Provenance study of miocene to pliocene sand and sandstone intervals recovered on Expedition 317 in the Canterbury Basin, South Island, New Zealand

Integrated Ocean Drilling Program (IODP) Expedition 317 drilled several sites on the Canterbury Margin. Three of these sites were located on the shelf (U1353, U1351, U1354) and one site on the slope (U1352), forming a west (U1353) to east (U1352) transect across the shelf. This study analyzed the sandy intervals from the Pliocene to Miocene units and compared them with previous petrographic studies and onshore samples from rivers and outcrops to ascertain the key controls on sand sedimentation on the shelf and slope. Thirty-eight offshore samples from Expedition 317 cores recovered at sites U1353, U1351, and U1352 and two onshore samples from the Ashburton River and Blue Cliffs Formation were petrographically analyzed using the Gazzi-Dickinson method of point counting. Sand grains were classified using schemes developed by Bender-Whitaker (2013) and Marsaglia and Nolasco (2016). The variation of composition on the Canterbury shelf and slope can be broken into at least three different petrofacies. The first petrofacies constitutes the entirety of the Site U1353 section and the uppermost units at Sites U1351 and U1352. It is related to a high compositional variability on the shelf, first defined by Bender-Whitaker (2013) in the Pleistocene units at these sites. The second petrofacies is related to a schist provenance signature characteristic of drift sediments deposited on the slope, by way of contour currents, increases in sedimentation rate, and changes in eustasy. This petrofacies, defined by Marsaglia and Nolasco (2016) at nearby Site 1119 as having high mica and higher-grade metamorphic lithic fragment content, is present in onshore outcrops of the Blue Cliffs Formation interpreted by others as drift deposits, and at Site U1352 in an interval with drift-like seismic reflectors. Lastly there is potential unroofing trend related to Miocene initiation and uplift of the Southern Alps at the base of Site U1352. These tectonic events initiated erosion of sedimentary units covering Torlesse, transitional schist-Torlesse, and schist directly onshore of Expedition 317 sites. A high low-grade lithic content and an increased proportion of biotite characterize the sand provenance signature of this potential unroofing sequence, which culminates in quartoze sand.