The deposition of turbidite sediments in an area of active salt/shale tectonics involves a complex, two-way interaction since loading by sediments induces ductile flow in the substrate but this, in turn, alters the topography controlling the turbidity currents. This study developed a 3d numerical model to investigate this interaction. The resultant program was used to test a possible model for the observed topography and turbidite sand distribution in an area West of Shetland.
Turbidity currents, and their deposits, were modelled using the depth-averaged equations described in Waltham et al (2008). Flow of the ductile substrate was modelled using the thin-film equations described in Waltham (1996) except that the overburden was assumed to have a Coulomb, rather than elastic, rheology in which pressure from a surface load is distributed, at the base of the overburden, by a Gaussian-like point-spread function as demonstrated experimentally in (Bräuer et. al. 2006). Successful modelling of the ductile-tectonics/sedimentation interaction requires simulation of hundreds of successive turbidity currents and a great deal of project time was taken up with improving the accuracy and robustness of the modelling algorithms to allow this to happen without any numerical stability issues.
West of Shetland Constraints
Constraints were based upon seismic interpretations and wire-line log interpretations provided by DONG together with palaeo-bathymetric reconstructions provided by MVE.
The broad features that a numerical model needed to reproduce were:
- Linear depocentres a few km wide which are sub-parallel to the basin margin and separated by ~5km.
- Hundreds of metres of sand in depocentres but only a few tens of metres of sediment between them.
- Turbidity current supply from a channel on the eastern margin which is ~2km wide and ~100m deep.
- A geometry at the base of the deposits which has a 500m scarp on the eastern margin, bathymetry which deepens westwards by a kilometre over a distance of ~30 km and ridges between depocentres which are up to 500m high.
- Significant sand distribution across ~1000 km2.
Preliminary Modelling from MVE
Before undertaking complex, syn-sedimentary modelling of ductile shale tectonics, it is important to establish that the observed sediment distribution cannot be reproduced in the absence of simultaneous seafloor deformation. This work was undertaken at MVE and the results indicated that the sediment distribution could only be satisfactorily reproduced by introducing multiple source locations (none of which corresponded to the observed channel). It was therefore sensible to attempt modelling using a dynamic, rather than static, seafloor.
No attempt was made to simulate the observed geometries in detail. Instead, the objective was to investigate a possible, general scenario to see if it reproduced all the broad features discussed above. The components of this scenario were as follows:
- Shale deposition across uneven topography (produced, for example, by prior rifting) to give a ductile layer with variable thickness.
- Regional tilting to produce westward deepening of the basin.
- Tilt-generated slumping of ductile shale to give scarps separated by flatter zones.
- Turbidite deposition on flat zones producing heterogeneous loading.
- Ductile shale movement away from these loads to produce linear depocentres separated by growing shale-walls.
Bräuer, K, Pfitzner, M, Krimer, D.O., Mayer, M, Jiang, Y and Liu, M, 2006. Granular elasticity: Stress distributions in silos and under point loads, Physical review, E74, DOI: 10.1103/PhysRevE.74.061311.
Waltham, D, 1996. Why does salt move ?, Tectonophysics 282, 117-128.
Waltham,D, Jaffey, N, MacLean, S & Zampetti,V, 2008. Combined Structural Reconstruction and Stratigraphic Modelling of Turbidite Prospects using 3D Seismic Data. Petroleum Geoscience, 14, 1-9.