Sedimentary processes and sequence stratigraphy of a Quaternary siliciclastic shelf-slope system; insights from sand provenance studies, Canterbury Basin, New Zealand

We combined sand petrofacies with lithofacies to characterize sedimentation within unconformity-bounded sequences at Integrated Ocean Drilling Program (IODP) Expedition 317 sites drilled across the Canterbury shelf to the slope, located off the east coast of the South Island, New Zealand. Differentiation of the relative influence of along- and across-shelf sand supply in this system is made possible by the unique aspects of the onshore geology. Northern rivers draining mainly Torlesse composite terrane lithologies are dominated by lower-grade metamorphic lithic fragments, whereas central rivers, draining the Torlesse to schist (semi-schist) transition (Haast/Otago Schist), contain more higher-grade metamorphic lithic fragments, and the southern rivers contain sand that is quartzofeldspathic and mica rich, having been derived predominantly from coarse schist. Differences are documented in onshore river sand that allow for the provenance classification of 38 offshore sand samples from IODP Expedition 317 cores into four provenance groups based on their likely bedrock sources: (1) Torlesse, (2) Torlesse-schist transition, (3) schist, or (4) mixed. The distribution of sand composition in the 0-0.62 Ma sections of the shelf and slope sites indicates a dynamic system where shore-parallel and shore-perpendicular processes alternated on the shelf, and shore-perpendicular processes dominated at the slope site. When sand compositions are placed in a sequence-stratigraphic context, they indicate an evolving paleogeography through time. Significant sand provenance shifts are linked to falling sea level, with Torlesse-schist transition compositions characteristic of regressive systems tracts. Torlesse compositions are supplied to the sites during falling sea level and sea-level lowstands, when fluvial and coastal geomorphology promotes influx from the north. Mixed compositions characteristic of transgressive systems tracts are likely products of littoral- and shelf-current mixing and potential influx of schist detritus from the south.