In situ modification of modern submarine hyaloclastic / pyroclastic deposits by oceanic currents: an example from the Southern Kermadec arc (SW Pacific)

Wright, I.C. (2001) In situ modification of modern submarine hyaloclastic / pyroclastic deposits by oceanic currents: an example from the Southern Kermadec arc (SW Pacific). Marine Geology, 172 (3-4), 287-307. (doi:10.1016/S0025-3227(00)00131-6).

Abstract

Submarine volcaniclastic deposits, both modern and ancient, pose a conundrum in distinguishing between syn- and post-eruptive processes. High-standing, submarine volcanic edifices of the late Quaternary southern Kermadec arc (SW Pacific) are point sources of pyroclastic/hyaloclastic deposits that are bathed and modified by a complex current system of the South Pacific gyre flowing southeast along the northern margin of New Zealand, which in part comprises the anticyclonic flow of the warm-cored East Cape Eddy (ECE). Flow of the ECE across the southern Kermadec arc provides a present-day case of extensive and in situ, post-eruptive, textural modification of modern pyroclastic/hyaloclastic deposits on the crests and upper flanks of submarine stratovolcanoes. Photographic observations (and limited textural data) from seven Kermadec volcanoes reveal pervasive evidence of sediment winnowing (including crag and tail structures, scour and moating around volcanic blocks, coarse sand-granule lag deposits, epifaunal deflection, lineated mud streaking, and moulded bioturbation mounds) and asymmetric current-ripple bedforms at water-depths of at least 1500 m. All bedforms indicate increasing current speed at progressively higher elevations (decreasing water-depth) for each volcano. Current-ripples mostly have discontinuous, asymmetric, shorted-crested, linguoid–lunate forms below 1000 m water-depth, progressing to semi-continuous, asymmetric, shorted-crested, linear-sinuous forms above 500 m. Current elutriation of the Kermadec deposits progressively removes fines with decreasing water-depth resulting in relatively fines-depleted, volcaniclastic sands and granules. This post-eruptive process overprints syn-eruptive processes that notionally generate more comminuted fine-grained clasts with decreasing water-depth as phreatomagmatic explosive eruptions become more vigorous. Current-elutriation also modifies volcaniclastic detritus prior to subsequent removal by episodic, mass-gravity flow. In addition the sand-granule traction load, driven by current-flow, moves sediment nearly continuously to gully and rill heads for removal down-slope, independently of syn-eruptive sediment flux. The underlying observation is that volcaniclastic deposits rarely reflect just syn-eruptive processes, and that significant in situ current-elutriation of at the least surficial pyroclastic/hyaloclastic eruption products can occur on submarine volcanoes.
Threshold current velocities, derived assuming unidirectional flow over cohesionless sand-lapilli grainsizes, and accounting for bed friction, yield current velocities (at 100 cm above the bed) of ?15 cm s?1 for water-depths >1500 m through to 70 cm s?1 for depths <500 m at the crests of Rumble III and V volcanoes. Estimated velocities are consistent with short-term current velocities of 30–40 cm s?1, measured directly from either acoustic doppler current profile data or relative geostrophic flow, since the latter do not account for seafloor topographic intensification. The variable hydrographic climatology of the ECE, known from sea-surface dynamic heights and repeat CTD surveys, is possibly recorded by seafloor substrates as evinced by worm-trails post-dating ripple formation and differing orientations of winnowed structures and ripples.

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