Abstract:
Off the east coast of South Island, New Zealand, 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 at Ocean Drilling Program (ODP) Site 1122 on Leg 181. The 617 m section, which is divided into three units and nine subunits, consists of a Quaternary fan turbidite sequence that transitions downward into a Pleistocene to Pliocene mixed turbidite to contourite facies, which unconformably overlies a Miocene contourite-pelagic succession. Petrographic analysis (point counts) of 55 fine to very fine sand samples across the cored interval shows them to be quartzofeldspathic with moderate mica and minor metamorphic lithic components (mean values = Q (sub 44) F (sub 44) L (sub 12) , Qm (sub 50) K (sub 3) P (sub 47) , Lm (sub 70) Lv (sub 4) Ls (sub 26) , total%mica (sub 14) ). Mean recalculated parameters for Site 1122 units and subunits cluster on QFL, QmKP, and LmLvLs ternary plots with little compositional variation. 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 1122 sand samples, except that thicker beds tend to contain slightly more metamorphic rock fragments. The generally homogeneous composition of Site 1122 sand indicates that it may have had a relatively uniform source back into the early Miocene. Thus, the up-section change from sandy contourite to turbidite deposits at Site 1122 is not reflected in sand composition. This suggests that the sand provenance remained constant while the depositional processes of sand at Site 1122 changed. Sand detrital modes at Site 1122 most closely match those of the Clutha River, especially in terms of QFL, QmKP, mica, and lithic proportions, suggesting that it was a major source of sand at Site 1122. However, admixing of sand from the Waitaki River and other sources cannot be completely ruled out.