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.

Death Valley Day 0.5 - Devonian Spring Mountain Carbonates

For about ten days, eight grad students (myself one of them) and two profs bopped around Death Valley, taking in the absolutely phenomenal geology of the place. Honestly, Death Valley is sort of a mecca for geologists. It pretty much has everything you could want: Proterozoic-Cambrian transition, the Sauk and Tippecanoe super-sequences, miogeoclinal carbonates (plus a few lonely siliciclastics here and there), extensional tectonics (turtlebacks and offset fans!), some of the slickest alluvial fans anywhere (both in section and on the surface), and evaporative playas. This should come as no surprise because, as my grandmother used to say, “For good geology, gimme the basin-and-range any day of the week!” (not really).

Anyway, we flew into the festering pit of decadence and hubris that is Las Vegas, and immediately made our way for the Spring Mountains, where we set up camp just below the snow line. Come dawn, we found that we’d pitched our tents next to some pretty neat outcrops, so our first stop was an hour of poking around some pretty nifty carbonate parasequences, shown in the picture below.


This picture was actually taken by a friend of mine, with a camera that I understand cost approximately $6,000,000,000, and therefore, takes very large and very good pictures. Anyway, you can see two “cycles” capped by thick, dark grey limestones. The lower portion of a cycle is made up of thinnly bedded carbonate with some disseminated sponge spicules and rare floating quartz grains, whereas the upper capping unit is made up of thick carbonate mud and silicified stromatoporoids, which were a spongey sort of reef builder in the Devonian. They’re shown in the picture below, again courtesy of my friend with the golden camera (I was selfishly saving my limited memory card for some alluvial fan stuff).


At the VERY top of the “cycle” picture above, we found a calci-clastic unit that exhibited some pretty nice (you know, for limestone) hummocky- and swaley-cross stratification. I think the picture below was mine, actually.


So the cycles seemed to be showing a lower, thinnly bedded limestone that was separated from overlying reef deposits by a sharp surface, which was itself sharply overlain by storm influenced reworked carbonates. Maybe this represents a shallowing upwards carbonate parasequence? Alternatively, I guess it could also maybe show a back-reef, reef, fore-reef trend? This is why I like siliciclastics; they do what the hydrodyanamics tells them to do.

Anyway, I think I’ve used up my allotted digital volume of images as dictated by Blogger, so I’ll have to post the rest of Day One later.

Milankovitch Cycles and Stratigraphy

Part of my research deals with the controls on sedimentation, and how stratigraphic packages of rock can be used to reconstruct patterns of erosion, sediment transport, and deposition. This area of sedimentological research has been going on for quite some time, of course, with lots of different workers producing lots of data (and interpretations) that seem to point in many different directions. One of the frameworks employed has been the interpretation of repeatable packages of sedimentary rocks (i.e., stratigraphic cycles) as having been caused by Milankovitch cycles in the climate. Wikipedia has a pretty good summary of Milankovitch cycles at: http://en.wikipedia.org/wiki/Milankovitch_cycles.

Milankovitch cycles attempt to explain how variations in the orientation and orbital pattern of the Earth result in changes in the amount of incoming solar radiation (insolation). The original work (Milankovitch, 1941; translated in 1969) uses this theoretical framework to explain the cyclicity observed in glacial-interglacial periods (the Ice Ages). That’s all well and good, and I think it holds up pretty well when used to explain the past few hundred thousand years of ice dynamics, since the mechanism (insolation changes) can be reasonably hypothesized to strongly effect ice volumes. What I’ve always had a problem with seems to be the desire to extend the Milankovitch cycle into the stratigraphic record where we KNOW that there were no large scale ice sheets (The Eocene, or the Cretaceous, for example).

Finally, what are the actual impacts in a SEDIMENTOLOGICAL sense that these Milankovitch cycles would be expected to have? How would they modulate erosion or transport, and what would their impact be on the rock record? So far, I haven’t found a Milankovitch-supporter who can adequately provide me with an answer.