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Southampton Marine and Maritime Institute

Earth Surface Dynamics

Published: 1 March 2011Origin: Geography and Environmental Science

Origins of hummocky cross-stratification: a field and flume study

Supervisor: Prof. Paul Carling

Funding: Statoil and School of Geography

The issue
Recent research suggests that short-lived flow instability events may be an important agent of shelf sediment transport. If this is correct, scientific understanding of how sand is transported and distributed within shelf settings and how it is transported towards deep water environments could change considerably- with clear implications for the identification and successful exploitation of hydrocarbons. It is the aim of this project to rigorously test the instability hypothesis using hydraulic flumes together with field analysis of proposed instability deposits (Hummocky cross stratification).

Geography will split the fees and maintenance costs of the studentship 50:50 with Statoil but the latter will be requested to fund flights and maintenance costs associated with visiting the hydraulic flume facility, undertaking field analysis and undertaking an internship with Statoil. The school of Geography will secure use of the Hydralab hydraulic flume (unsubsidised cost c. x NOK).

Proposal
Hummocky cross stratification (HCS) is a common but poorly understood structure which is most commonly found in ancient shelf facies. Although it has long been assumed to represent the deposit of large-scale ripples, recent research suggests that it is actually easier to explain it as resulting from short-lived instability events (Quin, 2011). If this is the case, there are potentially important implications for understanding of marine sediment transport. The hitch with an instability hypothesis, however, is that the nature of the proposed instabilities is currently entirely speculative and for this reason this project will attempt to shed light on the issue by investigating whether these instabilities exist and whether they are capable of generating HCS. These questions will be addressed by testing a series of flow combinations in flume experiments including interactions between oscillatory, density and shore-parallel flows. The study will also investigate the development of resonance between oscillatory motion generated by surface gravity waves and the natural turbulence associated with shear in density and along-shelf currents. In parallel with the flume experiments, field outcrop analysis will be undertaken on classic HCS outcrop (e.g. Utah) to confirm and quantify reported bed organisation patterns. Thus the proposal should define the exact environmental conditions associated with the development of hummocky strata.

If it can be proven that instability events do have the potential to generate HCS, for example by interaction between oscillatory and density currents (such as hyperpycnal flows), this finding would have major implications for understanding of sediment transport in shelf settings and beyond. This information would ultimately be used by oil companies as input in attempts to numerically model shelf and palaeo-delta development, to make estimation of sand extent offshore from known palaeo-shorelines, or as a tool in regional exploration. There are potentially also intriguing implications for the generation of turbidite successions which are the focus of much current frontier hydrocarbon exploration. For example, would an instability mechanism facilitate transport of sand across shelves and into deep water regardless of timing within the eustatic cycle and unrelated to the extent to which deltas have prograded towards shelf margins? If so, what implications might this have for the distribution of deep water sand and how might this be linked to factors such as palaeoclimate and palaeogeography which influence the presence and distribution of rivers which are likely to generate density flows (i.e. hyperpynal flows)? This information could be used to drive source-to-sink exploration concepts in frontier deep water and/or sub-salt exploration. Such
knowledge could also change the way some deep-water turbidite successions are interpreted (i.e. shifting the emphasis from surge to continuous drive) and thus potentially influence the input mathematics of the flow models used to generate sand distribution models in field development. In addition to these geological issues, if the turbulence associated with density flows does resonate with motion associated with surface gravity waves (one of the issues which will be investigated) there may be implications for marine engineering and safety (oil platforms, offshore windfarms etc).

Training
Detailed physical understanding of flow dynamics coupled with experience of seeing and interpreting sedimentary deposits (i.e. from core, outcrop and seismic) is key to identifying future hydrocarbon potential particularly in frontier deep water exploration (sub-salt etc). The student who undertakes the project will therefore gain a unique skills-set beneficial to oil company exploration, field development or research. The project also offers key learning for Statoil employees associated with the project. Given the importance of grounding the project within other disciplines, it is suggested that the graduate student should attend Statoil courses examining classic turbidite successions (for comparison). In addition, it would be attractive to organise an internship within a deep water asset or research group at Statoil to ensure that the graduate student is familiar with the challenges faced by oil company exploration and also to ensure that the results of the project are properly linked to the needs of Statoil.

Reference
Quin, J.G. (2011) Is hummocky cross stratification formed by large-scale ripples? Sedimentology (in press).

Candidates must have or expect to gain a first or strong upper second class degree, in an appropriate discipline, not necessarily Geography. Details on how to apply are available from Julie Drewitt, Graduate School Administrator, School of Geography University of Southampton, SO17 1BJ, Telephone 023 8059 2216, email J.A.Drewitt@soton.ac.uk . Informal enquiries may be made to Prof Paul Carling (email p.a.carling@soton.ac.uk ).

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