Sedimentological and stratigraphic criteria to distinguish between basin-floor and slope mudstones: Implications for the delivery of mud to deep-water environments

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Kévin Boulesteix , Miquel Poyatos-Moré , Stephen S. Flint, David Hodgson, Kevin G. Taylor, Rufus L. Brunt


Deep-water mudstones overlying basin-floor and slope sandstone-prone deposits are widely interpreted as hemipelagic drapes deposited during extended periods of sand starvation. However, the processes of mud transport and deposition, and the resulting facies and sedimentary architecture of mudstones in different deep-water environments, remain poorly understood. This study documents the sedimentology and stratigraphy of basin-floor and slope mudstone units intercalated with sandstone-prone deposits of the Laingsburg depocentre (Karoo Basin, South Africa). The mudstone units have been mapped for 2500 km 2 and investigated using macroscopic and microscopic descriptions from a continuous core dataset. Basin-floor mudstones exhibit a repeated and predictable alternation of bedsets dominated by low-density turbidites, and massive packages dominated by debrites, with evidence of turbulent to laminar flow transformations. Slope mudstones exhibit a similar facies assemblage, but the proportion of low-density turbidites is higher, bioturbation is more pervasive, and no repeated or predictable facies organisation is recognised. Regional mapping evidences a gradual basinward tapering of all mudstone units, consistent with the distal part of basin margin clinothems, and suggests a dominant line-source of mud delivery beyond the shelf edge. However, the well-ordered and predictable facies organisation of the basin-floor mudstones also suggest the presence of local point sources from active slope conduits, responsible for the deposition of compensationally-stacked muddy lobes. The lack of a predictable facies organisation in slope mudstones suggests deposition took place in a more variable range of sub-environments. For the first time we present a set of sedimentological and stratigraphic criteria to distinguish between submarine slope and basin-floor mudstones, which may provide an important tool to refine palaeogeographic reconstructions of other deep-water successions. This study suggests that deep-water mud can be delivered dominantly by sediment gravity flows through both line-and point-source supply, during periods of up-dip sand storage, challenging the model of basin-floor sediment starvation.



Physical Sciences and Mathematics


depositional processes, Karoo Basin, mudstones, basin-floor, slope


Published: 2020-10-22 10:37

Last Updated: 2020-10-22 17:37


CC BY Attribution 4.0 International

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