Sunday, August 7, 2011

Brittle synsedimentary deformation!

Synsedimentary deformation is pretty common; pretty much anywhere you go in the stratigraphic record, regardless of age, sediment type, or environment, there will almost always be some sort of sedimentological chaos present. While structural deformation (i.e., deformation due to tectonic processes after burial) is cool and all, synsedimentary deformation (where the chaos occurs contemporaneously with deposition) is, on a purely objective level, much cooler; just think about a delta front slump, the collapse of a point bar, or the huge deformation related to a mass transport complex! And, while the deformation is itself kinda cool to envision, the potential stratigraphic effects that such local reorganizations can have are even more amazing!

Synsedimentary deformation, like all deformation, comes in two flavors - ductile (i.e., folding) or brittle (i.e., faulting). Ductile deformation is probably the most familar style of sysedimentary chaos-ification, and can range in scale from individual mm-scale laminae slipping and folding all the way to huge fold-and-thrust belts at the toes of submarine fans. However, I get the feeling that brittle deformation is maybe more common in the rock record than is often reported, especially at the smaller end of the scale.

The picture below is from the cretaceous Ferron Ss, at the famous and picturesque Muddy Creek, Utah, and shows some examples of synsedimentary faulting we're talking about. These are in deltaic sediments:

Here's another, from the same section! Nifty, huh?

One more from the Ferron, for good measure!

And, finally, here's syn-sedimentary deformation showing both the faulting AND some subtle "growth-strata" like compensation/adjustment in sedimentation as a result of the deformation, all in one! These are from fluvial strata in the Eocene Green River Formation.

The importance of these brittle processes can't be overestimated, since synsedimentary extensional faulting is an easy way to produce a whole lot of space in a short amount of time. This can produce some dramatic effects, since these little extensional grabbens are ideal sediment ponds. What's especially cool is that you can see synsedimentary deformation directly influencing sedimentation patterns in the rock record (in fact, that's how you distinguish it from the boring ol' structural variety of faulting). You can see how the angle of sedimentation changes as you move up; in otherwords, there's initially some steeply dipping sediments that fill the little pond, with subsequently shallower sedimentation as the wound "heals".

The picture below shows the fault chopping off some sediment below, and juxtaposing more massive sandstones in the hanging wall. Importantly, this deformation is localized, and overlying and underlying sediment don't show any additional deformation - that's how you know it's not related to structural/tectonic deformation, but in fact a process going on at the same time as deposition.

One more, showing the juxtaposition of more massive sands in the hanging wall right against the fault!

Finally, one last picture, showing the (sub-seismic!) outcrop scale deformation possible in some successions. Here, again from the Muddy Creek, are some of the weirdly angled "healing" sediments:

You can imagine how, from a resource perspective, these sorts of things could really complicate your reservoir, and you'd never even know about it!

If you're interested in reading a pretty nifty paper that goes into the palinspastic retrodeformation of these extensional synsedimentary systems in the Ferron, take a look at Bhattacharya and Davies (2004). It's a pretty fun paper that makes a neat study of the linked sedimentology, stratigraphy, and deformation going on in these rocks, AND it's available as a free pdf right here. It's a fun read!


Bhattacharya, J.P., and Davies, R.K., 2004, Sedimentology and structure of growth faults at the base of the Ferron Member along Muddy Creek, Utah, In: T.C. Chidsey, R.D. Adams, and T.H. Morris (eds.) The Fluvial-deltaic Ferron Sandstone: Regional-to-Wellbore-scale outcrop analog studies and applications to reservoir modeling, AAPG Studies in Geology 50. P. 279-304.


Matt said...

Are these extensional features deformation bands? Or are they "regular" fault like structures?

Eric said...

It depends on the magnitude of the displacement...some of them are pretty much just little def bands, without much displacement, but OTHERS can have some pretty substantial throw, as evidenced by "weird" lithofacies that get deposited inside the grabbens.

The extensional features occur where a big ol' plug of deltaic sands get deposited on the underlying offshore mudrock. The sand overloads the wet mud, and you get all sorts of foundering, resulting in extension in one spot and (presumably) contraction farther down in the delta toes. So the features occur (predictably) within discrete stratigraphic and paleoenvironmental locations.

Zane Jobe said...

wow, these are great photos - thanks for sharing!

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