Author: Simon Blondel (OGS Trieste, Italy)

Cover image: Tilted Halokinetic sequences in the Emirhan Minibasin, Silvas Basin, Turkey (photo from Vedat Esen archive).

In the previous post, we briefly summarized how salt layers flow and discussed the importance of scaling. This week we will talk about the mechanisms and parameters that govern salt tectonic. The topics presented are based on the “Salt Tectonics: Understanding Rocks that Flow” conference, organized by the Geological Society in London last October 2019.

Parameters governing Salt tectonics

Last time we discussed how Salt flow but we haven’t explained why. Before we review this, you need some background knowledge about salt structures and you need to know the vocabulary we are using to describe them. I will not go through it because it would simply be too long but you can check-out Figure 1 or you can read the peper “Terra Infirma: understanding Salt Tectonics”, by Michael Hudec and Martin PA Jackson, freely available here.

Figure 1: Block diagram showing schematic shapes of salt structures. Structural maturity and size increase toward the composite, coalesced structures in the background. (a) Elongated structures rising from line sources. (b) Structures rising from point sources. (from Hudec and Jackson (2007), simplified from Jackson and Talbot (1991)).

The salt structures resulting from halokinesis (the movement of salt bodies) are described in terms of shape (diapirs, turtle-back, canopied etc…), depth, heights, length, setting, and if we possess rock samples, composition. Based on these observations, we try to estimate the intensity and the history of the deformation. We are trying to answer the following questions:

  • Why and when does salt start flowing?
  • What are the parameters that will define the intensity and the rate of salt-related deformation?
  • Can we mathematically express the rate and intensity of salt deformation via analytical or empirical laws?

Answering these questions help us to predict the distribution of rock properties in the subsurface and the evolution of salt-related structures with time. The results are used for petroleum, geothermal and storage industries.

Nowadays, differential loading is considered the dominant force driving salt flow (i.e. salt rocks will flow from areas where loading is the highest toward areas where it is the lowest), while the strength of the overburden and the boundary friction at the edge of the salt rocks are inhibiting it (Hudec and Jackson, 2007). Three mechanisms of loading may act on a salt layer (Figure 2):

  • Gravitational loading applied by the salt unit’s weight itself and by the overburden;
  • displacement loading applied during tectonic extension, tectonic shortening or prograding sediments;
  • thermal loading applied by expanding heated salt.
Figure 2: Examples of gravitational and displacement loading in salt tectonics. (a)Salt flow triggered by the differential loading applied by the overburden; (b) Salt flow triggered by differential loading applied by the salt’s weight unit itself at different elevation head; (c) Salt flow triggered by displacement loading during shortening; (d) Salt flow triggered by displacement loading during extension (modified from Hudec and Jackson, 2007).

Which loading is most important depends on the depth of salt burial, geometry of the salt body, tectonic setting, and thermal conditions of the salt (Jackson and Hudec, 2017).

There are a lot of parameters that drive and influence these mechanisms. Their relative role on salt tectonics is still poorly understood, yet essential to correctly interpret salt kinematics and the distribution of structural domains within salt basins. The key parameters enumerated during the conference were:

  • The thickness and lithology of the salt unit
    • It defines how much material can be mobilized, how strong is gravitational loading, how viscous salt is, and how much resistance is opposed to salt flow by viscous shear forces;
    • Fernando Borràs (Universitat de Barcelona) suggested that the thickness ratio between salt and overburden is the main parameter controlling the wavelength of the deformation in salt-bearing fold-and-thrust belts;
  • The topography of the underlying basement rock
    • The dipping of the base salt defines how it responds to gravity loading, while its rugosity influence how easily salt flows;
  • The lithology and thickness of the overburden
    • It determines how much gravitational loading is applied on the salt unit, but also how stiff the overburden is, hence how it will accommodate salt rocks deformation;
    • Syn-kinematic sedimentation can apply locally additional vertical or horizontal loading on salt, therefore influencing strongly the deformations style;
  • The geologic settings during and after the deposition of the salt (extension, compression, strike-slip)
    • It indicates the stress regime applied through time and space and how much displacement loading may have been applied to the salt;
  • The thermal gradient
    • Changing temperatures cause volume changes of the salt unit, affecting its density and its buoyancy, and potentially triggering thermal loading.

As you can see the mechanism of salt deformation has many variables. These variables are inter-dependant, highly heterogeneous in space and time, and it is expensive to collect enough data to measure their evolution accurately. In addition to this, while these parameters influence halokinesis, they can also be partially controlled by it. Indeed, salt related structures modify the basin’s topography, with lows accumulating halokinetic sediments in minibasins and highs blocking sediment transport (Figure 4). These new highs can in turn be eroded or provide substrate for reef development, increasing the sediment supply that loads the salt. This inter-dependency between the halokinesis and the parameters influencing it must be taken into account to understand the dynamic of salt tectonic.

To measure the preponderance of these parameters on halokinesis, the first step is to perform a detailed interpretation of the data available (cores, well-log, seismic, field measurements and samples, satellite pictures). The analysis of the data will provide a good overview of the salt distribution in space. For time It’s more difficult because the time scale of these processes spans far beyond humans’ range, so we need models and experiments that can reproduce the deformation of rock salt at our scale and confirm our hypothesis (see next post). Two main parameters have retained my attention, because I believe they were the most discussed ones during the conference: the topography of the underlying basement and the influence of syn-kinematic sedimentation.

