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

Abstract

Deep-water mudstones overlying basin-floor and slope sandstone-prone deposits are often interpreted as hemipelagic drapes deposited during sand starvation periods. However, mud transport and depositional processes, and resulting facies and architecture of mudstones in deep-water environments, remain poorly understood. This study documents the sedimentology and stratigraphy of basin-floor and slope mudstones intercalated with sandstone-prone deposits of the Laingsburg depocentre (Karoo Basin, South Africa). Sedimentologic and stratigraphic criteria are presented here to distinguish between slope and basin-floor mudstones, which provide a tool to refine palaeogeographical reconstructions of other deep-water successions. Several mudstone units were mapped at outcrop for 2500 km² and investigated using macroscopic and microscopic core descriptions from two research boreholes. 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, and no repeated or predictable facies organisation is recognised. The well-ordered and predictable facies organisation of basin-floor mudstones suggest local point sources from active slope conduits, responsible for deposition of compensationally stacked muddy lobes. The lack of predictable facies organisation in slope mudstones suggests deposition took place in a more variable range of sub-environments (i.e. ponded accommodation, minor gully/channel-fills, levees). However, regional mapping of three mudstone units evidence basinward tapering and similar thicknesses across depositional strike. This geometry is consistent with the distal part of basin margin clinothems, and suggests laterally extensive mud delivery across the shelf edge combined with along-margin transport processes. Therefore, the sedimentology and geometry of mudstones suggests that mud can be delivered to deep-water dominantly by sediment gravity flows through point source and distributed regionally, during periods of up-dip sand storage. These findings challenge the common attribution of deep-water mudstones to periods of basin-floor sediment starvation.