Cyclic subsidence and uplift in the Mississippi Delta

The Mississippi Delta is really one of those iconic depositional settings within sedimentary geology; when folks talk about deltas, consciously or unconsciously most people get a picture of that big, beautiful, stereotypical Bird's Foot protruding out into the Gulf of Mexico. And not without good cause, of course. From a purely aesthetic viewpoint, the Mississippi Delta is just damn pretty, especially from space, as evidenced by the Earth as Art Landsat image of the Mississippi Delta, below:



And scientifically, the Mississppi Delta has experienced a fair amount of study. Coleman (1988) provided a nice summary of the evolution of the Delta, demonstrating the complexity and extreme variability of the individual delta lobes. The image below is from Coleman (1988, his Figure 2 on p. 1000), and is the iconic illustration of how quickly the individual delta lobes of the Mississippi system switched location:


Coleman (1988) pointed out that the Mississippi system switches the locus of deltaic deposition on average every 1500 years. And keep in mind that each of those lobes covers ~35,000 km2, and is somewhere around 15-25 m thick. That's a lot of sediment in a pretty short amount of time! These pulses of deltaic avulsion and deposition have always been ascribed to the usual suspects in sedimentology: sea-level change, sediment supply changes, and subsidence in the delta.

Interpreting how these forcers interacted with the Mississippi delta system makes up a fair component of the literature, and has provided some interesting insights and entertaining arguments for many years. A recent paper by Blum et al (2008) has revealed a previously unknown driver of change within the deltaic system: cyclic uplift and subsidence driven by changing sediment volumes in the lower Mississippi valley.

Blum et al (2008) point out that the subsidence recorded along the Gulf Coast is different, depending on where you measure it. The figure below is from Blum et al (20088, their Figure 1 on p. 676). Notice how the Alabama and Texas coasts are pretty different from the Valley edge subsidence patterns. Of course, this has been recognized before. Tornqvist et al (2004) interpreted this signal as a result of ongoing glacio-isostaic adjustments. Using marshland peats as baselines, and correcting for the subsidence pattern, Tornqvist et al (2004) reconstructed a sea-level curve for the Mississippi delta.



However, an unexpected result of the Tornqvist model was a phase of "unacceptably high" rate of uplift in the peat benchmarks during the mid-holocene, corresponding to a mid-Holocene sea-level high. Tornqvist et al (2004) did not think that a phase of such large-scale uplift was vary realistic, and discounted it.

However, Blum et al (2008) may have identified a viable mechanism for rapid uplift and subsequent subsidence in the Mississippi Delta. Using the same data points and subsidence curves as Tornqvist et al (2004), Blum et al (2008) preformed a series of 1-D and 3-D isostatic modelling exercises that explain the observed uplift pattern (shown below is their Figure 3, on p. 677).



They interpret a phase of melt-water discharge during the last interglacial as having driven erosion and sediment removal out of the lower Mississippi Valley, followed by a period of Delta construction and valley filling. According to their isostatic models, this 2-phase erosion and then construction in the Lower Mississippi Valley produces up to 9 m of uplift that would effect 150 km of coastline! In other words, the sea-level signal recorded in the Mississippi Delta is a relative sea-level curve (of course), but in addition to having to deconvolve eustasy and sediment compaction, we also have to care about erosion and sedimentation in the attached lower Mississippi Valley as a cause of isostatically driven surface deflection! Pretty neat (and complicated)!

Blum et al (2008) point out that this isn't a Mississippi-only thing, either; deltas are attached to rivers, and in the big ones, we need to be aware of what the record of sedimentation and erosion is. In other words, changing the sedimentary volume drives not only the source-to-sink mass balance of clastic delivery, but can also have an effect on uplift and subsidence patterns in the system.

WORKS CITED:

Blum, M.D., Tompkin, J.H., Purcell, A., and Lancaster, R.R., 2008, Ups and downs of the Mississippi Delta: Geology, v. 36, p. 675-678.

Coleman, J.M., 1988, Dynamic changes and processes in the Mississippi Delta: Geological Society of American Bulletin, v. 100, p. 999-1015.

Tornqvist, T.E., Gonzalez, J.L., Newsom, L.A., Van de Borg, K., De Jong, A.F.M., and Kurnik, C.W., 2004, Deciphering Holocene sea-level history on the U.S. Gulf COast: A high-resolution recrod from the Mississippi Delta: Geological Society of America Bulletin, v. 116, p. 1026-1039.

Book Cliffs Channelform

Just a quick post of a nifty photomosaic from a roadcut, north of Price, UT. Behold (to quote The Bard: "Methinks that thou shouldst Clicketh, and lo! It doth have a greater size")!

A nice, simple, straightforward, channelform complex, with a couple-or-three accretionary macroforms forming the bulk of the channelform sandstone body. Note the differential compaction of the underlying coals in relation to the hefty sands that got emplaced over it!

