraction. Foraminifera are the dominant bioclasts, with pelagic more common than benthic varieties. The average proportions of monocrstalline quartz, plagioclase and potassium feldspar are similar in the very fine and fine sample sets (average QmKP of ∼25:12:63 each). In contrast, the very fine sand fractions have, on average, higher lithic (QFL%L of 16 vs. 3%), mica (Totalgrain%M of 11 vs. 5%), and dense mineral (Totalgrain%D of 9 vs. 3%) content as compared with the fine sand fractions. Stratigraphic sections correlated by previous workers with younger clinoform drape (Unit I) and older mounded drift (Unit II) seismic facies showed no distinct compositional differences in their sand fractions. The composition of Site 1119 drift sand is, however, distinctly different from modern onshore river sand. Common metamorphic lithic fragments and chlorite grains suggest a southern Otago schist provenance, however the high proportion of Ca-plagioclase, a non-schist mineral phase, requires a contribution from Torlesse Group or Cenozoic volcanic rocks. Previous workers have linked grain size variation of the silt fraction in this drift deposit to glacial processes resulting from tectonic uplift along the nearby Alpine Fault plate boundary; this may help explain the compositional mix. We are also comparing and contrasting the Site 1119 drift sand compositions with those from along-strike slope-clinoform and shelf facies recovered at Integrated Ocean Drilling Program Expedition 317 sites to the south, in order to place the Site 1119 results in a larger, more detailed source-to-sink context.