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Masters Thesis

Sand provenance of the Bounty submarine fan off the eastern coast of South Island, New Zealand

The submarine Bounty Fan is a major depositional feature off the eastern coast of South Island New Zealand that was recently selected as an alternate focus site for the MARGINS Source-to-Sink initiative. The Bounty Fan lies in the most seaward axial deep of the Bounty Trough, a remnant continental rift. The north levee of the Bounty Channel was cored on Ocean Drilling Program (ODP) Leg 181. The 617 meter section consists of three Miocene to Pleistocene units divided into nine subunits. Fifty-five samples were selected from intervals of very fine to fine sand ranging in thickness from 1 to 70 em. The samples were sieved for the sand-sized fraction. Stained thin sections of the sand concentrates were point counted (400 points) using the Gazzi-Dickinson method. The samples are quartzofeldspathic, dominantly composed of monocrystalline quartz and feldspar, with moderate mica and minor metamorphic lithic components. Bioclasts, including calcareous and siliceous microfossils, are present in varying amounts. Both opaque and non-opaque dense minerals are rare; the most common dense mineral is epidote. Trends in sand compositional parameters such as QFL, QmKP, and LmLvLs show that mean values for Site 1122 units and subunits cluster, with little compositional variation. Other lithic and mica parameters (e.g., proportions of biotite, muscovite, chlorite, and various metamorphic lithic types) also show little compositional variation among the units and subunits of Site 1122. Furthermore, there are no significant trends among thickness, grain size, composition, and depth of Site 11 22 sand samples, except that thicker beds tend to contain slightly more metamorphic rock fragments and that deeper, older units tend to contain slightly less potassium feldspar. The overall homogenous composition of Site 1122 sand indicates that it had a relatively uniform source. Possible sediment sources for sand at ODP Site 1122 include onshore rivers (Clutha, Waitaki, and Rangitata). One sample of fine grain sand from each of these rivers was petrographically analyzed. The Waitaki and Rangitata river samples are quartzolithic, dominated by metamorphic lithic fragments, moderate quartz and minor feldspar, whereas the Clutha River sample is quartzofeldspathic, dominated by quattz, with lessor feldspar, and minor lithic fragments. Sand detrital modes at Site 1122 closely match those of the Clutha River sample, especially in terms of QFL, QmKP, mica and lithic proportions suggesting it was the major source of sand at Site 1122. An upsection change from sandy contourite to turbidite deposits at Site 1122 is not reflected in sand composition. This suggests that the sand source remained constant while the depositional mode of sand at Site 1122 changed.

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