LECTURE 18: DELTA AND COASTAL DEPOSITIONAL PROCESSES
Note: to provide continuity, the first few paragraphs below are repeated from condensed lecture notes
that were broadcast prior to Quiz 2.
On the part of a delta above sea level (the delta plain) the river system behaves as described above.
However, in a very large delta, such as the Mississippi delta, which is by far the best known and
understood in the world, more complex activity is occurring. As the river dumps sediment at its mouth
; it builds the land surface forward into what was formerly the sea. The river must flow
farther and farther to reach its mouth.
Simultaneously, the delta is
(sinking) because (i) fresh goopy mud when first deposited is
80% water, which gets squeezed out through
, and (ii) the entire delta region is subsiding by
. Dumping of sediment on the earth's crust imposes an enormous load (a million
tons of sediment every day in the Mississippi delta) formerly not present on the crust, which
consequently sinks under its own weight. Of these two processes that cause subsidence, compaction is
by far the more important.
At any one moment of geologic time the river will be depositing sediment in a local prograding
. Meanwhile the delta elsewhere is subsiding and being reclaimed by re-invasion of the sea.
Subsidence causes the gradient to the sea to become steeper, encouraging the river to abandon its
currently active delta lobe and switch into another area, there to deposit a new lobe.
For the past 600 years the Mississippi River has deposited only in a small tract, the "Birdfoot Delta." A
major distributary, the Atchafalaya River, branches off the master stream and it reaches baselevel (the
Gulf of Mexico) in one-third the distance as that traveled by the master stream. The Atchafalaya is
threatening to take over as the master channel (leaving New Orleans abandoned), and it already would
have done so if the Corps of Engineers had not installed a system of restraining locks and dams.
Growth, abandonment, subsidence, and reoccupation accompanied by renewed deposition, which are
processes of the
, proceed continually in a large delta.
As fresh but muddy Mississippi River water mixes with salty Gulf of Mexico water, the tiny clay
particles clump together and settle to the bottom as an enormous mass of mud, which with burial will
transform into shale. Because the deposit is offshore, in front of the delta plain, it is called the
Marshes between distributaries grow with dense vegetation that, upon burial, may transform into lignite
or coal. In the late Paleozoic when the Ouachita Mountains occupied central Texas, rivers drained
toward the west, building deltas into an inland sea. Marsh deposits in these former deltas are now mined
Sediment is transported along a coast in a