THE DYNAMIC EARTH: A BLOG ABOUT GEOLOGY AND THE EARTH SCIENCES

Monday, November 30, 2009

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.

3 comments:

BrianR said...

Great summary ... now I don't need to read the paper :)

Steve Gough said...

Very nice work, thanks!

palmcop said...

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