AAPG Salt Basins Technical Interest Group – Q&A From Online Seminar by Oliver Duffy (AGL) July 21st 2020
Seminar Title: The Subsidence and Mobility of Minibasins: Insights from Natural Examples, Physical Models, and Numerical Models
replay: https://youtu.be/k9XLUSVOHdA

9 the whole minibasin stroy starts with sedimentation above thick pervasive layer of salt - can you think about mechanisms focusing sedimentation in particular areas?
As mentioned after the talk, this is an important question that people have been grappling with for a long time. One possible way could be if dense carbonates are deposited in patches near the top of the ‘salt’ sequence, perhaps that could set off localized density-driven subsidence that could then focus the later clastics? However, this is not likely to be a widespread process. Beyond that, there is the concept of ‘pods’ in the North Sea which many people are familiar with, that invokes basement faulting as a driver of topographic relief and subsequently localized deposition. However, as far as I am aware, the pod model does not explain the development of circular to ovate minibasins that are surrounded by polygonal salt walls (as is typical of many minibasin provinces).

10 Rotation of the minibasins is only driven by tectonic or any other mechanism?
In Tim Dooley’s physical models, minibasins rotate during shortening due to the induced flow of salt and the differential traction that is exerted on the outer surface of the minibasins. It should be noted that in the physical models, the minibasins are circular in map view. We speculate that some shapes of minibasin may be easier to rotate (or may increase how much and for how long they rotate) than others. Minibasins also rotate as they collide with other minibasins or the base of salt and pivot around these obstacles.
There is more detail on this in the pre-print of a manuscript that is available to download at: https://eartharxiv.org/n59z4/
Although this has not been proven, I would speculate that minibasins translating down salt-detached slopes could rotate. I can imagine scenarios where minibasins in such settings collide with base-salt relief or other minibasins that would then allow them to pivot and rotate without a tectonic influence. It would be interesting to think about if the downslope flow of salt alone on salt-detached slopes is sufficient to drive rotation of minibasins.

11 Excellent. Is it only the salt and sediments density contrast which create minibasins? or tectonic forces have any play to creat mini basin other than reshapping + "Thanks Olly for the great talk. My question is: How do you explain the minibasin development and subsidence quite early before the minibasin pile’s average density becomes greater than that of salt?"
Thankyou!. There are a variety of mechanisms proposed by Hudec et al (2009) – see full citation below. As well as tectonic scenarios etc, in some cases you can get fill/cover on top of salt that is of high density lithologies (carbonates/evaporites). This may set off density-driven subsidence while the fill is thin
HUDEC, M.R., JACKSON, M.P. & SCHULTZ-ELA, D.D. (2009) The Paradox of Minibasin Subsidence into Salt: Clues to the Evolution of Crustal Basins. Geological Society of America Bulletin, 121, 201-221.

13 "Also, how would you expect minibasins to interact with topography associated with carbonate platforms and pinnacles."
Researchers have studied how a translating overburden can interact with base salt topography. This topography could ancient volcanoes, salt feeders or carbonate features mentioned. An example of such a paper is:
FERRER, O., GRATACOS, O., ROCA, E. & MUNOZ, J.A. (2017) Modeling the Interaction between Presalt Seamounts and Gravitational Failure in Salt-Bearing Passive Margins: The Messinian Case in the Northwestern Mediterranean Basin. Interpretation-a Journal of Subsurface Characterization, 5, Sd99-Sd117.

In the case of minibasins interacting with carbonate platforms and pinnacles, I’d imagine the same principles of minibasin obstruction outlined in the talk would apply (if the minibasins had subsided deep enough) i.e. minibasins would collide with, and be buttressed from further movement by the platforms and pinnacles if they had subsided deep enough. It would be neat to see an example of such interaction, as I am not quite sure of the relative scales of the features you describe.

Also – see answer (and refs) to the question below regarding the complex spatial and temporal patterns of extension and shortening that can occur as supra-salt stratigraphy translates over a rugose base of salt (not that in those cases there are no minibasins and no collisions as the overburden translates above the relief).

14 "Have you seen some of these examples in the Brazilian pre-salt? and if so, what's the main setting here? The same upslope shortening - downslope extension pairity? Ulisses. Thank you."
I have not had the opportunity to look at much data from offshore Brazil. Saying that, I see no reason why the obstructed minibasins principle would not apply to offshore Brazil, if minibasins are thick enough to interact with the base of salt. However, I am aware that papers published in recent years based on offshore Brazil, show additional aspects at play as supra-salt stratigraphy translates over a rugose base of salt. This results in complex spatial and temporal patterns of extension and shortening. You may already be aware of these, but in case not, here are a couple of papers that may be of use (see references within):
PICHEL, L.M., JACKSON, C.A.L., PEEL, F. & DOOLEY, T.P. (2020) Base‐Salt Relief Controls Salt‐Tectonic Structural Style, São Paulo Plateau, Santos Basin, Brazil. Basin Research, 32, 453-484.
DOOLEY, T.P., HUDEC, M.R., PICHEL, L.M. & JACKSON, M.P. (2020) The Impact of Base-Salt Relief on Salt Flow and Suprasalt Deformation Patterns at the Autochthonous, Paraautochthonous and Allochthonous Level: Insights from Physical Models. Geological Society, London, Special Publications, 476, 287-315.

