The role of intermediate-depth currents in continental shelf-slope accretion: Canterbury Drifts, SW Pacific Ocean

Abstract

Abstract The Late Oligocene to Recent Canterbury Drifts were deposited in water depths between c . 400 and c . 1500 m by northward-flowing, cold, intermediate-depth water masses: Sub-antarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW) and their predecessor current flows. Drift accumulation started at c . 24 Ma, fed by terrigenous sediment derived from the newly rising Alpine Fault plate boundary in the west, which has built a progradational shelf-slope sediment prism up to 130 km wide at rates of eastward advance of up to 5.4 km Ma -1 . Gentle uplift associated with the nearby plate boundary has exposed older Late Oligocene and Miocene drifts onland (Bluecliffs Formation). Ocean Drilling Program Site 1119, located 100 km offshore at a water depth of 394 m, penetrated a 428 m thickness of mid-Pliocene to Pleistocene (0–3.9 Ma) drift located just seaward of the eastern South Island shelf edge. Uniquely, these large (>60 000 km 3 ), regionally extensive, intermediate-depth sediment drifts can be examined in outcrop, in marine drill-core and at the modern sea bed. The drifts comprise planar-bedded units up to several metres thick. Some sand intervals have sharp, erosive bases and normally graded tops into overlying siltstone; others are symmetrically graded with reverse-graded bases and normally graded tops. Bioturbation is moderate and rarely destroys the pervasive background, centimetre-scale, planar or wispy alternation of muddy and sandy silts displayed by Formation Micro-Scanner imagery. These features are consistent with deposition from rhythmically fluctuating bottom currents. Texturally, the drifts are poly modal quartzofeldspathic silty sands, sandy silts, silts and silty clays, with varying admixtures of benthic and biopelagic carbonate and silica. Miocene samples are mostly dominated by coarse silt (45–60 µm) and very fine sand (70–105 µm) grain-size modes, whereas strong fine silt (11–13 µm) and very fine silt-clay (<5 µm) modes become dominant after c . 3.1 Ma in the Late Pliocene, consistent with an increasing input of glacially ground material. Over the Plio-Pleistocene part of the succession, the sand-silt lithological rhythmicity occurs in synchroneity with Milankovitch-scale climate cycling, with periods of inferred faster current flow (sand intervals) mostly corresponding to warm, interglacial periods. Northward drift dispersal has helped cause the seaward growth of the eastern South Island shelf-slope system since the Late Oligocene probably by clinoform progradation and by episodic welding of mounded slope drifts onto the pre-existing sediment prism. Such along-slope, contourite drift accumulation occurs even in the absence of mounded drifts on seismic profiles, and represents a previously underemphasized mechanism for the progradation of shelf-slope clinoforms, worldwide. The Canterbury Drifts vary in thickness from c . 300 m near the early Miocene shoreline, where they were accumulating in limited shallow-water accommodation, to c . 2000 m under the modern shelf edge. Mounded drifts first occur in the Middle Miocene, at c . 15 Ma, their appearance perhaps reflecting more vigorous intermediate water flow consequent upon the worldwide climatic deterioration between 15 and 13 Ma. At Site 1119, a further change from large (>10 km wide) to smaller (1–3 km wide) mounded slope drifts occurs at c . 3.1 Ma, marking further cooling and perhaps the inception of discrete SAMW flows and initiation of the Subantarctic Front.