Fringe or background: Characterizing deep-water mudstones beyond the basin-floor fan sandstone pinchout

Abstract

Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages (< 0.7 m thick) of background mudstones. Genetically related bedsets stack to form bedset packages, which are bounded by thicker (> 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets (< 0.7 m thick) are interpreted to mainly represent autogenically driven lobe abandonment due to up-dip channel avulsion. The thicker background mudstones separating bedset packages (> 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change.

Flow‐process controls on grain type distribution in an experimental turbidity current deposit: Implications for detrital signal preservation and microplastic distribution in submarine fans

“…Three probable explanations are proposed: First, microplastics were held in suspension and bypassed the sand‐prone parts of the BFD to the siltstone‐prone fringe (Figure 15) (sensu Spychala et al 2020). Spychala et al (2020) document a siltstone‐prone fringe which extends at least 270% further than the sand‐prone part of the deposit in similar experimental sediment gravity flow deposits; Boulesteix et al (2020) show siltstone‐prone parts of lobes extending at least 18.5 km beyond the sand pinchout; and Stow et al (1990) recognised finer‐grained turbidites in the Bengal Fan which had travelled at least 2,500 km. If microplastics do bypass the sand‐prone parts of lobes en‐masse these examples suggest they are probably deposited over vast areas of the sea floor adjacent to continental margins, where they may also be reworked and re‐concentrated by bottom currents (Kane et al, 2020).…”

Interpretation and Reconstruction of Depositional Environment and Petroleum Source Rock Using Outcrop Gamma-ray Log Spectrometry From the Huai Hin Lat Formation, Thailand

“…The depositional environment of the measured section is interpreted as an overall lacustrine environment with two subenvironments: deep lake basin containing mudstonedominated facies (FA1), and sublacustrine fan comprising sandstone-dominated facies (FA2). The deposition of mudstone and siltstone of the FA1 suggests low energy environment deposited by suspension fall-out or low-density/dilute turbidity currents (Bouma, 1962;Middleton, 1993;Boulesteix et al, 2020), representing relatively deep lacustrine environment or in the lacustrine basin floor. The presence of plant fragments including leave and wood debris indicates non-marine terrestrial sources.…”

“…Sandstones mostly consist of medium to fine grains with little to no coarse-grained and conglomerate sediments. The majority of sandstone bodies both structured and structureless facies can be interpreted to represent the fan lobes (Prélat et al, 2009;Dodd et al, 2019), possibly in medial fan, while the interbedded sandstone and siltstone facies suggests relatively distal fan fringe of the sublacustrine fan (Dodd et al, 2019;Boulesteix et al, 2020).…”

“…The pattern of the total gamma-ray log shows cylindrical or box shape with only small fluctuations and often Deposited by high-density turbidity current with high sediment load fallout (Lowe, 1982;Miall, 1996), Ta turbidite (Bouma, 1962 Deposited by turbidity currents ranged from low-to high-density and concentration (Bouma, 1962;Lowe, 1982 Deposited from suspension fall-out and low-density or dilute turbidity currents (Bouma, 1962;Middleton, 1993). Deposited in low energy condition from suspension and settling of mud, low density or dilute turbidity currents (Bouma, 1962;Middleton, 1993;Boulesteix et al, 2020). Low energy setting deposited from suspension fall-out (Bouma, 1962) with more calcareous components.…”

Substrate Entrainment, Depositional Relief, and Sediment Capture: Impact of a Submarine Landslide on Flow Process and Sediment Supply

“…The laterally extensive character, mudstone dominance, and overall eastward (downdip) fining and thinning trend of thin-bedded sandstones of Unit 1 suggest deposition from low-energy sediment gravity flows in distal areas (e.g. Mutti, 1977), with possible distal lobe fringe deposits (Boulesteix et al, 2020). The discrete intervals of deformed bedding found in the upper parts of Unit 1 represent a post-depositional sheared zone linked to the overlying Unit 2.…”