"The Thing on the Fourble Board" - Halloween Geology!

It's Halloween again and, just like last year, I've got some geology- themed horror for you all to enjoy! As we all know, Science is the unholy pursuit of Knowledge That Man Was Never Meant To Know. And the MOST blasphemous of all sciences is geology, seeing as how we seek to understand the inner workings of untold epochs and the secret histories of Earth's unimaginably ancient past; I mean, that's a given, right? An exploration of the horrors that lurk in the dark recesses of the ancient Earth form the center piece of the radio play "The Thing on the Fourble Board", an episode of the horror show "Quiet, Please" from 1948 (downloadable from the link I just gave you).

The radio show takes place on an oil derrick (as evidenced by the the title: a "fourble" is a catwalk on a derrick that's four pipelengths high off the bushing) somewhere in Pennsylvania. After drilling deeper than anyone had ever drilled before, the rough necks come across...something from deep within the Earth.

Anyway, "The Thing on the Fourble Board" is probably one of the best, legitimately spooky pieces of horror radio out there (especially the weird vocalizations of the The Thing itself). And it takes place on an oil derrick (and, come to think of it, would be one hell of a "Safety Moment" back at Oil Company HQ). Anyway, sit back and enjoy some Halloween themed geology with "The Thing on the Fourble Board".

Happy Halloween!

Hawk vs Duck

Just a quick picture, taken from the loading bay in our department, of a hawk, enjoying a leisurely meal...

...actually, it's a little ominous, I reckon.

Kiwi Research Information Service

Lookit here! There's a one-stop gateway to all sorts of tasty, free (i.e., SOCIALIST), open-access research documents from a whole bunch of New Zealand research institutes anduniversities. This, of course, includes a fair bit of geology relating info, too. Nifty and convienient, which is what I would expect from Middle Earth.

Above: New Zealand Petroleum Research Institute

Remember when Science was rad?

Whew! A busy start to a semester, what with seminars and teaching and paper work and writing and oh yeah GSA isn't that far off so gotta get that talk done (well, started, actually). In amongst all this bullshit that you've got to put up with in schoolin', it's sometimes hard to keep sight of the reason why you got yourself into the mess that is grad school in the first place, you know?

There's a series of Op-Eds at The New York Times website, ostensibly for incoming freshmen, about how to get the most out of your college experience. All of them are written by big-wig academic types with hugely famous reputations, and some of the advice is a mixed bag (and some is hilariously predictable, given the writer, fer' instance: Harold Bloom thinks people should read the classics. Who'd a thought!?!). Anyway, there is kind of a neat one by Nancy Hopkins, on the excitement of Your Chosen Field, which is pretty good, I think.

Now get back to work!

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

SedPortal

Well, sadly, I'm back from the field...however, having to deal with the dismal, post-field depression is somewhat mollified by the soothing, minty balm that is the geoblogonetosphere. ANYWAY, just thought I'd point out a recent (?) addition to that august electron-based sed/strat portion of the web that I (and a few others) reside in. Boyan Vakarelov is a lecturer in Sed/Strat at the University of Adelaide, and also runs the website SedPortal, which has some nifty stuff. Welcome to the HiveMind, Boyan!

Nifty stuff from the Utah Geological Survey

Hup! Just a quick post about this thing that I recently found. The Utah Geological Survey has made up some of them there KMZ files that the Google Earth Folks are so hyped-up about, letting you drape a 3-D geological map over the terrain around St. George, Utah. Pretty spiffy, really. They also have a virtual field trip that makes use of the crazy 3-D map, which is a pretty neat thing. All in all, a pretty cool use of the ol' Google Earth.

Anyway, back to work!

Might as well post a picture, while I'm at it...

Since I'm in town, using that there internets to do my patented "Last Minute Abstract Submittal" to GSA, I thought I'd slap up a picture of some nifty soft-sed deformation. Here you go (in the words of the Prophet, "Click it, and it shall be Bigger"):



This picture is from some Cretaceous deltaic stuff exposed on the Rock Springs Arch, a little east of that shining beacon of metropolitanism, Rock Springs Wyoming. A picture of this very outcrop is in the "Roadside Geology of Wyoming", making it famous, I guess. That same book probably also tells you the name of the unit it's in, which escapes me now (I THINK it's Mesaverde Group, but I can't get any finer scale than that).

Anyway, you can see the nice folding going on in these slippery, mudrock and sandstone prodelta interbeds. If you look REALLY close, near the core of the fold, there's a little red stripe of something...that's my rockhammer for scale. The metal post on the right is a Speed Limit Sign, which is maybe 6 or so feet tall? So it's a good sizes feature. If you're every heading out on I-80, this outcrop is right on the westward-side of the interstate, a few miles before you get to Rock Springs proper. Take a look!