Grain size matters: late Quaternary dynamics of a sediment-routing system in Canterbury Basin, South Island, New Zealand

ABSTRACT The adjustment of sediment-routing systems to Quaternary environmental variations and the transmission of resultant sedimentary signals to the depositional sink is a matter of intense debate because of the potential for loss of the signal due to autogenic processes. We analyze the provenance of three different grain-size fractions from continental-shelf strata to reconstruct the dynamics of the southern Canterbury sediment-routing system (SRS), South Island of New Zealand, during the late Quaternary. We use the distinctive bedrock mineralogy in the system to reveal differing dispersal SRS pathways for each fraction. Mineralogical characterization of the sand-, silt-, and clay-size fractions from IODP Site U1353 indicates variations in sediment provenance at different timescales, reflecting the various processes of sediment generation and transport to the continental shelf that change over time. Sand-size sediment shows a marked change in composition around 340 ka (MIS 10/9) that resulted from a shift in the dominant mode of fine-sand dispersal from marine along-shore to fluvial-regressive. This may be related to decreased fluvial storage capacity leading to enhanced sand dispersal to the coast via advective suspended-load transport. The composition of silt and clay fractions fluctuates at glacial–interglacial timescales indicating supply of sediment from different areas of the proximal Waitaki River Basin, related to: i) changing onland storage capacity and sediment fluxes during glacial cycles, and ii) modulation of along-margin sediment dispersal due to changes in continental-shelf accommodation during sea-level cycles. Finally, we use observations of modern sediment transport rates of the three size fractions to quantitatively explore the relative abilities to buffer, record, and shred externally (climate) forced signals of sediment flux. This study shows the importance of considering various grain-size fractions in the study of sediment-routing systems as they can reflect earth-surface dynamics at different timescales.