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 basinfloor 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 km 2 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

Mud‐rich low‐density turbidites in structurally‐controlled intraslope mini‐basin: The influence of flow containment on depositional processes and sedimentation patterns (Szczawa, Oligocene, Polish Outer Carpathians)

“…This way, FA1 (thick and very thick turbidites) may represent periods of relative sea‐level fall (lowstand and early transgression) and intensified remobilization of shallow‐marine deposits into slope area. In turn, FA2 (thin and medium turbidite mudstones) may represent periods of relative sea‐level rise (highstand) and reduced intensity of redeposition into the slope (for discussion on sea‐level control on sediment transfer to slope area, see, for example, Deik et al ., 2021; Boulesteix et al ., 2022; Brooks et al ., 2022, and references therein). Indeed, the FA2 mudstone bed packages show relatively high degree of bioturbation intensity, which can be interpreted to represent periods of significantly reduced sediment input (Boulesteix et al ., 2022).…”

“…In turn, FA2 (thin and medium turbidite mudstones) may represent periods of relative sea‐level rise (highstand) and reduced intensity of redeposition into the slope (for discussion on sea‐level control on sediment transfer to slope area, see, for example, Deik et al ., 2021; Boulesteix et al ., 2022; Brooks et al ., 2022, and references therein). Indeed, the FA2 mudstone bed packages show relatively high degree of bioturbation intensity, which can be interpreted to represent periods of significantly reduced sediment input (Boulesteix et al ., 2022). Avulsions between delta lobes on the shelf or high‐frequency climatic changes (humid‐to‐arid) which control the delivery from fluvio‐deltaic system to slope (see, for example, Nakajima et al ., 2009) could be considered as the cause of changes in intensity of sediment input.…”

Submarine Crevasse Lobes Controlled by Lateral Slope Failure in Tectonically-Active Settings: An Exhumed Example From the Eocene Aínsa Depocentre (Spain)

“…Here, background sedimentation refers to hemipelagic settling, and thin dilute sediment gravity flow deposits not visible to the naked eye in outcrop (Boulesteix et al, 2019(Boulesteix et al, , 2020(Boulesteix et al, , 2022. These accumulations of thin beds are interpreted as the product of flowstripping of the dilute upper parts of channelised turbidity currents into the overbank areas (Piper and Normark, 1983;Peakall et al, 2000;Keevil et al, 2006;Kane et al, 2007;Kane and Hodgson, 2011;Hansen et al, 2017a).…”

Autogenic and allogenic controls on turbidite lobe stacking pattern and architecture: The case of the Turonian to Coniacian deep‐water turbidite lobe complexes (Dønna Terrace, offshore Mid‐Norway)

“…Facies Tu4 is composed of the final terms of the Bouma sequence [Te and, less commonly, Td (Bouma, 1962)]. However, recent microscopic studies revealed that bedded deep-water mudstones may have deposited within a slightly more energetic environment than previously considered (Boulesteix et al, 2019(Boulesteix et al, , 2020(Boulesteix et al, , 2022. Facies Tu4 is interpreted to be mainly deposited by suspension fallout of low-density turbidity flows.…”