Mixed sand–mud bedforms produced by transient turbulent flows in the fringe of submarine fans: Indicators of flow transformation

Abstract

The fringe of fine‐grained deep‐marine systems often exhibits complex sedimentary facies and facies associations, because the presence of clay promotes the development of transient turbulent flows with complex depositional properties. Relatively little is known about the variation of current‐induced sedimentary structures found within these facies. This study provides the first comprehensive description and interpretation of mixed sandstone–mudstone bedforms observed in the fringe of the mud‐rich submarine fan that makes up the Aberystwyth Grits Group and Borth Mudstone Formation (Wales, UK). Using textural and structural descriptions, 158 bedforms in sediment gravity flow deposits were characterized into three main types: ‘classic’ sandy current ripples, large current ripples and low‐amplitude bed‐waves. The sandy current ripples comprise clean sandstone, with average heights and lengths of 11 mm and 141 mm, respectively. The large current ripples are composed of mixed sandstone–mudstone and possess greater dimensions than the sandy current ripples, with an average height of 19 mm and an average length of 274 mm. The low‐amplitude bed‐waves are long thin bedforms composed commonly of mixed sandstone–mudstone, with an average height and length of 10 mm and 354 mm, respectively. The large current ripples and low‐amplitude bed‐waves are strikingly similar to experimental bedforms produced under decelerating mixed sand–mud flows and are interpreted to form beneath transitional flows with enhanced and attenuated near‐bed turbulence, respectively. From the fringe to the distal fringe of the fan, the dominant bedform type changed from sandy current ripples, via large current ripples, to low‐amplitude bed‐waves, suggesting that the flows changed from turbulent to increasingly turbulence‐modulated. It is proposed that the flow Reynolds number reduced, reflecting this flow transformation, from a combination of constant or decreasing flow height, flow deceleration from sediment deposition, and increasing flow viscosity due to the shear‐thinning nature of clay‐rich suspensions. Large current ripples and low‐amplitude bed‐waves are likely to be common in the fringe of other submarine fans. The presence and spatial trends in mixed sand–mud bedform types may be an important tool in interpreting fan fringe environments.

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

“…Similar apparently homogeneous mudstones from other basin-floor successions are also commonly interpreted as hemipelagites (e.g. Hesse 1975;Mutti 1977;Stow and Piper 1984;Baker and Baas 2020). However, basin-floor background deposits are usually highly bioturbated in distal (yet well-oxygenated) basinfloor environments, due to relatively low depositional energy, leading to destruction of the primary sedimentary fabric (e.g.…”

“…Normally graded mudstones are commonly reported as deposits of distal basin-floor fan successions (e.g. Hesse 1975;Pickering 1981;Etienne et al 2012;Grundvåg et al 2014;Baker and Baas 2020;Pickering et al 2020). These deposits may represent the distal expression of sediment gravity flows that deposited their sand fraction in more proximal slope or basin-floor environments.…”

“…These deposits may result from flow transformation from turbulent to laminar along the flow path (e.g. Kane et al 2017;Baker and Baas 2020)…”

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

“…The relatively high proportion of debrites on the basin-floor mudstones of the deep-water Laingsburg succession is consistent with a distal position, more prone to flow transformations from turbulent to laminar (e.g., Haughton et al, 2003;Talling et al, 2004;Hodgson, 2009;Kane et al, 2017;Baker and Baas, 2020). The high mud content on the seafloor during the interpreted relative sea-level highstand periods in the Karoo Basin (Flint et al, 2011) and its incorporation in sediment gravity-flows may have favoured flow transformations by enhancing the damping of flow turbulence (Baas and Best, 2002;Baas et al, 2011).…”

“…Some of the mudstone clasts are relatively well rounded ( Figure 8A, B), which suggest progressive disintegration and abrasion in sediment gravity flows with a component of turbulence (e.g., Haughton et al, 2003;Fonnesu et al, 2015;Boulesteix et al, 2019). Therefore, we propose that some of the massive mudstones (F3) may represent the distal expression of flows that underwent transformation from turbulent to laminar along their flow path (e.g., Baas et al, 2011;Kane et al, 2017;Baker and Baas, 2020). The intense to complete bioturbation of F3 suggests deposition under lower sedimentation rate compared to F1 and F2, associated with a relatively low frequency of flow events (e.g., Wetzel, 1984;Heard and Pickering, 2008;Gingras et al, 2011).…”