Using Laser-Diffraction Grain-Size Analysis and End-Member Modelling Analysis (EMMA) to understand laminar to turbulent flow transitions in deep-water systems

Hybrid beds or linked debrites are deposits that form under bi- or tri-partite flow conditions, involving transitions from turbulent to laminar flow conditions. Often, hybrid beds occur with distal or lateral flow transformation following significant entrainment of a muddy substrate and/or declining turbulent energy. Hybrid beds have been noted to make up significant proportions of deposits within basin-floor setting worldwide, most commonly within the distal fringes of lobe systems. The stratigraphic distribution of hybrid beds has been linked to the character of the supply slope and seafloor relief, where hybrid beds are invoked to develop during periods of disequilibrium in out-of-grade slopes.

The mechanisms of formation and evolution of flows that deposit hybrid beds have been significantly studied and debated over the past decades. The aim of this study is to utilize techniques not commonly used in this area, namely Laser-Diffraction Grain-Size Analysis and End-Member Modelling Analysis (EMMA) to undertake a detailed study of lateral and down-dip changes within targeted deposits, to interpret more accurately how and when turbulent to laminar fluctuations occur, as well as adding quantitative analysis to previously established qualitative models. Samples were taken from the Marnoso-Arenacea Formation, the Castagnola Formation, and the Gottero Formation.

Initial findings show significant correlation between grain-size distribution from similar facies across different formations. Despite variability in the median grain-size between formations, similar end members can be extracted from each, indicating deposition under similar flow conditions.