15 "i am guessing that, at least in the GOM, extensive drilling of minibasins allowed some sort of strat control and age. Have ppl, on a broader sense, did some work on estimating the timing between the ""sinking"" of adjacent minibasins that evolved/deformed at different times and rates?"
Yes, this is an excellent question. We have applied this approach to a number of minibasins in the Northern Gulf of Mexico. Unfortunately was not enough time to fit this material into the talk. One of the interesting things about this setting is that supra-canopy minibasins in the GoM are subsiding above a rugose base of salt (feeders etc) and they are also translating downslope. So the controls on minibasin subsidence have the potential to get very complex! This contrasts with the numerical models we showed where the base of salt is flat. Hopefully, a pre-print on that research will not be too long away.
17 Salt flow velocity model in complex shapped mini basins is based on pure salt (Halite) or evaporite mixtures? If it is pure salt then what will the impact of evaporite mixtures on complexity of mini basins?
Good point. Yes, in the numerical models the salt is halite. More details are available in the paper:
FERNANDEZ, N., HUDEC, M.R., JACKSON, C.A.-L., DOOLEY, T.P. & DUFFY, O.B. (2020) The Competition for Salt and Kinematic Interactions between Minibasins During Density-Driven Subsidence: Observations from Numerical Models. Petroleum Geoscience, 26, 3-15.

One would anticipate that all being equal, the subsidence rates would vary as the salt changes composition (density), and that when the salt is layered or shows lateral variations in composition, subsidence patterns could become more complex. Ongoing work is tackling this issue using physical modelling, numerical modelling and natural examples, so insights will be forthcoming soon.

19 The shift of the bowl and wedge axis orientations. Can be due to sediment feeders shift or and fill and spill phenomenon?
So far, we have a fairly good understand of some of the general causes of transitions from bowls-to-wedges and the directions of tilts:
1) minibasins will tilt away from pressurized salt (the pressurized zones may be created due to shortening or the subsidence of adjacent minibasins)
2) minibasins tilt as they pivot on primary welds or secondary welds with other minibasins. These processes could be active synchronously. As minibasins rotate on secondary welds, this may cause the location of subsidence to rotate as well
3) minibasins subside quicker over thicker salt, and so will tend to tilt towards areas of thicker salt.
References on this topic include:
FERNANDEZ, N., HUDEC, M.R., JACKSON, C.A.-L., DOOLEY, T.P. & DUFFY, O.B. (2020) The Competition for Salt and Kinematic Interactions between Minibasins During Density-Driven Subsidence: Observations from Numerical Models. Petroleum Geoscience, 26, 3-15.
ROWAN, M.G. & WEIMER, P. (1998) Salt-Sediment Interaction, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico. AAPG bulletin, 82, 1055-1082.
JACKSON, C.A.L., DUFFY, O.B., FERNANDEZ, N., DOOLEY, T.P., HUDEC, M.R., JACKSON, M.P. & BURG, G. (2019) The Stratigraphic Record of Minibasin Subsidence, Precaspian Basin, Kazakhstan. Basin Research.
HUDEC, M.R., JACKSON, M.P. & SCHULTZ-ELA, D.D. (2009) The Paradox of Minibasin Subsidence into Salt: Clues to the Evolution of Crustal Basins. Geological Society of America Bulletin, 121, 201-221.
https://eartharxiv.org/n59z4/

So we think the above are some of the main control. However, the effect of shifts in the locus of sediment feeders through time on the direction of tilt is not really understood. Though we might ask the question, how much tilt influence would a say 50-100m thick channel system have on a 7 km thick minibasin? Seems likely it would be negligible…. but I think more work needs to be done on these relationships.

20 "Really good story, easy to follow as you built up the levels of complexity. I liked the early explanation of the roll of density that you returned to on your final slide. The models illustrate this is a very 3D flow. why is the salt lower density with depth? Is Temperature playing a roll?"
Thankyou. Yes, I suspect that the slight decrease of salt density with depth is a function of increased temperature with depth.
22 "Are minibasins typical occurance outside salt systems?
Interesting thought. I am aware that minibasins can develop in shale systems:
RUH, J.B., VERGÉS, J. & BURG, J.-P. (2018) Shale-Related Minibasins Atop a Massive Olistostrome in an Active Accretionary Wedge Setting: Two-Dimensional Numerical Modeling Applied to the Iranian Makran. Geology, 46, 791-794.

Though I don’t know how common of a phenomenon that is, or if minibasins can occur in other scenarios.

secondly overpressure of the salt as a result of movements of the minibasin, would it in any way cause further deformation in the salt body"
This question relates to question 17 too. As minibasins subside it will force salt out from beneath. That salt flows somewhere, so will internally deform the salt body. As to the details of this, we will find out once ongoing work is completed on this topic. However, some examples of flowing halite-rich units within layered evaporite sequences (i.e. the salt body) are illustrated in this pair of papers:
JACKSON, C.A.-L., JACKSON, M.P., HUDEC, M.R. & RODRIGUEZ, C.R. (2015) Enigmatic Structures within Salt Walls of the Santos Basin—Part 1: Geometry and Kinematics from 3d Seismic Reflection and Well Data. Journal of Structural Geology, 75, 135-162.
DOOLEY, T.P., JACKSON, M.P., JACKSON, C.A.-L., HUDEC, M.R. & RODRIGUEZ, C.R. (2015) Enigmatic Structures within Salt Walls of the Santos Basin—Part 2: Mechanical Explanation from Physical Modelling. Journal of Structural Geology, 75, 163-187.

24 "Congratulations for the excellent presentation.
Could Minibasins rotation generate heat for alter salt properties at their vicinities? Therefore, allowing different movement behavior for different Minibasins."
This is an interesting and unusual question. I like it! My instinct suggests extent of any lithological change due to the rotation and shear would be over a very narrow zone that I doubt would extend to influence adjacent minibasins. But that is a guess!
26 Can the participants get access to the presentation video? Thanks
Yes, this video will be available on youtube (https://www.youtube.com/watch?v=k9XLUSVOHdA). If there are any further questions or further discussion, please feel free to email at [email protected]