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Production of Coupled Sand-Mud Deposits by Remobilization in Subaqueous Transitional Flows

Abstract

Turbidite-debrite couplets have been interpreted as the product of deposition from spatially separated turbidity currents and debris flows or a current transitioning from turbulent to laminar flow over significant time and space. We present results from 3D laboratory experiments demonstrating that a single sediment-gravity flow can also develop turbidite-debrite-like couplets by early-stage remobilization sediments that are still in the process of being deposited. This remobilization appears common to flows composed of mixtures of sand and mud with viscosities and strengths measurably greater than water, but not so high as to fully suppress the settling of sand through the depositional current. The dewatering of the early sand deposit acts to lubricate the basal portion of the increasingly muddy cap of the flow, causing it to accelerate downslope, triggering a secondary flow with a sediment composition distinct from the primary mixture. While remobilization of an upper muddier interval by dewatering of a basal sand deposit is the primary autogenic process observed, minor flow acceleration of the uppermost muddy cap is triggered where its thickness exceeds its strength. The flow, sedimentation, and remobilization processes are all imaged using ultrasonic transceivers. After all transport ceases the final deposit is also acoustically imaged. This 3D imaging is supported by an extensive coring and sediment sampling scheme. The collected cores and grain-size data show sand-mud couplets that in 2D section are qualitatively similar to interpreted turbidite-debrite couplets in natural systems. Recent advances in visualization techniques have enabled mapping of seismic geomorphology on ultrasonic data cubes and reveal unprecedented deposit patterns. Sediment deposition and remobilization by a single-event flow produces various sedimentologic patterns including clean, structureless basal sands, structureless sandy muds, thin sand-mud bands, and reverse grading in the muddy capping layer. We interpret the composite internal deposit morphology to reflect the spatial heterogeneity occurring within a vertically segregated flow where divisions with different rheological properties interact during both transport and deposition of sediment. This work highlights the presence and complex spatial organization of argillaceous and clean sands in subaqueous lobes, and contributes to a better understanding of palaeoenvironments and property distributions in these settings.