Facies and bed type characteristics of channel‐lobe transition deposits from the Oligocene‐Miocene Tajau Sandstone Member, Kudat Formation, Sabah, Malaysia

Abstract

Channel‐lobe transition zones (CLTZs) are identified in many modern deep‐water systems, but few exhumed examples have been identified. Exposures of the Oligocene‐Miocene Tajau Sandstone Member (TSM), Kudat Formation, northern Sabah, Malaysia, provides the opportunity to document a CLTZ from an active basin margin. This work provides the first detailed field‐based sedimentological logging to produce a quantitative database on facies, sedimentary structures, bed type, and statistical analysis. This is particularly important to produce a robust stratigraphic framework of the TSM. Sedimentary facies support interpretation of subaqueous sediment density flows, and key features, including scour‐fills, antidunes, and dune‐scale bedforms, suggest changes in gradient and/or flow confinement and the development of hydraulic jumps. Eight bed types are recognized including: (a) tripartite beds with a debrite (BT1), interpreted as hybrid event beds recording downslope flow transformation between turbulent and laminar states; (b) beds with a mixture of depositional, erosional, and bypass features (BT2, BT4, BT5), interpreted as recording the transitions between supercritical and subcritical flow conditions triggered by hydraulic jumps; (c) bipartite beds with a basal massive sandstone overlain by fine‐grained facies (BT3), interpreted as hyperconcentrated flow deposits with evidence of downcurrent flow transformation; (d) bipartite beds with a basal high‐density turbidite sharply overlain by a low‐density turbidite (BT6), interpreted as turbidites with evidence of sediment bypass; (e) basal tractive structures capped by fine‐grained facies as the product of reworking of very coarse‐ to coarse‐grained sediments caused by lateral spreading of turbulent flows; and (f) Bouma Tbcde sequences (BT8) interpreted as high‐to‐low‐density turbidites. Our depositional model for the TSM comprises: (a) an aggradational channel‐lobe transition zone (CLTZs/BTA 1 and BTA 2) which was dominated by hydraulic jumps and sediment bypass; and (b) stacked lobe (i.e., lobe‐axis/BTA 3a, lobe‐off axis/BTA 3b, frontal lobe fringe/BTA 3c, and distal lobe fringe/BTA 3d) located in the northern and southern parts of the study area, which is dominated by tabular, sheet‐like elements bioturbated with the Nereites ichnofacies. The aggradational stacking of CLTZs deposits observed in the TSM may be explained by high subsidence rates combined with high sediment supply rates associated with a tectonically active margin setting.

Submarine Channel Mouth Settings: Processes, Geomorphology, and Deposits

“…The preservation of a thicker stratigraphic record for CLTZs has been explained by high aggradation rates (e.g., Pemberton et al, 2016; Figure 10), tectonically-active settings (Ito, 2008;Mansor and Amir Hassan, 2021;Brooks et al, 2022), rapid abandonment or avulsion of feeder channels before erosion into the CLTZ (e.g., Hofstra et al, 2015;Brooks et al, 2018), or a large-scale passive margin setting allowing more net aggradation (Navarro and Arnott, 2020). Nonetheless, these studies also interpret lobes and channel-fills as part of the stratigraphic succession, which points to an aggrading and interfingering succession where the CLTZ is relatively fixed, and preserved as surfaces and thinner stratigraphic units as part of a thicker succession.…”

“…Detailed outcrop research on CLTZs has primarily focused on large, relatively tectonically-quiescent basins associated with mature passive margins or thermal sag basins, and influenced by glacial-interglacial cycles, such as the Karoo Basin in South Africa (e.g., Van der Merwe et al, 2014;Hofstra et al, 2015;Brooks et al, 2018). Therefore, it is important to understand if these same models can be applied to tectonically-active basins (e.g., Mansor and Amir Hassan, 2021;Brooks et al, 2022), such as forearc or retroarc foreland basins, formed in different climatic conditions in both modern environments and in the rock record. Indeed, tectonically-active basin-fills are likely to host CMEZs given the steeper slopes.…”

“…There is a growing literature on CLTZs interpreted from ancient outcrops (see compilation by Navarro and Arnott, 2020), with recognition criteria proposed to support links between sedimentary processes and deposits (e.g., Bravo and Robles, 1995;Ito, 2008;Pyles et al, 2014;Hofstra et al, 2015;Pemberton et al, 2016;Postma et al, 2016;Brooks et al, 2018;Hofstra et al, 2018;Postma et al, 2021). Preserved stratigraphic successions of interpreted CLTZs range from thick successions of aggradational beds in close association with scour-fill features (e.g., Pemberton et al, 2016;Brooks et al, 2018;Mansor and Amir Hassan, 2021;Brooks et al, 2022) to single surfaces that separate lobes from overlying channel-levee systems (e.g., Gardner et al, 2003;Hodgson et al, 2016). The wide range of expressions and dimensions (Navarro and Arnott, 2020) point to a number of parameters and configurations that control the transfer, and preservation, of channel mouth settings into the rock record.…”

Channel‐lobe transition zone development in tectonically active settings: Implications for hybrid bed development

“…Hybrid beds develop through flow transformation following significant entrainment of a muddy substrate and/or declining turbulent energy (Amy & Talling, 2006; Haughton et al, 2003, 2009; Hodgson, 2009; Lucchi & Valmori, 1980; Muzzi Magalhaes & Tinterri, 2010; Southern et al, 2015), and are commonly observed within the distal fringes of lobe systems (Fonnesu et al, 2018; Haughton et al, 2003; Hodgson, 2009; Kane & Pontén, 2012; Kane et al, 2017). Recently, studies have shown that hybrid beds can also develop in base‐of‐slope settings (Baas et al, 2021; Brooks et al, 2018a; Fonnesu et al, 2018; Ito, 2008; Mansor & Amir Hassan, 2021), and are generally associated with frontal lobes (Mueller et al, 2017; Spychala et al, 2017a, 2017b, 2021). The stratigraphic distribution of hybrid beds has been linked to the character of the supply slope and seafloor relief, where hybrid beds are invoked to develop during periods of disequilibrium in out‐of‐grade slopes (Haughton et al, 2003, 2009; Hodgson, 2009; Pierce et al, 2018; Spychala et al, 2017a, 2017b).…”

“…Nevertheless, outcrop studies (Brooks et al, 2018a(Brooks et al, , 2018bHofstra et al, 2015;Ito, 2008;Pemberton et al, 2016;Pyles et al, 2014; Van der Merwe et al, 2014) have helped develop recognition criteria for CLTZs, which include: a thin stratigraphic expression; amalgamated erosional features; coarse-grained/mudclast lag deposits; aggradational bedforms (i.e. sediment waves); soft-sediment deformation; interfingering of up-dip and down-dip deposits; and sand-rich hybrid beds in proximal lobes (Brooks et al, 2018a;Mansor & Amir Hassan, 2021;Stevenson et al, 2015). Detailed outcrop research on CLTZs has often focussed on large, relatively tectonically quiescent basins associated with mature passive margins, or thermal sag basins, such as the Karoo Basin in South Africa (Brooks et al, 2018a;Hofstra et al, 2015;Van der Merwe et al, 2014).…”