Stratigraphic patterns in slope minibasins

A few months ago some former colleagues and myself have published a paper on the stratigraphic evolution of minibasins that are present on many continental slopes (Sylvester, Z., Cantelli, A., and Pirmez, C., 2015, Stratigraphic evolution of intraslope minibasins: Insights from surface-based model: AAPG Bulletin, v. 99, no. 6, p. 1099–1129, doi: 10.1306/01081514082.) We used a simple model that investigates the interplay between subsidence and sedimentation and helps in the understanding of how stratal termination patterns relate to variations in sediment input and basin subsidence. Conventional sequence stratigraphy focuses on what is happening on the shelf and the shelf edge; and the origin of stratal patterns that characterize the typical ‘slug diagram’ are not trivial to relate to the three main parameters that influence continental-margin stratigraphy: changes in sea level, sediment supply, and subsidence. Our minibasin model is simpler because (1) it only has two parameters: sediment supply and subsidence (the impact of sea level – if any – is included in the sediment supply curve); and (2) sedimentary layers are deposited horizontally during each time step. The basic idea is to investigate the geometries of a system that consists of a depression that deepens through time and sediment is deposited in it with a certain rate. The figures that follow are color versions of the model cross sections that were published in the paper. The originals are actually interactive (you can zoom in and pan if you want to, which is useful if you want to check some details, especially along the basin margins). However, I could not get WordPress to display these in an interactive format; you can check out the interactive version of this post on Github. I have used the excellent, d3.js-based – and recently open-sourced – Plotly to create the interactive plots. A few brief comments follow so that the plots make some sense even if you don’t read the paper.

There are two types of layers in the model: (1) the ones that correspond to the sediment supply that comes into the basin from updip, mostly in the form of turbidity currents – these are shown in yellow color in the plots below; and (2) thinner ones that reflect an overall finer-grained background sedimentation – these are shown in brown color. The latter have a constant thickness across the basin. It is important to note that, while it is true that all the sand is supposed to be in the yellow units, not all of the yellow units are coarse-grained. In fact, slope minibsains have overall a much lower sand content than what a ‘yellow = sand’ assumption would imply in these plots. The blue dots are pinchout points, locations where the yellow layers terminate. In the paper we adopted the idea that ‘onlap’ refers to the case when these termination points move toward the basin edge, and ‘offlap’ corresponds to stratigraphic intervals where the pinchout locations migrate toward the center of the basin.

Sedimentary fills of slope minibasins commonly show strata that either converge toward or onlap onto the basin margins (or show a combination of the two). The first two cross sections are examples of ‘pure’ convergence and well-defined onlap. In the first case (convergence), both sediment input rate and subsidence are constant and have similar orders of magnitude, and, as a result, the locations where sedimentary layers terminate are stationary. The basin never gets very deep, as sedimentation keeps up with subsidence.

Screen Shot 2015-12-19 at 4.40.33 PM

Obviously animations are much better at conveying the idea of sedimentation taking place while the basin is subsiding (the background sedimentation is set to zero in the animation; the lower panel is a chronostratigraphic – or Wheeler – diagram):

movie_1_convergence

For the formation of onlap, a relatively deep basin and high sediment input are needed. The model below has the same overall sediment volume as the previous one, but it was deposited in half the time. Subsidence was kept constant, just like in the convergence model.

Screen Shot 2015-12-19 at 4.42.09 PM

And here is the animation for the onlap scenario:

movie_2_onlap

The next two plots illustrate what happens if the sediment input varies following a sinusoidal curve with three cycles, while subsidence is constant. In the first model, there is enough accommodation in the basin to keep all the sediment; in the second model, the sediment input exceeds the space available in the basin and each cycle has an upper part during which some of the sediment bypasses the basin. If you zoom in, you can see that both onlap and offlap are present in both cases.

Screen Shot 2015-12-19 at 4.44.14 PM

Screen Shot 2015-12-19 at 4.46.05 PM

And the animation for the no-bypass scenario:

movie_3_sinusoidal_input_no_bypass

Next we looked at what happens if sediment input is kept constant, but subsidence varies (following a sinusoidal function). It is tempting to think that the result must be similar to the case of variable sediment input and constant subsidence, but that is *not* the case. In  previous models onlap surfaces overlie condensed sections across the whole basin. In this new model, onlap surfaces are restricted to the basin margin and they correlate to sections with high sedimentation rate at the basin center.

Screen Shot 2015-12-19 at 4.46.51 PM

It is a cool idea that stratal patterns might tell you whether variability in sediment supply or deformation is more important. However, if both parameters are changing through time, with similar magnitudes, it is practically impossible to tell what is going on:

Screen Shot 2015-12-19 at 4.47.44 PM

This animation gives you an idea how can these two cases transition into each other, as sediment input goes from constant to sinusoidal and out-of-phase with subsidence:

movie_7_constant_to_out_of_phase_sediment_input

So far all the simulations were based on sinusoidally varying parameters; what happens though if sediment supply is closer to an ‘on-off’ function? You can see the result below: each sedimentary cycle only shows onlap, the offlapping upper part is missing.

Screen Shot 2015-12-19 at 4.48.35 PM

The next two plots are attempts to reproduce the stratal patterns seen in two well-studied minibasins in the Gulf of Mexico: Brazos-Trinity Basin 4 and the Auger Basin.

Screen Shot 2015-12-19 at 4.49.19 PM

Screen Shot 2015-12-19 at 4.50.08 PM

Finally, we looked at the common scenario of multiple linked minibasins, with sediment bypassing the updip basin serving as the sediment input for the basin downdip. The classic idea about this depositional setting is called ‘fill and spill’: the updip basin has to be filled first before any sediment gets into or spills the second basin. A ‘static’ version of this idea, with pre-formed basins and no subsidence during sedimentation, is illustrated in the plot below, with three sediment input cycles:

Screen Shot 2015-12-19 at 4.50.46 PM

The animated version looks like this:

movie_9_static_fill_and_spill

However, this static fill and spill is probably the exception rather than the rule: most minibasins keep subsiding while sedimentation is taking place. So, in contrast with the previous model, the large-scale fills of the basins are time-equivalent, while at the scale of individual sediment input cycles the fill-and-spill model still works.

Screen Shot 2015-12-19 at 4.50.58 PM

Now let’s animate this:

movie_10_dynamic_fill_and_spill

For more details on these models, see the paper in AAPG Bulletin. Ten related animations are open access and available on the AAPG website. And, again, the plots are a lot more interesting if you can zoom, pan and rescale, so you should really check out the intended format of this post.

3 thoughts on “Stratigraphic patterns in slope minibasins

  1. Hi, I really like your post and blog. I am curious, are the IPython files available somewhere? I am trying to learn Python for teaching purposes and I would like to know how you did the calculations and plots. I understand if they are not, just asking.
    Best regards
    FER

  2. Hi Fernando — so far I haven’t made the IPython notebooks available. The minibasin model code unfortunately is not public at this point; but I do plan to upload the notebook for plotting.

  3. I really enjoyed this post. This is extremely relevant to my work here at Penn State for my masters! A quick google image search led to me your website and I found your model animations fascinating and insightful. I’m working on characterizing slope minibasins in the Kumano forearc basin offshore Japan using seismic stratigraphy. I understand there may be certain limitations to your model, but I wonder if you have looked into varying the direction of sediment input or somehow applying differential uplift to the minibasins? Please feel free to send me an email. I’d love to get some insight from you.

Leave a Reply to zsylvester Cancel reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s