PhD: A field study of the End-Devonian mass extinction and source-rock formation in Nevada and the Canadian Rockies

Summary

This fieldwork-based project will study outcrops in Nevada and the Canadian Rockies and investigate the environmental changes at the time of a little known but major mass extinction that affected a broad range of groups, including trilobites, brachiopods and especially fish. The losses are associated with major sea-level and climate variations, including an intense glaciation and the spread of black shales across equatorial seas. Sedimentary and palaeontological studies conducted during fieldwork will be backed up with petrographic and ecosystem modelling studies in order to better understand this poorly understood ancient crisis.

The Hangenberg Event

The end-Devonian saw spectacular, high amplitude environmental changes that include the shut down of carbonate platforms in equatorial latitudes and the formation of extensive black shale horizons. Many of the latter are of immense economic importance in North America where units like the Woodford Shale, Pilot Shale and Exshaw Shaw are major sources of oil and shale gas (Hemmesch et al. 2018). The interval also saw short-lived, high amplitude sea-level variations associated with intense glaciation in the southern hemisphere. The effects on life were equally spectacular: a series of crises beginning with a mass extinction, named the Hangenberg Event after anoxic facies of this age developed in Europe. Whilst severe on the seabed the extinction losses in the water column were much worse and the Hangenberg crisis marks the greatest losses of fish in their long history, all major planktonic groups were also severely affected. Recovery was exceptionally slow perhaps due to the widespread development of hostile conditions that produced further source rocks in the earliest Tournasian (e.g. Alum Shales). It may also reflect the time required for the reconstruction of complex ecosystem structure following such a severe biotic crisis. In many sections the extinction losses are reported from within the black shale successions and not at the base of such units where they typically abruptly overlie platform carbonates (Kaiser et al. 2016; Wignall 2019). This raises questions about the causes of the extinction and their interrelationship with the nature and timing of source rock development, all within an overall context of major sea-level fluctuations.

The project will reconstruct the detailed depositional history of latest Devonian-Carboniferous sections in two regions of western North America that have spectacular natural outcrops: Nevada and Utah, in the Basin-and-Range of the US and the Rockies of Canada. The former region saw the Devil’s Gate Limestone onlapped by the black shales of the Pilot Shale whilst the latter region saw the cessation of limestone deposition (Palliser Formation) followed by the extensive development of the Exshaw Shale that straddles the Devonian-Carboniferous boundary (Morrow & Sandberg 2008; Caplan and Bustin 1999). Fieldwork will involve sedimentary logging and construction of correlation panels to produce a sequence stratigraphic model that places black shale development in context.

Older source rocks in the region formed during lowstand and early transgressive intervals when limited basin connectedness restricted circulation and created oxygen-poor conditions (Harris et al. 2018). In contrast, the latest Devonian source rocks are much more widespread and so may have developed at different points on the eustatic curve and be related to other controlling factors such as climate. Analysis of the black shales will include petrographic studies to determine their sedimentology and the assay of pyrite framboid populations: a powerful tool for determining past oxygen levels (Bond & Wignall 2010). The nature of carbonate platform shutdown will also be assessed by focussing detailed petrographic studies on the transition between the topmost limestone and basal-most black shales.

Fieldwork will also include the collection of fossil data (trilobites, brachiopods, conodonts) to determine the course of the mass extinction. There are numerous purported causes of the crisis, including cooling/regression and anoxia and water column nutrient changes. The combination of fossil and petrographic/sedimentological data will allow the competing causes to be tested. The student will also apply recently developed ecological food web modelling approaches to investigate the ecosystem complexity throughout this time interval in combination with diversity, extinction and origination rate analyses (e.g. Dunne et al. 2008).

In summary, the project will address the following questions:-

1) How did relative sea-level variations affect the shut-down of carbonate platform productivity and the subsequent development of extensive hydrocarbon source rocks?

2) How do the major sedimentary-environmental events of the latest Devonian relate to the Hangenberg mass extinction? For example, are the losses related to the loss of shallow-water carbonate habitats or to water column redox changes during the development of black shales?

Objectives

The project will have three main objectives to address the questions above.

1) Construct a sedimentary and sequence stratigraphic history of the Devonian-Carboniferous successions in the Great Basin of the western USA (sections in Nevada) and the Canadian Rockies (sections in western Alberta).

2) Evaluate depositional environments of the study sections using a combination of field observations and detailed petrographic study (including pyrite framboid size analysis by backscatter SEM analysis).

3) Compile a palaeontological record of the Hangenberg crisis using fossil data collected from the field and literature compilations to assess the timing of the crisis compared to contemporary environmental changes. Ecosystem modelling during and after the crisis will be applied to evaluate the importance of collapsing food webs, especially within the water column where the losses were at their most severe.

Potential for high profile outcome

The project investigates one of the most severe (and least studied) crises in Earth history, it ranks alongside the big five mass extinctions in most metrics and yet has received a tiny fraction of research interest compared to them. The project will also provide insight into the formation of some of the most economically important source rocks of North America. We therefore expect the project to result in several high-profile papers suitable for high-impact journals (e.g. Science, Nature Geoscience).

Training skills

The interdisciplinary nature of the research will ensure the student is equipped with a broad range of skills pertinent to employment in both industry and academia, including sedimentary facies analysis in combination with sequence stratigraphic analysis, petrographic skills and also computer modelling of large datasets using R.

Devonian-Carboniferous black shales (Exshaw Shale Formation) overlying platform carbonates at Goat Creek in the Canadian Rockies.

Bibliography

Bond, D.P.G. & Wignall, P.B. 2010. Pyrite framboid study of marine Permian-Triassic boundary sections: A complex anoxic event and its relationship to contemporaneous mass extinction. Bulletin Geol. Soc. America 122, 1265-1279.

Caplan, M.L. & Bustin, R.M. 1999. Devonian-Carboniferous Hangenberg mass extinction event, widespread organic-rich mudrock and anoxia: causes and consequences. Palaeogeog. Palaeoclim. Palaeoecol. 148, 187-207.

Dunne, J.A. et al. 2008. Compilation and network analysis of Cambrian food webs. PLOS Biology 6, e102.

Harris, N.B., McMillan, J.M., Knapp, L.J. & Mastalerz, M. 2018. Organic matter accumulation in the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, from sequence stratigraphic analysis and geochemical proxies. Sedimentary Geology 376, 185-203.

Hemmesch, N.T., Harris, N.B., Mnich, C. & Selby, D. 2018 A sequence stratigraphic framework for the Upper Devonian Woodford Shale, Permian Basin, west Texas. Bull. AAPG 98, 23-47.

Kaiser, S.I., Aretz, M. & Becker, R.T. 2016. The global Hangenberg Crisis (Devonian-Carboniferous transition): a review of a first-order mass extinction. In: Becker, R. T., Konigshof, P. & Brett, C. E. (eds). Devonian Climate, Sea Level and Evolutionary Events. Geological Society, London, Special Publications, 423, 387–437.

Morrow, J.R. & Sandberg, C.A. 2008. Evolution of Devonian carbonate shelf-margin, Nevada. Geosphere 4, 445-458.

Wignall, P.B. 2019. Extinction. Oxford University Press.