Variable character and diverse origin of hybrid event beds in a sandy submarine fan system, Pennsylvanian Ross Sandstone Formation, western Ireland

Abstract

Hybrid event beds comprising both clean and mud‐rich sandstone are important components of many deep‐water systems and reflect the passage of turbulent sediment gravity flows with zones of clay‐damped or suppressed turbulence. ‘Behind‐outcrop’ cores from the Pennsylvanian deep‐water Ross Sandstone Formation reveal hybrid event beds with a wide range of expression in terms of relative abundance, character and inferred origin. Muddy hybrid event beds first appear in the underlying Clare Shale Formation where they are interpreted as the distal run‐out of the wakes to flows which deposited most of their sand up‐dip before transforming to fluid mud. These are overlain by unusually thick (up to 4·4 m), coarse sandy hybrid event beds (89% of the lowermost Ross Formation by thickness) that record deposition from outsized flows in which transformations were driven by both substrate entrainment in the body of the flow and clay fractionation in the wake. A switch to dominantly fine‐grained sand was accompanied initially by the arrest of turbulence‐damped, mud‐rich flows with evidence for transitional flow conditions and thick fluid mud caps. The mid and upper Ross Formation contain metre‐scale bed sets of hybrid event beds (21 to 14%, respectively) in (i) upward‐sandying bed set associations immediately beneath amalgamated sheet or channel elements; (ii) stacked thick‐bedded and thin‐bedded hybrid event bed‐dominated bed sets; (iii) associations of hybrid event bed‐dominated bed sets alternating with conventional turbidites; and (iv) rare outsized hybrid event beds. Hybrid event bed dominance in the lower Ross Formation may reflect significant initial disequilibrium, a bias towards large‐volume flows in distal sectors of the basin, extensive mud‐draped slopes and greater drop heights promoting erosion. Higher in the formation, hybrid event beds record local perturbations related to channel switching, lobe relocations and extension of channels across the fan surface. The Ross Sandstone Formation confirms that hybrid event beds can form in a variety of ways, even in the same system, and that different flow transformation mechanisms may operate even during the passage of a single flow.

Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

“…Hybrid event beds (Haughton et al, 2003(Haughton et al, , 2009) are most commonly identified toward the bases and fringes of lobe deposits (e.g., Hodgson, 2009;Talling, 2013). However, they can form in any environment where mud and mudstone clasts are entrained into the turbulent flow, damping turbulence, and developing high-concentration to pseudo-laminar flow conditions (e.g., Ito, 2008;Haughton et al, 2003;Talling et al, 2004;Baas et al, 2011;Pierce et al, 2018).…”

Early burial mud diapirism and its impact on stratigraphic architecture in the Carboniferous of the Shannon Basin, County Clare, Ireland

“…The lower portion of the succession, the Shannon Group ( Fig. 1B; Clare Shale, Ross Sandstone and Gull Island formations), represents deep-marine and slope depositional environments (Hodson & Lewarne, 1961;Rider, 1974;Gill, 1979;Chapin et al, 1994;Wignall & Best, 2000Lien et al, 2003;Martinsen et al, 2003Martinsen et al, , 2017Pyles & Jennette, 2009;Pyles et al, 2011Pyles et al, , 2014Pierce et al, 2018). The Pennsylvanian (Namurian) Central Clare Group overlies these deposits ( Fig.…”

Turbidite, debrite, and hybrid event beds in submarine lobe deposits of the Palaeocene to middle Eocene Kapit and Pelagus members, Belaga Formation, Sarawak, Malaysia

“…The shallow channelized turbidite beds are interpreted as representing the down‐dip termination of distributary channels (Johnson et al, ; Terlaky, Rocheleau, & Arnott, ). There is increasing evidence that gravity flow transformations, from turbidity flow to cohesive and laminar flow, are the dominant bed type in the distal ends of submarine fan systems and the fringes of individual lobes (e.g., Barker, Haughton, McCaffrey, Archer, & Hakes, ; Fonnesu et al, ; Grundvåg et al, ; Haughton et al, , ; Hodgson, ; Ito, ; Kane & Pontén, ; Lowe & Guy, ; Pierce et al, ; Spychala, Hodgson, & Lee, ; Sylvester & Lowe, ; Talling et al, ). However, they are not exclusively restricted to the distal parts and can also be present in proximal areas of lobes, where they are interpreted as the product of enhanced erosion and flow deceleration due to processes occurring in the channel‐lobe transition and due to basin confinement (e.g., Fonnesu et al, ; Haughton et al, ; Ito, ; Jackson et al, ; Mueller et al, ; Patacci et al, ; Sylvester & Lowe, ; Terlaky et al, ; Vinnels, Butler, McCaffrey, & Lickorish, ).…”

“…The predominance of the mudstone bed type (BT1), the mainly dm-scale thickness of individual sandstone beds and the absence of bed amalgamation, indicates a more basinward or lateral position relative to BTA 2. At the individual lobe scale, hybrid event beds are common elements of the lobe fringe area (Fonnesu et al, 2018;Grundvåg et al, 2014;Haughton et al, 2003Haughton et al, , 2009Hodgson, 2009;Ito, 2008;Kane & Pontén, 2012;Pierce et al, 2017;Spychala, Hodgson, & Lee, 2017;Talling et al, 2004). Bed thinning upward patterns may indicate either lobe/fan retreat (i.e., retrogradation), across-strike lobe/fan shifting (i.e., avulsion), or a combination of both (Prélat & Hodgson, 2013;Spychala, Hodgson, & Lee, 2017).…”