TY - JOUR

T1 - Integrating transitional-flow signatures into hybrid event beds: Implications for hybrid flow evolution on a submarine lobe fringe

AU - Łapcik, Piotr

AU - Baas, Jaco

PY - 2024/9/4

Y1 - 2024/9/4

N2 - Alongside turbidites and debrites, hybrid event beds are now recognized as a common occurrence in deep-marine environments. Yet, many variations in the standard H1–H5 facies model of Haughton et al. (2009, Marine and Petroleum Geology, v. 26, p. 1900–1918) have been described since its introduction, with the role of transient-turbulent flows, i.e., flows that are transitional between fully turbulent turbidity currents and fully laminar debris flows, being particularly enigmatic. Based on a comprehensive dataset collected from the lobe fringe and distal fringe of a submarine fan (Silurian Aberystwyth Grits Group and Borth Mudstone Formation, West Wales, United Kingdom), transitional-flow signatures were integrated into the standard hybrid-event-bed model. These signatures include muddy sandstones and sandy mudstones with large ripples (formed by turbulence-enhanced transitional flows), low-amplitude bed waves and heterolithic lamination (formed by turbulence-attenuated transitional flows), and banding (formed by turbulence-enhanced to turbulence-attenuated transitional flows). The field data reveal that: (a) H1 divisions are generated by turbulent flows that form not only massive, structureless facies but also plane-parallel-laminated and ripple-cross-laminated facies; (b) H2 divisions are formed by transitional flows that form banded facies, but also facies with large ripples and low-amplitude bed waves, as well as heterolithic facies; (c) H3 divisions are formed by laminar debris flows of varied rheology; (d) H4 divisions can form from both tractional turbulent and transitional flows; and (e) H5 divisions can be hemipelagic, deposited from the dilute tail of the flow or originate from cohesive freezing of a late-stage muddy debris flow. Based on embedded Markov-chain analysis, the vertical stacking of facies in the five principal hybrid-event-bed divisions suggests a transformation from turbidity current via transitional flow to debris flow (H1 to H3), followed by a repetition of this transformation in the H4 and H5 divisions, but in overall finer-grained sediment. In addition to this complete extended facies model for hybrid event beds, three incomplete bed types could be defined: turbulent-flow-prone, transitional-flow-prone with an H3 division, and transitional-flow-prone without an H3 division. The sedimentary successions in the study area reveal a basinward change from predominantly turbidites and turbulent-flow-prone hybrid event beds via a mixture of turbulent-flow and transitional-flow signatures in hybrid events beds to H3 missing hybrid event beds with transitional-flow and muddy-debrite signatures. Hence, sediment gravity flows became increasingly muddy and cohesive from lobe fringe to lobe distal fringe.

AB - Alongside turbidites and debrites, hybrid event beds are now recognized as a common occurrence in deep-marine environments. Yet, many variations in the standard H1–H5 facies model of Haughton et al. (2009, Marine and Petroleum Geology, v. 26, p. 1900–1918) have been described since its introduction, with the role of transient-turbulent flows, i.e., flows that are transitional between fully turbulent turbidity currents and fully laminar debris flows, being particularly enigmatic. Based on a comprehensive dataset collected from the lobe fringe and distal fringe of a submarine fan (Silurian Aberystwyth Grits Group and Borth Mudstone Formation, West Wales, United Kingdom), transitional-flow signatures were integrated into the standard hybrid-event-bed model. These signatures include muddy sandstones and sandy mudstones with large ripples (formed by turbulence-enhanced transitional flows), low-amplitude bed waves and heterolithic lamination (formed by turbulence-attenuated transitional flows), and banding (formed by turbulence-enhanced to turbulence-attenuated transitional flows). The field data reveal that: (a) H1 divisions are generated by turbulent flows that form not only massive, structureless facies but also plane-parallel-laminated and ripple-cross-laminated facies; (b) H2 divisions are formed by transitional flows that form banded facies, but also facies with large ripples and low-amplitude bed waves, as well as heterolithic facies; (c) H3 divisions are formed by laminar debris flows of varied rheology; (d) H4 divisions can form from both tractional turbulent and transitional flows; and (e) H5 divisions can be hemipelagic, deposited from the dilute tail of the flow or originate from cohesive freezing of a late-stage muddy debris flow. Based on embedded Markov-chain analysis, the vertical stacking of facies in the five principal hybrid-event-bed divisions suggests a transformation from turbidity current via transitional flow to debris flow (H1 to H3), followed by a repetition of this transformation in the H4 and H5 divisions, but in overall finer-grained sediment. In addition to this complete extended facies model for hybrid event beds, three incomplete bed types could be defined: turbulent-flow-prone, transitional-flow-prone with an H3 division, and transitional-flow-prone without an H3 division. The sedimentary successions in the study area reveal a basinward change from predominantly turbidites and turbulent-flow-prone hybrid event beds via a mixture of turbulent-flow and transitional-flow signatures in hybrid events beds to H3 missing hybrid event beds with transitional-flow and muddy-debrite signatures. Hence, sediment gravity flows became increasingly muddy and cohesive from lobe fringe to lobe distal fringe.

U2 - 10.2110/jsr.2024.023E

DO - 10.2110/jsr.2024.023E

M3 - Article

JO - Journal of Sedimentary Research

JF - Journal of Sedimentary Research

SN - 1527-1404

ER -