Physical Geology – Deltas – Summary


>>Dr. Doug Elmore: This is the summary for
deltas. Deltas are the end of the line. In this picture of the Mississippi River and
Delta in Louisiana, you can see the Mississippi River coming down here past New Orleans just
South of Lake Pontchartrain and out to the current Birdfoot Delta down here. Deltas are
sights of deposition, or sights of progradation, where we add land. What happens is that sediment
is being carried in the river when the river enters the ocean or when the water enters
the ocean, it deposits sediment. Why? Well because the velocity slows down. When the
velocity slows down the courser material falls out of the water and you get deposition. There are several major processes that operate
on a delta, three major processes. Fluvial or river processes, wave processes, and tides.
Here in this picture you can see some of the major subdivisions of a delta. The distributary
channels right here (highlighted on map), the distributary mouth bars or DMB’s, these
are located right here (highlighted on map). They’re where the distributary channels actually
enter the ocean or the Gulf of Mexico in this case. There are delta front sheet sands, which
are spread out in front of the delta here. There’s the prodelta, which is out in front
of the delta. In between the distributary channels you find interdistributary bays.
Here’s another block diagram that shows some additional environments on a fluvially dominated
delta like the Mississippi. You have the distributary channels (highlighted in blue), you have levees
(highlighted in green), which are right next to the channels. Levees are created during
flood events. They build up a mound that makes the channel bigger and can help prevent flooding.
There are also crevasse splays (highlighted in yellow), which are places where the levee
has been breached. You can see one right here breaching occurs during major floods and the
sand and the water then spread out into the interdistributary bay. And you can also see
the interdistributary bay. In addition in the block diagram you can see the various
rock types and some of the features, the sedimentary structures that are found in these delta environments. There’s some very important processes that
operate at the distributary mouth bar and out into the prodelta. One of the most important
is the segregation of sand from mud. What happens is the sand is carried in the river
over here. When it gets to the gulf or to the ocean velocity slows down, you deposit
the sand principally the bed load, and that’s where you form the DMB. But the mud is carried
out into the ocean, goes into salt water and out there a process called flocculation causes
the clay particles to clump together, and then they’re deposited. So as a result you
have sand near the shoreline but out farther you have lots of mud. And that mud has a lot
of water in it. There is some large-scale delta processes that are very important. One
of the first ones is progradation, or adding of land. As more and more sand is deposited
it moves out into the ocean and eventually it will be deposited over mud, so you get
sand deposition on top of mud. Now, remember that mud has a lot of water in it. You put
something heavy on top of the mud and it’s going to cause compaction and that will cause
sinking. Think of a sponge with water in it. If you put something heavy like a block of
wood on top of it, it’s going to sink down, expel some of the water, but it’s going to
sink. That’s sort of what happens to the mud. Now so long as there’s more sedimentation
then there is sinking you will still get progradation, or adding of land. Eventually however, the delta will build out
so far that there’s a shorter and steeper pass to the ocean. The water will follow the
steeper and shorter path. This is called avulsion. And you can see it here in this figure. Here
is the original river and the delta and then at some point the delta, the river will avulse
and seek a shorter and steeper pass to the ocean and a new delta will form. Avulsion
is a very important process on deltas. But what are the consequences of this process?
The major consequence is that the old delta lobe sinks and the new delta lobe forms, as
seen in this figure. Why does the old delta lobe sink? Well remember that you always have
compaction on deltas. Because you have sand deposited on top of mud. If you’re no longer
getting sedimentation on the old delta because all the sediment is going to the new delta
you only have sinking. So eventually the old delta lobe will sink, but a new delta lobe
will form. This map shows the positions of seven previous
lobes on the Mississippi River. In the last 4-5,000 years the Mississippi has evolved
seven times with new lobes forming. So this is a very common process that operates on
the Mississippi Delta.