Tuesday, September 1, 2009

Basins: depositional vs. structural

One thing I've noticed when teaching Historical Geology at NOVA and GMU over the past four years is that students get confused between basins. There are depositional basins and structural basins, and they're not the same thing, though they both sag downwards in the middle. The other day while driving out to the Blue Ridge for a hike, a lightbulb went off above my head. I knew what I needed was a graphic that explicitly laid out the processes responsible for each structure, and their development over time. I jotted down a reminder to myself on the lid of the Starbucks coffee cup in my car's cup-holder.

When I got home, I translated the scrawled reminder into action. In my spare time over the past couple of days, I've been composing the basin graphic with CorelDraw. Here's what I drew:

Basins_comparison

Depositional basins result when there's a low spot on the Earth's crust. Water flows into these crustal sags, carrying sediment with it. Gradually, they can fill in. Sedimentary inputs are shown with arrows. (They can also self-perpetuate, as the heavy sediment keeps the crust sagging downward at that location.) Layers stack up according to superposition: oldest on the bottom, youngest on the top.

In contrast, structural basins have a different story. There, we start with an accumulation of sedimentary layers, and then we deform them into a basin shape. This deformation is the result of tectonic stresses which warp the rock layers. Erosion can then attack the downwarped strata, planing the "nested cups" shape down to a roughly horizontal ground surface. Sedimentary outputs are shown with arrows. The resulting outcrop pattern is somewhat like a bull's-eye, with the youngest layers exposed in the middle and the oldest layers exposed on the outer part of the structure.

In a depositional basin, the downward central sag comes first, and the stack of sediment is a result of that sag. In a structural basin, the stack of strata comes first, and the central downwarp is produced second.

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If any educators want a larger version of this graphic for use in teaching, let me know. I'll happily e-mail you one. Also, if anyone would suggest any modifications to the graphic to make it more accurate or more useful for communicating these ideas, I'd be happy to get that feedback.

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8 Comments:

Anonymous BrianR said...

cool! I love basins ... two questions:

- So, is your structural basin equivalent to a large synform then? Driven by compressional deformation and not subsidence?

- What are some good examples of a structural basin?

September 1, 2009 10:56 AM  
Blogger Eric said...

You can subdivide sedimentary basins one more step down, as well; modern hydrological basins versus stratigraphic basins, right?

A modern drainage basin is one where rivers are flowing into it, whereas a sedimentary basin is a 3-D volume of sedimentary rock, recording a phase of deposition from the past. In this way, we can talk about the passive margin sediments of the Cordilleran miogeocline (which have been uplifted as mountains), as representing a sedimentary basin (just an old one).

September 1, 2009 11:12 AM  
Blogger Suvrat Kher said...

very nice graphic!

just a thought that your structural basin model paradoxically requires a long period of tectonic quiescence after the initial downwarping is produced. So the development of an erosional landform is a requirement of your model.

September 1, 2009 11:27 AM  
Blogger CJR said...

Personally, I think this is one of those instances where the students have some right to be confused. It's a conflation of form and process: calling a structure which was never a site of sediment deposition a 'basin' seems rather silly to me.

Also, I can't recall encountering this terminology in my structure classes. Is it a US thing?

Nonetheless, it is a nice graphic.

September 1, 2009 11:40 AM  
Blogger Lockwood said...

Basins can be both, can't they? I.e., a structural basin can become a locus of deposition. Your graphic implies that most sediment leaves the basin area; that's simply not true in much of the US west.

September 1, 2009 2:57 PM  
Anonymous Anonymous said...

The comments and graphic (nicely done, BTW) raise more questions, I think, than the graphic answers, though that's not necessarily a bad thing, for the sake of classroom discussion:

- As Eric points out, a third type of "basin" needs to be addressed: the hydrologic/"drainage" basin (net erosion) vs. the sedimentary basin (net deposition); both terms are important in geology.

- Looking at the graphic--as a student--I might ask "why are the (grey and green) beds in the depositional basin curved? Don't sediments normally get deposited horizontally?" to which you might reply "because they are downwarped by sediment loading", to which I might then ask "but if they're downwarping, doesn't that make it a structural basin?". As Lockwood notes, it would actually be both.

- Similarly, looking at the initial stage of the structural basin, with its three layers of sediment, a student could fairly ask: "where did the sedimentary layers come from?" Didn't they have to be deposited in a depositional basin initially? In which case the structural basin is a secondary form overprinted on a primary depositional basin" (form vs. process, as Chris notes).

- Without careful explanation, the graphic might lead a student to think that "structural basins" are synforms, i.e. fold structures. But how about the well-known example of the Basin and Range province in the western US, where the "basins" are flat-lying but fault-bounded grabens, accumulating sediment. Is this a "structural" basin, or a "depositional" basin? Or both?

Cheers,
--Howard

September 1, 2009 4:53 PM  
Blogger Geology Happens said...

I agree that many students get confused. I try to use the words geological basin vs topographic basin to show the difference. And yes your graphics are awesome. I am busy right clicking them as we speak.

September 1, 2009 10:34 PM  
Blogger Callan Bentley said...

Thanks for the great comments, everyone. The discussion continues in today's post.

September 2, 2009 8:38 AM  

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