Armored Mudballs!

"Quiet water conditions" is the depositional mechanism most often evoked to explain the presence of fine-grained mud in the rock record.  Mud, generally made up of clay minerals, is defined on the Udden-Wentworth grain size scale as particles smaller than ~0.00015 inches, or around 0.003 mm.  That's pretty small, and any amount of turbulence or motion in a water column will keep particles of that size suspended for quite a while.  However, as I've discussed before, there are some hints that a fair portion of mudrock in the stratigraphic record records higher energy conditions; one of the nifty qualities of clay minerals is that they're weakly charged, and can aggregate together into larger "chunks" that behave as hydrodynamically heavier clasts.  Sometimes, the features that would record this sort of aggregate/flocculate behavior of mudclasts is subtle or cryptic, especially once muddy sediment gets compacted and flattened out.  Sometimes, however, the evidence for muddy clasts is obvious!

The picture below is from the Eocene Cathedral Bluffs Member in SW Wyoming/NW Colorado, a pretty thick succession of fluvial/alluvial sediments that form some fairly picturesque vistas in that area.  The channelform sandstones around here are pretty coarse - usually upper Medium at the small end, and sometimes even getting into coarse and very coarse sands (we're pretty near their source area!).  Anyway, in among the fluvial sandstones, are things like this:

That's a gravel-sized clast, maybe 50 mm across or so, made up entirely of clay, and coated on the outside by a nice armor of sandy grains glommed onto the outside!  In a moment of refreshing clarity, the official science word for these things is the surprisingly restrained term "armored mudball".  Nifty, huh?  Here's another shot:

You can see that these muddy clasts are floating in a coarse sandy matrix.  Because these mudballs are so large, they form a nice erosional lag at the base of the channels.

Here's a close up of some of the gravel-sized mudballs:

 

The shot below shows the muddy interior of these little fellows:

Neat, huh?  Like almost all things in sedimentary geology, mud is a lot more fun once it starts gettin' pushed around by turbulence!

Non-Newtonian Pool Party!

Yet more crazy YouTube videos of weird, geologically relevant stuff! This time...it's non-newtonian fluids!

Newtonian fluids are the sort of everyday, hum-drum fluids we generally interact with (like coffee, or beer). If you exert some force over area on a newtonian fluid, it deforms pretty much instantly in proportion to the force applied to it. Another way of saying this is that newtonian fluids have a constant viscosity.

BUT non-newtonain fluids have a variable viscosity; generally, this viscosity varies nonlinearly as a function of either the amount of shear stress (force over an area) applied OR as a time-dependant function. Examples of non-newtonian fluids include things like ketchup (which gets stuck in the bottle until you shake it; the sudden application of force to the ketchup drastically reduces the viscosity, letting it flow out of the bottle) and whipped cream (which experiences an increase in its viscosity as a function of applied shear stress).

If you are lucky enough to be a geologist, then you actually have more opportunities than most to interact with non-newtonian fluids. Drilling muds and clays, used to lubricate drill bits, are examples of non-newtonian fluids. Even cooler, of course, are things like debris flows, which behave as viscous, non-newtonian fluids.

You can make your own shear-thickening non-newtonian fluid right at home, by mixing ~2 parts corn starch with 1 part water; get it good and gloppy, and you've got a fluid that, when you gently push against it, behaves just like water, but when you smack it hard, it behaves more like plastic. Try it out! It's insanely fun!

And, if you are really ambitious, you can fill up a whole giant tub of the stuff, and run across it, like these nuts did (the whole video is in Spanish, so that's a little tricky; still, it's fun!):



AND, just for more fun: here's a video of some of the "do-it-yourself home-made cornstarch non-newtonian fluid", put on a speaker!

Time-lapse video from underneath a glacier

How's this for rad:



Pretty slick, huh? It's a clip from some NOVA Special on PBS. Just a neat video to help you make it through the afternoon!

Mud as Sand!

Mud is, as we all know, an important component in the sedimentary rock record, and in some successions, completely dominates the record. The picture below is from around Grand Junction, Colorado, USA, and shows a thick package of the Mancos Shale, deposited in the Western Interior Cretaceous Seaway. I don’t care who you are or where you’re from, that’s a lot of mud!



Anyway, mudrocks (encompassing silt- and clay-sized grain populations) occur across the depositional landscape, from floodplains to abyssal plains, and are easily the third-best grain size out there (the others, sand- and gravel-sized particles, are tied for first in the “best grain-size” category). And that grain-size holy trinity (mud, sand, gravel) represents the methodological hydrodynamic triumvirate that most sed/strat types deploy (almost casually) in the field: mud is deposited in low energy conditions, sand takes some energy to shift around, and gravels need quite the push to get moving through a system.

