Senin, 20 Juni 2011

DEFINITION OF FLUVIAL

Meaning “of the river,” referring to different and

diverse aspects of rivers. The term may be used to refer to the

environment around rivers and streams or, more commonly,

to refer to sediments deposited by a stream or river system.

Sediments deposited by rivers tend to become finergrained

and more rounded with increasing transport distance

from the eroded source terrain, typically an uplifted

mountain range. The sediments also tend to become more

enriched in the more stable chemical components such as

quartz and micas and depleted in chemically vulnerable particles

such as feldspars.

Stream channels are rarely straight, and the velocity of

flow changes in different places. Friction makes the flow

slower on the bottom and sides of the channel, and the

bends in the river make the zone of fastest flow swing from

side to side. The character of channels changes in different

settings because of difference in slope, discharge, and load.

Straight channels are very rare, and those that do occur have

many properties of curving streams. The thalweg is a line

connecting the deepest parts of the channel. In straight segments,

the thalweg typically meanders from side to side of

the stream. In places where the thalweg is on one side of the

channel, a bar may form on the other side. A bar (for example,

a sand bar) is a deposit of alluvium in a stream. Most

streams move through a series of bends known as meanders.

Meanders are always migrating across the floodplain by the

process of the deposition of the point bar deposits and the

erosion of the bank on the opposite side of the stream with

the fastest flow. The erosion typically occurs through slumping

of the stream bank. Meanders typically migrate back and

forth, and also down-valley at a slow rate. If the downstream

portion of a meander encounters a slowly erodable

rock, the upstream part may catch up and cut off the meander.

This forms an oxbow lake, which is an elongate and

curved lake formed from the former stream channel. Braided

streams consist of two or more adjacent but interconnected

channels separated by bars or islands. Braided streams have

constantly shifting channels, which move as the bars are

eroded and redeposited, during large fluctuations in discharge.

Most braided streams have highly variable discharge

in different seasons, and they carry more load than meandering

streams.

River and stream channel deposits tend to be composed

of sands and gravelly sands that exhibit large-scale threedimensional

ripples, shown in cross section as cross-bedding.

These cross-bedded sands are common around the inner

bends of channels and mark the former positions of point

bars, and they are commonly interbedded with planar bedded

sands marking flood stage deposits and gravelly sands

deposited during higher flood stages. The tops of channel

deposits may be marked by finer-grained sands with smallscale

ripples and mud drapes forming flaser-bedding,

interbedded with muds, and grading up into overbank floodplain

deposits. This upward-fining sequence is characteristic

of fluvial deposits, especially those of meandering streams. In

contrast, braided stream deposits show less order and are

characteristically dominated by bed load material such as

gravel and sand. They include imbricated gravels, gravels

deposited in shallow scours, and horizontally bedded sands

and gravels deposited in bars.

Fluvial channel deposits form a variety of geometric patterns

on a more regional scale. Shoestring sands form an

anastomosing pattern of river channels enclosed in overbank

shales and muds, formed by meandering and anastomosing

river channels. Sheets and wedges of fluvial sediments form in

front of uplifted mountain chains in foreland and rift basins

and may pass basinward into deltaic or shallow marine sediments

and mountainward into alluvial fan deposits. The type

of tectonic setting for a basin may be deduced by changes or

migration of different fluvial facies with stratigraphic height.

Fluvial sediments are widely exploited for hydrocarbon

deposits and also are known for placer deposits of gold and

other valuable minerals.

See also CLASTIC ROCKS; DRAINAGE BASIN; FLOOD;

RIVER SYSTEM

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