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Sea-Floor Sunday #62: Giant scour features in Gulf of Cadiz

March 21, 2010

This week’s Sea-Floor Sunday image is from a geoscience blog that I saw for the first time a few months ago — The Lisbon Structural Geologist, which is written by Filipe Rosas, who is — you guessed it — a structural geologist in Lisbon, Portugal. It’s a great blog that you should add to your feed if you haven’t already.

A few days ago Filipe posted some images from a paper he is a co-author on now in press for Marine Geology that summarizes mapping and characterization of the deep sea floor in the Gulf of Cadiz. The features of interest are some rather large erosional scours (5 km across) that remind me of scours on the Monterey submarine fan that I’ve posted about here.

Scour features on sea floor near Gulf of Cadiz (figure from Duarte et al., in press, Marine Geology)

Check out Filipe’s post about these features here — or go directly to the Duarte et al. Marine Geology paper in press here.

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11 Comments leave one →
  1. March 21, 2010 3:29 pm

    Thanks a lot for your interest and for this post, and also for the clarification regarding Fildani’s gender! Sorry for the confusion. Myself and Joao Duarte, we´re thinking about doing some numerical modeling using COMSOL software to test some hydrodynamic models suggested for the generation of this kind of scours (although we have to finish some other stuff that we have between hands right now). When we do so, if we get some interesting results we’ll let you and Andrea know. Who knows, maybe we could do some comparative Monterey vs. Golf of Cadiz approach, and try to get a broader picture of what is really going on…

  2. March 23, 2010 6:51 am

    Filipe … sounds good. If you’d like to contact me about possible collaboration or just trade some ideas don’t hesitate to contact me (see the ‘about’ page).

  3. Julien permalink
    March 25, 2010 6:44 pm

    Great pictures and job. It reminds me also the plunge pools along the California slope / Makran – quite similar processes of interaction between turbidity currents and thrust propagation.

    Something I wonder, however (but I might readed your paper too quickly), is the characteristics of the sedimentary lobe at the break of slope (slope-break deposits). You called it “progadational” lobes while in my opinion it rather looks like retrograding (updip/ upcurrent) lobes that progressively fill the depression while the knickpoint migrate upslope ?
    It would be also interesting to know how your seismic data showing the sed / tectonic evolution through times are correlated to the tectonic and/sedimentary phases of activity / inactivity in the Cadix area (which are more or less known).

    Thanks to both of you for sharing this very interesting study !

  4. March 26, 2010 1:43 am

    Dear Julien,

    Thank you for your nice comments.
    I must say that you are right. To be precise it should be upslope retrogradation. However, we used upslope progradation towards the scarp. We wanted to refer the direction in relation to the scarp. But considering the general slope of the area it is in fact a retrogradation :)

    Your second comment is also right to one major point. In fact, we wanted to do exactly that, since we mostly work with tectonics. However, there is no high resolution seismic data and cores available. This was a kind of preliminary work to present and discuss the data, but more investigation is needed because there are a lot of open questions.

    We know that one of the major tectonic events was in the Late Miocene with emplacement of the accretionary wedge and major olistosthromes. Some authors argue that the accretionary wedge is not active. However, this scours seem to have formed in the Plio-Quaternary and are probably active today. On other hand, the scours seat on top of blind thrusts that seem to route in the wedge decollement. So it could be that they are the expression of the recent wedge propagation. But they are also in the hanging wall of another major fault (Horseshoe Fault).. So we need a more detailed tectonic map of the area to constrain the structures that may play a major role in the formation of the scours.

    We tried to fallow these ideas in the beginning of the work but we found that a lot of turbidity activity is also related with oceanographic processes, like the accumulation of the sediments in the shelf by the Mediterranean Outflow Water (a contourite current – MOW). Without cores it is difficult to really correlate the turbiditic events that may be related with tectonic events (such as earthquakes). If we had a core and we could date the tectonic events on it, then we could correlate them with other known events such as the 1755 earthquake for example. One other hand, we could identify events that are not related with earthquakes but could be related with climate events that could trigger major shifts in the MOW.

    The problem with this features is that they are at great depths (almost 5000 meter) and the ships that colected the data were not prepared to operate at such depths. So we need to organize a cruise for that :) We hope that this paper may open some doors..

    So, there are a lot of things that can be done around this features :)
    Thanks again for you interest. It is great to have the opportunity to discuss this topic. Specially because make us see things from others perspectives. So if you all have any questions or ideas don’t hesitate to contact us :)

    Thank you Brian for the post :)

  5. March 26, 2010 7:48 am

    João and Julien,

    I’m glad to see people discussing papers here on the blog — this is exactly the kind of interaction I’m hoping to see more of on the internet (and why I maintain this blog).

    I haven’t read the paper yet so I can’t comment specifically. But, regarding dating turbidite deposits — this can be very tricky. More often than not, we end up getting dates that constrain the timing of events but it is very difficult to pinpoint it precisely because even if you find date-able material in the deposit itself it almost always is resedimented and, thus, doesn’t represent time of deposition. More typically, we can bracket the gravity-flow deposits by dating pelagic/hemipelagic material stratigraphically above or below.

    I wrote a paper where we were able to constrain the timing of turbidity current events down to centennial time scales, which is pretty good, but that still leaves some uncertainty if you’re trying to correlate to a very specific instantaneous event (e.g., earthquakes).

    I’m looking forward to reading the paper!

  6. March 29, 2010 1:53 am

    Brian,

    I agree with you that dating turbidites is tricky. On Friday I was talking with my other supervisor (Pedro Terrinha) and he also said the same thing, that often you do not get the dates right because of the resedimentation.. So we need to constrain the dates with the bottom and top hemi-pelagic sediments as you said..