There has been some recent, ahem, erosion, of that venerable concept of quiet-water suspension fall-out of mud, however; recent work, both field-based and experimental, are beginning to show that muds may be a little more complicated than just the “fine-grained, laminated, organic rich --- 50 meters thick” nonsense that gets slapped down in your field book.

Schieber and Southard, 2009 pulled off a rather nice, simple flume experiment using mud (in the 10 – 20 micron range) in both fresh and saltwater. They were able, through careful use of time-lapse photography, observations on ripples that became attached to the flume wall, and by quickly draining the slurry from the flume, to capture a variety of ripple formsets made entirely out of mud! The data repository for the paper has some slick movies of the mud ripples forming and migrating, as well. The picture below is their Figure 1, on pg. 484. That there’s a ripple, a-yup.


Anyway, the neat-o thing about this ripple is the kind of sediment transport these muds are experiencing. It was possible, for example, that the muddy ripples just looked like sandy ripples, but were being deposited as fine-grained, turbid slurries, which would still be a fairly low energy condition for ripple formation, right? However, Shieber and Southard 2009 have shown that, in fact, the muds rapidly flocculate into silt and sand-sized particles, which are then transported at comparable velocities to regular ol’ sand and silt grains.

Of course, the implications are pretty obvious: previously interpreted quiet-water offshore muds, for example, might not be as quiet-water as we thought. And, post-depositional burial and compaction might result in the general obliteration of these ripple cross-laminations and bedforms from the record, and the superficial appearance of horizontally laminated mudrock. That really changes your interpretation of hydrodynamics, sedimentation rate, and how the mudrocks fit into whatever lithofacies association scheme you’ve cooked up for your rocks.

Wright and Marriott, 2007, came to the same general conclusions regarding mudrock in the Lower Old Red Sandstone (South Wales, UK). These mudrocks generally lack the stereotypical “fine-scale laminations” of most mudrocks; because of this, the assumption has been that these muddy units have been altered by soil formation. However, Wright and Marriott 2007 point out that these mudrocks are often interbedded with gravel-lens that have sharp contact (above and below) with the mudrock, that there are sharp truncations that separate mudrock from other mudrock, and that there are some faint, large scale architectural components associated with these mudrocks, reminiscent of accretion packages in fluvial macroforms. The picture below sums up, diagrammatically, their own field-based evidence for these interesting mud associations; it’s Wright and Marriott’s Figure 3, on pg. 95.



Wright and Marriott (2007) point out that this is sort of a big deal. The interpretation of muddy deposits as relatively continuous, flat-lying overbank deposits with lots of pedogenesis is FUNDAMENTALLY different from the interpretation of mudrock deposited as sand-sized aggregates within a channel complex. Hydrodynamics, Time, and Sedimentation Rate…there are big differences between these two models. And, from a practical side, if I’m depositing mud-rich plugs in an active channel as macroforms, that is going to really change fluid migration paths for hydrocarbons within the ostensibly permeable and porous channel complex sandstones.

Anyway, kind of a neat thing to think about, next time your haulin’ ass over the mudrocks to get up there at the obviously more interesting sandstones. Maybe these fine-grained bedload phases are more common than we realize?

WORKS CITED

Schieber, J., and Southard, J.B., 2009, Bedload transport of mud by floccule ripples – direct observation of ripple migration processes and their implications: Geology, v. 37, p. 483-486.

Wright, J.P., and Marriott, S.B., 2007, The dangers of taking mud for granted: lessons from Lower Old Red Sandstone dryland river systems of South Wales: Sedimentary Geology, v. 195, . 91-100

Dolphins: Nature's Fluid Hydrodynamicists

Jumpin' Cats! Have you guys seen this:



How awesome is that!?!

These dolphins blow a bubble into a turbulently roiling eddy, which causes it to become a bubble-ring (similar to a Gandalf-style smoke ring, actually) and then, being dolphins, decide to play with it in the most ridiculously cute fashion possible. Hydrodynamically, these rings are a kind of toroidal vortex, which in this case is made up of a combination of air and water moving as a kind of propagating vortex. Here are some pictures of human-made bubbles, with a discussion of the physics involved in these things.

EDIT: Turns out a geo-blogger had already talked about this: check out Riparian Rap!

Kinematic Theory of Unsteady Seperation

One of the problems with so much of our understanding of hydrodynamics (and, more broadly, all Earth Processes) is that we have to make so many damn generalizations and simplifications. Of course these are important first steps in developing greater insight into the world, but sometimes, you just want to have a firm grasp on what the hell is going on, you know?

