Running Water - The Changing Earth: Exploring Geology and Evolution - page 312

much larger, far more complex, and consider-
ably more important as potential areas of natu-
ral resources. In fact, depending on the relative
importance of stream (or river), wave, and tide
processes, geologists identify three main types of
marine deltas (

◗

Figure 12.13 Floodplain Deposits

Deposition
(lateral accretion)

Erosion

Figure 12.15). Stream-dominated
deltas have long fi ngerlike sand bodies, each de-
posited in a distributary channel that progrades
far seaward. The Mississippi delta is a good ex-
ample. In contrast, the Nile delta in Egypt is wave
dominated. It also has distributary channels, but
the seaward margin of the delta consists of is-
lands reworked by waves, and the entire margin
of the delta progrades. Tide-dominated deltas are
continuously modified into tidal sand bodies
that parallel the direction of tidal fl ow.

◗

a Floodplain deposits formed by lateral accretion of point bars are made up
mostly of sand.

Alluvial Fans
Fan-shaped deposits of alluvium on land known
as alluvial fans form best on lowlands with ad-
jacent highlands in arid and semiarid regions
where little vegetation exists to stabilize surface
materials (

Floodplain

Figure 12.16). During periodic rain-
storms, surface materials are quickly saturated
and surface runoff is funneled into a mountain
canyon leading to adjacent lowlands. In the
mountain canyon, the runoff is confi ned so that
it cannot spread laterally, but when it discharges
onto the lowlands, it quickly spreads out, its
velocity diminishes, and deposition ensues. Re-
peated episodes of sedimentation result in the
accumulation of a fan-shaped body of alluvium.
Deposition by running water in the man-
ner just described is responsible for many
alluvial fans. In this case, they are composed
mostly of sand and gravel, both of which con-
tain a variety of sedimentary structures. In
some cases, though, the water fl owing through
a canyon picks up so much sediment that it
becomes a viscous debris flow. Consequently,
some alluvial fans consist mostly of debris-fl ow

◗

Channel

Natural levees

b The origin of vertical accretion deposits. During fl oods, streams deposit natural
levees, and silt and mud settle from suspension on the fl oodplain.

Floodplain

Channel

Natural levees

After fl ooding.

c

Topset
beds

Distributary
channel

Lake

Foreset
beds

◗

Figure 12.14 Prograding
Delta Internal structure of the
simplest type of prograding delta.
Small deltas in lakes may have this
structure, but marine deltas are
much more complex.

Bottomset
beds