    I think that try to get a correlation between the turbiditic events and earthquakes is a very long shoot. However, I know that the Spanish did some work on that. I’ll send you the papers if I find them.

    But concerning the Giants Scours, I think that it would be great if we could constrain the timing down to centennial time scales, because for now we have no idea if the scours are active or if they are fossilized structures. Some of them seem to be buried! And some of them seem to be very active! There are some major questions to solve: When did this process started? It is a cyclic or continous process at a geologic time scale? It is active at the present?

    It would be great if we could have at least an ideia when the process started, the regularity of this turbitic flows and if they are active or not. I would be happy if we could only have a relative dating.. :)

    Another major questions is to know if the bottom currents play an important role on the formation of these features or not at all.. If there are landslides and if there are mud volcanoes around the scours (we have evidences that there is at least one).

    To be honest, I am not a expert on processes related to turbidites.. So I hope that we can continue to exchange some ideas..

    For know it is a great pleasure to have this discussion with you.. and I’m learning a lot :)

    Best wishes..

  7. March 29, 2010 8:13 am

    Brian,

    I forgot to mention one important thing. In the Gulf of Cadiz region the convergence rates between plates are relatively slow (4-5 mm yr.).
    However, we had at least one major historical earthquake, in 1755, with an estimated magnitude of 8.5 to 9. Calculations showed that such events only happen every 2000 years. So, maybe it possible to identify these events on cores record even with low resolution.

    I don’t really know! What do you think?

    João

  8. Julien permalink
    March 29, 2010 2:14 pm

    Joao,
    From my knowledge of the Cadix area, past studies have shown that there is a strong interaction between turbidity currents and the activity of the MOW (i.e. contour currents). I think that the Sedimentary lobes and channels described by Hanquiez et al. (in press, Sedimentary Geology) are not so far from here.
    If you have some sedimentary cores in the area you would probably be able to build a stratigraphy quite easily. About 100 cores have been analysed at Bordeaux (and I guess it’s the same in Spain and Portugal!) and they could build a strong regional core stratigraphy for the Late Quaternary in either contourites, turbidites, and hemipelagites-dominated cores (14C, CaC03 evolution, XRF, delta18O)… Past climate and its influence on bottom current / turbidity current activity is also quite well constrained (work my Marches et al., Toucanne et al.). I’m sorry I cite only the work from my friends at the uni but I’m sure there’s plenty of work from the Portugese and Spanish teams as well ! I can send you the PhD thesis and related papers if you want.
    Cheers !

  9. March 29, 2010 7:46 pm

    João,

    you mention:

    “…for now we have no idea if the scours are active or if they are fossilized structures.”

    Again, I haven’t read your paper yet so forgive if I’m asking some basic things — but, my first question would be if you know if this area of the deep sea has been actively receiving sediment since the Last Glacial Maximum (~18-22 ka)? As you know some systems have been abandoned as sea-level rose and the feeder canyons become stranded and unable to feed the deep water any terrigenous sediment. Are there any submarine canyons that are presently connected to a coastal source of sediment?

    Also, I don’t know too much about this region but isn’t there a pretty strong current coming out of the Mediterranean?

    As for the field of turbidite paleoseismology … I remain cautious about how well it works.

  10. March 30, 2010 6:05 am

    Dear Julien,

    Thank you for the reference of the Hanquiez et al. paper. Actually the studied area in that paper is very close, and it is connected, to the Horseshoe Valley (the area where the Giant Scours are located). An interesting observation is that the Horseshoe Valley is feed by two drainage systems: one E-W that drains from the area that Hanquiez studied and another mainly N-S that drains the Portuguese Canyons. So it would be nice to compare the cores from this area with the ones you mentioned. I am aware of some of the work made by the French Team, but maybe I am not very up to date (because recently I’ve been more dedicated to the tectonic mapping of the region and analogue modeling, which is my PhD core). So I will check for more recent papers and it would be great if you could send me that PhD thesis that you mentioned.

    I just confirmed that we may have two cores available for future work in the scours, one inside the depression and another outside (There is one multi-core also and a couple more in the surrounding area).

    I already arranged with Filipe (and my other supervisor Pedro Terrinha) a meeting to discuss the possibility to submit a project to the National Science Foundation on this subject. We will contact you both, Julien and Brian, because if you are interested it would be great, and a very good opportunity, if we could collaborate in such a project.

    Brian,

    From what I remember, I think that the Portuguese Canyons are active. There are some Canyons connected to a coastal source of sediment, especially to the Guadalquivir Basin (the foreland of the Gibraltar Arc). But I can check with a colleague of mine that is doing his PhD on the Canyons (especially on their tectonic control). Nevertheless, I remember to read a paper in which Mulder shows that the activity on the Canyons is also controlled by the activity of the Mediterranean Outflow Water, which is responsible for the accumulation of large sedimentary deposits in SW Iberian shelf and upper slope.

    Julien and Brian,

    I was looking for recent papers of the French team (that Julien suggested) and I just found this one of Eulalia Gracia, with whom we also collaborated a lot in the past. I think it is right on some of the things we are talking :)

    Gracia et al., (in press in Quaternary Science Reviews). Holocene earthquake record offshore Portugal (SW Iberia): Testing turbidite paleoseismology in a slow-convergence margin

    Once again it is a pleasure to discuss this topic with both of you :)
    My best whishes
    João

  11. Julien permalink
    March 30, 2010 2:46 pm

    Well send you your e-mail on jbourget@asp.adelaide.edu.au, I’ll we forward you the 3 phD thesis I have on the cadix area from bordeaux, and talk further with possible future join work (this is a great idea).
    Cheers
    julien

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