Fringe or Background: Characterizing Deep-Water Mudstones beyond the Basin-Floor Fan Sandstone Pinchout

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) 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 prior to intense biogenic reworking. These observations challenge the idea that mud only accumulate from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets ( 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 within 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.