Figure 3: The basin-fill model with subsiding minibasins adjacent to salt diapirs (from Fernandez et al., (2019), modified after Barde et al. 2002)).

The topography of the underlying basement rock

Only recently we have been able to deliver high-resolution 3D seismic images beneath salt, so we are starting to apprehend how the dipping and the rugosity of the basement influence the salt deformation. Studies in the Santos Basin presented by Mark Rowan (Rowan Consulting Inc) and Leonardo Muniz Pichel (Imperial College London) have shown that base-salt relief is paramount in controlling the structural domains and their styles of structures (see next post). When the moving salt encounters a steep ascending basement high, it slows down and contracts around the obstacle (Figure 4). Contraction also occurs when the flow of salt decreases after accelerating down a basement high. The deceleration on the ascending side coupled with the acceleration on the descent slowly thins the salt above the structure and generates extension.   The topography of the basement rock also influence the distribution and the thickness of the salt. Salt accumulation focus around pre-existing depression resulting in an irregular thickness evaporites. Rodolfo Uranga (University of Barcelona) suggested that these phenomena partly controlled the  salt structures observed in the northwest African passive margin.

Figure 4: Synthesis diagram of final model results comparing structural style distribution associated with a pre‐salt horst, and pairs of tilted fault‐blocks defined (from M. Pichel et al., 2019).

The interplay between halokinesis and syn-kinematic sediment accumulation

We discussed it before, structures formed by mobile evaporites can control syn-kinematic carbonate and clastic deposition. With increasing thickness, these deposits can in-turn control the geometry of the evaporite bodies. Halokinetic sequences that form in minibasins around salt structures highs record well the interaction between the two. Leonardo Muniz Pichel (Imperial College London) presented his work where he studies the terminations of these sequences on diapirs to understand the complex interplay between salt rise, diapir geometry, roof thickness and sediment accumulation along salt walls. Based on field observation in the Basque-Cantabrian Basin (Spain), Zoë Cumberpatch (University of Manchester) suggested that deep marine facies distribution within halokinetic successions are heavily influenced by salt movements. Several researchers also emphasized the presence of carbonate deposits above salt diapirs, as depicted on Figure 5). Ramon Lopez Jimenez showed examples from field observations in El Gordo Diapir (Mexico), while Paolo Esestime (TGS) showed example on modern 3D seismic in offshore Gabon. Peter Gutteridge (Cambridge Carbonates Ltd) presented a case study in the Sureste basin, Mexico, where these stiff carbonate roofs can restrict the diapir growth.

Figure 5: Schematic illustration of an interpreted carbonate-dominated composite halokinetic sequences from the Bakio diapir flanks. CHS refers to composite halokinetic sequence (from Yohann et al., (2016)).

The other parameters were not neglected but I’ve just presented the two that I believe were the most highlighted ones. I hope that now you apprehend how complex one stage of deformation can be considering the many parameters that come into play… Now you must also consider that ancient salt giants have experienced several stages of salt tectonics. They underwent a multi-phased geological evolution, where the tectonic regime has changed from extension to compression and sometime shearing in between.  What we observe nowadays is the sum of these deformations, each one overprinting the previous one. For example, back to the northwest African passive margin example, Rodolfo Uranga’s (University of Barcelona) presented at least three phases of halokinesis triggered by different parameters: a first one driven by regional extension (second event on Figure 5), a second one driven by driven by sediment loading (third event on Figure 5), and a last one driven by regional shortening (fourth and fifth events on Figure 5).

Multi-phased salt tectonic makes it difficult for discerning how salt moved at each step, hereby complicating the estimation of the significance of the key parameters that governed salt tectonics. Each salt giant carries its own mysteries and controversies because of his own intricated history. In the next post I will wrap up, salt giant after salt giant, what has been discussed during the conference.

Figure 6: Evolutionary Chart of Salt structures in the Offshore Tarfaya Basin, NW Africa (from Rodolfo Uranga Moran, available at https://www.researchgate.net/publication/337001079_Evolutionary_Chart).

CITED REFERENCES

Fernandez, N., Hudec, M.R., Jackson, C.A.-L., Dooley, T.P., Duffy, O.B., 2019. The competition for salt and kinematic interactions between minibasins during density-driven subsidence: observations from numerical models. Pet. Geosci. 26, 3–15. https://doi.org/10.1144/petgeo2019-051

Hudec, M.R., Jackson, M.P.A., 2007. Terra infirma: Understanding salt tectonics. Earth-Sci. Rev. 82, 1–28. https://doi.org/10.1016/j.earscirev.2007.01.001

Jackson, M.P.A., Hudec, M.R., 2017. Salt Tectonics: Principles and Practice. Cambridge University Press.

M. Pichel, L., Finch, E., Gawthorpe, R.L., 2019. The Impact of Pre‐Salt Rift Topography on Salt Tectonics: A Discrete‐Element Modeling Approach. Tectonics 38, 1466–1488. https://doi.org/10.1029/2018TC005174

Uranga Moran, Rodolfo & Ferrer, Oriol & Zamora Valcarce, Gonzalo & Muñoz, Josep & Roca,  Eduard. (2019). Salt Tectonics characterization of the Offshore Tarfaya Basin, NW Africa.

Yohann, P., Christophe, B., Etienne, J., Matthieu, G., Michel, L., 2016. Halokinetic sequences in carbonate systems: An example from the Middle Albian Bakio Breccias Formation (Basque Country, Spain). Sediment. Geol. 334, 34–52. https://doi.org/10.1016/j.sedgeo.2016.01.013

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