Fine-grained contourite architecture and cyclic sedimentation in a late Miocene–Pliocene plastered drift system, Canterbury Basin, New Zealand

The late Miocene–Pliocene Tokama Formation at IODP Site U1352, Canterbury Basin, New Zealand, preserves a thick upper-slope mixed drift system developed beneath the Southland Current on a margin where Neogene glacio-eustatic sea-level fluctuations are well documented regionally. This study documents in detail the sedimentology, facies architecture and physical properties of this contourite succession, with particular emphasis on the anatomy of fine-grained contourite sequences and their cyclic organisation. Core description, ichnological logging and integration with shipboard data show that the drift is dominated by olive-grey, bioturbated silty marlstones arranged into recurrent partial and rare complete bi-gradational contourite sequences (C1–C5). These include both top-cut and base-cut variants, together with intervals of more complex stacking that reflect subtle shifts in bottom-current regime and sediment supply.

High-resolution digital core imaging was used to derive continuous greyscale and L* color-reflectance logs, which display systematic alternations between darker and lighter intervals at decimeter- to meter-scale. Spectral analysis of these records reveals prominent cyclicities across a range of relatively short wavelengths, indicating higher-frequency, repetitive changes in sediment character through time, but without uniquely constraining an orbital (Milankovitch) control. Thin-section image analysis (ImageJ) of selected samples provides initial quantitative grain-size constraints, indicating very fine silt mean sizes, poor sorting and mesokurtic to leptokurtic distributions. These data support assignment of the dominant facies to intermediate contourite subunits (C2/C4), with subordinate muddier (C1/C5) and sandier (C3) intervals.

The facies, ichnological and physical-property datasets demonstrate that the Tokama Formation at Site U1352 represents a large, plastered drift–terrace system shaped by persistent along-slope flow, punctuated by changes in bottom-current velocity, sediment input and redox conditions. The work refines recognition criteria for fine-grained contourites in mixed systems, highlighting the role of omission surfaces and missing time within contourite sequences, and provides a stratigraphic and process-based framework for subsequent grain-size and sortable-silt analysis of this drift.