Comments on: 150 years of changes of the Chandeleur barrier islands

By: BrianR

BrianR — Sat, 06 Feb 2010 18:33:13 +0000

Angie, I recommend that you go to your college’s library (or local library) and find the appropriate geologic maps for that area. Whenever I want to know what kind of rocks are in a certain place, that’s what I do. If those libraries don’t have that information then you might consult your state’s geological survey. Good luck with the project.

By: Angie

Angie — Sat, 06 Feb 2010 04:41:09 +0000

Do you have a current blog? I am working on a research paper for a Geology college course. I am looking specifically for information on whether there are igneous rocks on the Barrier Islands off the Coast of Mississippi. I’m not finding any definitive information. Would you be able to direct me? Thank you, Angie

By: BrianR

BrianR — Tue, 14 Oct 2008 03:15:15 +0000

swess … ah yes, that makes sense … the delta lobe avulses and then the sediment is redistributed into the barrier islands.

thanks for sharing your expertise

By: swess

swess — Mon, 13 Oct 2008 20:11:41 +0000

They primarily formed after the delta lobe was abandoned, at which point the wave energy overcame the influence of sediment deposition, so technically they developed from 2.5ka to present.

By: BrianR

BrianR — Mon, 13 Oct 2008 16:05:33 +0000

swess … thanks much for the comment … so, the Chandeleurs were constructed primarily during the St. Bernard lobe, which ceases ~2.5 ka?

By: swess

swess — Mon, 13 Oct 2008 15:56:53 +0000

As a delta progrades into open water it gradually loses efficiency (ie. slope) and will naturally seek a more efficient flow path. At this point the main channel of the river will divert to another course. A modern example is the Atchafalaya River. During the 50′s and 60′s it became apparent that if nothing was done that the Mississippi River would change course and favor the Atchafalaya. This spurred the construction of the Old River Structure to limit flow through the Atchafalaya to 30% or less. A consequence of this is that most of the sediment suspended in the river’s water is dumped off the continental shelf, doing no good for the subsiding and storm tattered wetlands along the coast.
The delta plain consists of several individual delta lobes that developed from 7ka to the present. The lobe that supplied sediment for the Chandeleur Islands was the St. Bernard delta which was active from about 4k to 2.5ka, at which point deposition shifted to the LaFourche lobe (think straight south of Baton Rouge). When the sediment supply is cut off, wave energy becomes the dominant force in the system and winnows the sediments, concentrating sand and carrying away clays and silts. The sand piles up and forms a barrier island. The Chandeleur Islands are in danger now because so much of their sand has been scattered onto the sea floor where wave energy is unable to round it up again and push it onto the shore. What you see in the latest pictures is the remaining marsh platforms standing in open water with all the sand removed. The marsh platforms are resistant to erosion because of their clay composition, but sand is easily carried away by storms. After hurricane Katrina in ’05 the islands did not look much different from after Ike, but the persistent scattering of sand will definitely have an effect on their long term existence.

By: Kim

Kim — Fri, 10 Oct 2008 16:46:21 +0000

There were a couple of good posters about the Chandeleurs at GSA on Wednesday. (Coastal process posters during the free beer.) I’ve lost my abstract cd (argh), and I don’t have the program with me. But I remember one study was by the USGS, and another study was by a woman from the University of… Virginia, I think?

By: BrianR

BrianR — Wed, 08 Oct 2008 21:50:25 +0000

Chris … yes, all the modifications of the river morphology (e.g., removal of sinuous bends, bank stabilization, etc.) have had a huge impact on sediment delivery/distribution. I don’t have a reference at my fingertips regarding differences in volumes of sediment … I’ll try and dig something up.

By: Chris Rowan

Chris Rowan — Wed, 08 Oct 2008 17:50:39 +0000

In this particular instance, isn’t the barrier island problem something to do with the way they’re trying to maintain the current river channel with levees, etc.? So all the sediment basically by-passes the wetlands/barrier islands and gets dumped further out in the Gulf of Mexico?

By: BrianR

BrianR — Wed, 08 Oct 2008 17:17:44 +0000

Thanks for the links Eric.

Regarding this very specific example of the Chandeleurs, the thing to remember is that those deposits are interpreted to largely be from an older delta lobe … that is, when the main branch of the river was delivering sediment to that location. It has since shifted (if I remember, about 2,000 yrs ago) to it’s current position. So, the point about less sediment delivery is also about *where* it is being delivered in addition to how much.

I could very well be mistaken on this delta-lobe history … I didn’t look it up … anyone feel free to correct me or add more info.