Fine-Grain Sediment Dispersal Pathways During the Late Pleistocene in Canterbury Basin, South Island of New Zealand
Villasenor, Tania; Jaeger, John M.
Decades of process-based studies of sediment dispersal on continental margins have identified key mechanisms that create muddy marine lithofacies, such as along- and across-shelf transport by surface plumes and in the benthic boundary layer. However, few examples exist of how these muddy shelf environments evolve over late Pleistocene changes in relative sea level and sediment supply. IODP Expedition 317 to Canterbury Basin New Zealand provides an opportunity to identify the processes forming sedimentary sequences where temporally evolving across- and along-margin sediment dispersal interact with both eustasy and tectonics to generate margin stratigraphy. We use X-ray diffraction bulk sediment mineralogy, grain size distributions, and downhole well-logs at mid-outer shelf and upper slope sites to evaluate the correlation of lithofacies with relative sea level and sediment sources during late Pleistocene glacioeustatic cycles. Distinctive mineralogy of two main source areas (southern Haast Schist vs proximal Torlesse Graywacke) allows for potential delineation of dispersal pathways. Results from principal component analysis (PCA) of the mineralogy and grain size data are compared to the core lithostratigraphy and natural gamma-ray log. Lithofacies correlate to changes in mineralogy and grain size, and are characterized by several cyclic transitions from coarser, more carbonate-rich sediment to muddy, terrigenous-rich material. The cycles are also indentified in natural gamma-ray logs. Sediment provenance is revealed by the second PC, which shows changes in the terrigenous material that compose the mud-rich intervals. A cycle consisting of an upsection increasing content of micas and fine-grain material transitioning to an overall coarsening and decreasing content of micas, suggests that there exists a Haast Schist (along-shelf dispersal) signature in muddier intervals, which is not apparent in coarser, more carbonate-rich intervals. These results imply that changes in relative sea level influence the mode of fine-grain sediment dispersal, favoring along-shelf mud transport during highstands and early regressive or falling stages and more across-shelf sand transport during later regressive or lowstand stages of sea-level. An overall upward trend of decreased content of micas, accompanied by high amplitude variability in the terrigenous material content, suggests that across-shelf sediment transport becomes more dominant with time.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013