Two recent papers, Weldon et al. 2008 and Lekien and Haller 2008, have made some impressive advances in our ability to describe and predict flow separation under unsteady conditions; the ramifications of this are pretty big, and if you've got a secret love of hydrodynamics (like me), it's pretty exciting stuff!

Fundamentally, flow separation occurs where a fluid is moving away from a solid boundary of some sort. In sedimentology, this is commonly illustrated by the behavior of a fluid flowing over a bedform, such as a dune in a river. While we have always had a general, qualitative appreciation of the effects of flow separation on bedform dynamics (i.e., back-flow eddies in the lee-sides of dunes, Kelvin-Helmholz instabilities in turbidity currents, etc), a quantitative description of the behavior has been lacking; as such, modeling these systems is difficult, and requires some arm-waving.

In fact, the nearest thing we've had to a kinematic solution for flow separation was Prandtl 1904. Using some mind-bogglingly complex math, ol' Prandtl was able to come up with a solution for laminar boundary separation in steady 2-D flows. Sentences like the previous one make sedimentologists involuntarily twitch: "laminar" and "steady" are both exceedingly rare in the natural world, making Prandtl's work useful for gross generalizations, but frustratingly weak in unsteady and turbulent flows.

Now, however, Weldon et al. and Lekien and Haller have both shown numerical and -most importantly- EXPERIMENTAL data that advances a kinematic theory of unsteady separation, allowing us to accurately predict separation points in unsteady flows. Furthermore, part of Weldon et al. (2008)'s work has shown that their kinematic theory accurately predicts flow separation in flows that are kinematically equivalent to turbulent flows.

Lekien and Haller (2008) also apply this particular kinematic theory to boundary separation models of the North Atlantic geostrophic current AND to boundary current separation and reattachment in Monteray Bay, based on field data collected from real live currents.

The public take on this (see here for MIT's own press release on the research) is focused on larger, non-geologic issues, such as increasing fuel efficiency by decreasing shear drag on cars. However, for selfish reasons, it will be particularly exciting to see where this sort of research leads to in the sed realm. We might finally start to home in on some robust models of mixing layer dynamics in sediment laden flows, or even (bestill my heart!) start being able to really interrogate bedform morphodynamics and evolution under realistically complex flow conditions!

WORKS CITED:

Weldon, M., Peacock, T., Jacobs, G.B., Helu, M., and Haller, G., 2008, Experimental and numerical investigation of the kinematic theory of unsteady separation: Journal of Fluid Mechanics, v. 611, p. 1 - 11.

Lekien, F., and Haller, G., 2008, Unsteady flow separation on slip boundaries: Physics of Fluids, v. 20

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.

Immense submarine debris-flow deposit

A particular interest (professional and otherwise…whatever that means) of mine is flow hydrodynamics. That is to say, how does sediment get transported across the surface of the earth, and what processes control the subsequent deposition of sediments. A recent paper by Talling et al. (2007, Nature v. 450, Nov. 22) is a prime example of how absolutely mind-boggling some of the processes are.

Talling et al. (2007) report on an interpreted submarine debris flow deposit, approximately 1500 km offshore of northern Africa. For those not enamored of the SI, 1500 km is about 930 miles, so you’d want to pack a lunch if you were travelling that far. Anyway, this deposit represents the longest run-out flow ever described on Earth. Neat, huh!?!

What’s really interesting, however, is the interpretation of the origin of this deposit. A submarine landslide, 1500 km shoreward of the eventual deposit, probably produced a kind of sediment-gravity flow called a turbidity current. These turbidity currents are flows that are turbulently supported (ergo the name…), meaning that the motion of the sediment-water mixture keeps the whole flow in suspension. Because of the density of the flow, it hugs the bottom of the seafloor, and moves down-gradient under its own weight. These flows are commonly very erosive, and can travel long distances. Talling et al. (2007) make the interpretation that a turbidity current travelled most of the 1500 km, but as it decelerated, it transformed itself into a debris flow. Debris flows are NOT turbulently supported; rather, they exhibit what’s called laminar flow (so NO turbulence in the flow). This transformation of the turbidity current is what resulted in a debris flow, which was rapidly deposited as a many-meter thick bed 1500 km from the source of the sediment.

An impressive paper, detailing the inherent complexity in unraveling the sedimentological record. And this is just a single event that happened only a couple hundred thousand years ago; now think about strata, many millions of years ago, divorced from their geographic context (in a fold-belt, for instance). What a great challenge!