Understanding geo-hazards associated with catastrophic deep-sea debris flows: a new approach using sole structures

Understanding geo-hazards associated with catastrophic deep-sea debris flows: a new approach using sole structures

 

Supervisors

Prof. Jeff Peakall (j.peakall@leeds.ac.uk), http://www.see.leeds.ac.uk/people/j.peakall

Prof. Dave Hodgson, http://www.see.leeds.ac.uk/people/d.hodgson

Prof. Jim Best (University of Urbana-Champaign, Illinois), https://www.geology.illinois.edu/people/jimbest/

Dr Jaco Baas (Bangor University), https://www.bangor.ac.uk/oceansciences/staff/jaco-baas

 

Highlights

  • An opportunity to understand how submarine landslides and debris flows (that collectively form mass transport deposits) move across the seafloor
  • Use sole structures to understand the nature of flow at the very base of these flows
  • Examine the evidence for a ‘lubricating’ layer, and the nature and prevalence of this
  • Novel experiments in the national environmental fluid dynamics laboratory, Leeds
  • Vibrant research community in the sedimentology / Earth surface processes group
  • Internationally leading supervisorial team
  • Fieldwork in the UK, with potential for work in Eastern Canada
  • Potential for visits to the University of Urbana-Champaign, Illinois, for experiments and training
View of sole structures
A range of sole structures, flutes, grooves, and more enigmatic features, revealing spatial and temporal changes in formative flow behaviour. Imaged from the Cloridorme Formation, Gaspé Peninsula, Quebec, Canada.

 

Understanding geohazards in submarine systems is challenging because we can only measure the most dilute of flows, and even then we have no data close to the flow-seafloor interface. In particular, the dynamics of the most powerful flows, such as debris flows, slumps and slides, are largely known from very small scale subaqueous experiments, and larger, but not necessarily applicable, subaerial experiments. Here, we identify a new approach that will help us understand the dynamics of natural high-concentration sediment gravity flows. Sole structures provide a record of the processes at the base of the flow, and understanding their formation, development and preservation provides us with an opportunity to look at bed processes, and identify the type and dynamics of the flows that formed them. This project will use these features to investigate whether a ‘lubricating’ layer exists at the base of these flows, and if so examine the nature and prevalence of this layer. The spatial distribution of sole structure also opens up new research potential in investigations of flow-substrate interactions, the recognition of different flow types and their transport processes in a range of natural flows.

 

Sole structures (flutes, tool marks, and a wide range of currently enigmatic features) are ubiquitous in deep-water clastic sediments, but at present geologists principally utilise them only as palaeocurrent indicators. This limited use is in sharp contrast with almost all other sedimentary structures, from which we interpret flow properties and thus help reconstruct ancient sedimentary environments. Indeed, it is astonishing that geologists have made such poor use of sole structures, given that other sedimentary structures are typically rare in deep-water sedimentary sequences, thus making environmental interpretations difficult, and limiting our ability to predict the character and spatial distribution of these sediments. This lack of progress in understanding sole structures reflects two key factors: a belief that they were all formed by low-concentration turbidity currents, and a near absence of research in this area since the early 1970s. However, since this time, our knowledge of deep-water sediment gravity flows (SGFs) has increased enormously, and we now recognise that there are a wide range of flows in these systems, from low-concentration, turbulent turbidity currents, through to high-concentration subaqueous debris flows, with a range of mud-rich transitional flows (see Baas et al., 2009, 2016) in between these end members. Furthermore, many SGFs change flow type spatially (down and across flow), and temporally, producing more complex, but common, deposits such as hybrid event beds (Haughton et al., 2009).

 

Recent work by the supervisors has demonstrated that the underlying assumption of the past ~65 years that all these sole structures are formed by classical low-concentration turbidity currents is fundamentally flawed (Peakall et al., 2020). Furthermore, we have been able to show that different sole structures can be linked to different SGFs, enabling us to greatly enrich the palaeohydraulic utilisation of sole structures, and to use these features to refine environmental interpretation and improve prediction of deep-water systems. A key implication of this recent work is that the classical Bouma sequence is wrong in its interpretation of many sole structures, and additionally that many of the more advanced models of hybrid event beds require modification. Whilst we have demonstrated the greatly increased utility of sole structures, this has opened up an entirely new field that is ripe for further study, offering huge opportunities to explore the formative processes and utility of sole structures. In particular, the near perfect preservation of these often delicate structures suggests that after their initial cutting, no further erosion took place. This observation suggests that a ‘lubricating’ layer was present at the base of the flows, and that sole structures can help us to elucidate this, with wide implications for the assessment of geohazards.

 

Aims and Objectives

The principal aim of the proposed PhD project is to improve understanding of the geohazards associated with deep-sea high-concentration flows, utilising a new tool in the form of sole structures. The basal flow conditions of mass transport flows are inaccessible in real-world settings, and thus are largely unknown, yet these are key to how debris flows, slides and slumps move across the seafloor. To advance our understanding an integrated laboratory and field approach is needed. The objectives include:

  • To understand how a range of sole structures initiate and evolve, and the nature of flow dynamics across these bedforms. Laboratory experiments using modern measurement technologies will be utilised to study these processes. A wide range of different types of experiment can be used dependent on the sole structures of interest.
  • To examine the nature of the lubricating layer via experiments
  • To understand the diversity of sole structures, their detailed morphology, and their spatial variability, in a range of deep-water settings. This will be achieved through fieldwork (UK based but with potential for work in Eastern Canada), recording sole structure type, using photogrammetric and drone-based techniques to collect morphometric and spatial information on sole mark distribution, and sedimentary logging to record bed types, grain size / sorting, and enable environmental interpretations.
  • To use the spatial and temporal relationships between different sole structures, to make big advances in understanding the processes operating in many subaqueous SGFs. In particular to examine the mechanics of debris flows, and the implications of this for geohazards.
  • To combine these approaches to develop new process models for the formation and environmental distribution of different sole structures.

 

PhD Schedule, Outputs and Training

This PhD will commence before the end of 2021 and run for 3.5 years. During this period, the student will be eligible for all the postgraduate training typically provided to students attending the University as part of the PANORAMA DTP. The student will receive training in relevant software packages, field based description, experimental techniques and data analysis, technical/scientific writing, and presentation of research to both scientific and public audiences. The student will be based in the Department of Earth and Environment at the University of Leeds, with potential for visits to the University of Illinois, Urbana-Champaign, USA, to both learn new approaches to experimental sedimentology and undertake selected classes to broaden their geological research background. The student will join the sedimentology / Earth surface processes group at Leeds that is one of the largest and most vibrant in the UK, with a very active group of doctoral and postdoctoral researchers.

 

 

References

Baas, J.H., Best, J.L., Peakall, J. and Wang, M. (2009) A phase diagram for turbulent, transitional, and laminar clay suspension flows. Journal of Sedimentary Research, 79, 162-183.

 

Baas, J.H., Best, J.L. and Peakall, J. (2016) Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand. Journal of the Geological Society, 173, 12-45.

 

Haughton, P., Davis, C., McCaffrey, W. and Barker, S. (2009) Hybrid sediment gravity flow deposits classification, origin and significance. Marine and Petroleum Geology, 26, 1900-1918.

 

Peakall, J., Best, J.L., Baas, J., Hodgson, D.M., Clare, M.A., Talling, P.J., Dorrell, R.M. and Lee, D.R. (2020) An integrated process-based model of flutes and tool marks in deep-water environments: implications for palaeohydraulics, the Bouma sequence, and hybrid event beds. Sedimentology, 67, 1601-1666.