Friday, May 9, 2008

Swaley Cross-Stratification Solidarity

Brain, over at Clastic Detritus ,has posted a nifty picture of some sweet, sweet Swaley Cross-Stratification. To show some solidarity, I too will post a couple of pictures of some SCS I’ve collected. Actually, both of these pictures are from the Book Cliffs, as well. If you ever find yourself in and around Price, Utah, I suggest you grab a map, your rock hammer, and a camera, and just spend some time wandering around all the BLM land and its fantastic geology.

The picture below has some SCS nicely exposed in the lower right of the outcrop; actually, if you trace the lower bounding surface over to the left a ways, you can just start to see it transition into hummocky cross-stratification. As Brian pointed out in his post, these two bedforms are often associated with one another. In fact, these bedforms are so characteristic, that they have often been evoked as “typical” of storm deposits. In some of the literature, HCS and SCS are often interpreted as unequivocal evidence for tempestites.

This next picture shows another nice little swale, hanging out with some structureless and planar laminated sands.

The SCS is all good and well, and some may be tempted to slap an interpretation of a storm-bed on there, but darned if some of the surrounding sands aren’t lookin’ an awful lot like some T a-b turbidites (with just the massive and planar laminated sections preserved of the more traditional Bouma sequence). But I thought SCS meant storms!?! Aren’t we in the middle-to-upper shore-face!?!

As Brian points out in his post, the flow hydrodynamics of these bedforms are related to high energy and rapid sedimentation. Dumas and Arnott (2006, Geology, p. 1073-1076) have further gone on to quantify the specific flow hydrodynamics related to these settings, demonstrating that oscillatory-dominant combined flow (meaning both back-and-forth AND directional currents) are responsible for the production of these bedforms. While it is true that storm currents can (and do) produce these bedforms, these same hydrodynamic conditions can also be produced by other processes, such as turbidity currents! The picture above is actually from a portion of the Book Cliffs recording the progradation of one of the famous clastic wedges, and there are turbidites and (potential) hyperpycnites associated with this interval.

Things are never as easy as they seem, and that goes double for facies models.


BrianR said...

Good points indeed.

Having looked at a lot of turbidites, I've definitely seen plenty of swaley-esque looking structures when you get into a traction-dominated section.

The grains really don't care about the environment, just the physics!

Is that the Panther Tongue that you are referring to? I still need to find the time to open up the hyperpycnite can of worms with a post one of these days.

Eric said...

I knew a carbonate worker who used to call siliciclastics "dumb", beacause they always followed orders (the hydrodynamics).

And yep, thats the ol' Panther Tounge. Some of the folks out there were hitting the ol' hyperpycnal bandwagon pretty hard, though I never could convince myself of their interpretations. There was no REAL evidence for the hyperpycnal stuff, as far as I saw (evidence in the vein of Mulder and Syvitski 1995, I mean). What is your opinion on that section?

BrianR said...

I've been to the Panther Tongue twice ... the first time was early in my education and either we didn't talk about that aspect or if we did, it didn't sink in obviously. The second time was more recent (a couple years ago) and we had a pretty good discussion about the origins of those beds.

There are clearly a lot of products of sediment gravity flows in those rocks. If I remember, there is evidence of some reworked bed tops and such as well.

So, the next step of interpreting the type of sediment gravity flow is where it's contentious, as you point out. After going to the Hedberg this year, which was focused on hyperpycnal flows/deposits, I realized what the problem is. Some stratigraphers have a very "loose" definition of what a hyperpycnal flow is - any gravity flow-dominated system that is fed directly by a river must be dominated by hyperpycnites. That would be the end-member in the loose definition. And then some have a stricter definition of both a hyperpycnal flow and an hyperpycnal flow deposit (e.g., Mulder et al). I tend to fall into the stricter category, as it seems you do as well.

I think we just don't know enough yet about gravity flow initiation mechanisms (not to mention flow transformation processes) to be able to say that certain kinds of deposits are always tied to a certain kind of initiation mechanisms.

For example, I'd think mouth bars would be continuously building up and failing during a high-magnitude river flood (that lasts days). Gravity flows from that constant failure would not be hpyerpycnal flows sensu stricto. The grains need to be in suspension from river directly to ocean/lake. But, some with the loose definition would say it is a hyperpycnal flow - to me, they are equating river-fed to hyperpycnal.

The Panther Tongue may indeed be hyperpycnal flow deposits ... I don't know ... but I think we should be much more rigorous what the criteria are for recognizing these deposits. Personally, I think more work needs to be done monitoring active river mouths as well as investigating Holocene deposits.

I'll have to put together a post about this stuff soon ... it's an interesting topic.