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Deposits of flows transitional between turbidity current and debris flow

Esther J. Sumner^1,*, Peter J. Talling^1,* and Lawrence A. Amy^1,

¹ Department of Earth Science, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK
² Institute of Petroleum Engineering, Heriot-Watt University, Riccarton Campus, Edinburgh EH14 4AS, UK

The relationship between submarine sediment gravity flows and the character of their deposits is poorly understood. Annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment-laden flows. Decelerating fast (>3 m/s) flows with fixed sand content (10 vol%) and variable mud content (0–17 vol%) resulted in only four deposit types. Clean sand with a mud cap that resembled a turbidity current deposit (turbidite) formed if the flow was turbulent when deposition began, or if the muddy fluid had insufficient strength to suspend the sand. The clean sand could contain structures if mud content was low (<6%) and the deceleration period was >300 s. Ungraded muddy sand with a mud cap that resembled a debris-flow deposit (debrite) formed if the flow became laminar before sand could deposit. Clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late-stage settling of sand from a muddy suspension. These deposits resemble enigmatic submarine flow deposits called linked debrite-turbidites. The experiments provide a basis for inferring flow type from deposit character for submarine sediment-laden flows.