Part IV: Clastic Depositional Environments.

A. Alluvial Fans.

B. Fluvial systems (Rivers and streams)(See Chp. 8 in Boggs, p. 215-220):

1.) Fluvial systems transport sediment into sedimentary basins (sedimentary basin = low area that can receive and store sediment)
2.) Fluvial systems are aggradational  (they fill low area)
3.) In map view channels range from: almost straight, high sinuosity, to braided channel patterns.

a. Straight channels- rare, occur in mud-rich sediment and occur in jointed rocks.
b. Anatomizing channels -- complex pattern.

i.) Low energy streams flowing through mud-rich material
ii.) Channels often separated by large, stable, vegetated islands.

c. Meandering channels:

i.) Asymmetrical flow velocity and turbulence causes erosion on outside of bends.
ii.) Mixed sand-mud systems

**See Fig. 8.9 in Boggs.

d. Braided channels:

i.) Overall relatively straight channel complex
ii.) Flow weaves around sand bars during low flow
iii.) Channel sand bars submerged during high low stages

** gravel-sand system

**See Figs. 8.10 & 8.11 in Boggs.

4.) Progression from source area:

Environment:	Alluvial fan	Braided Stream	Meandering stream	anastamosing stream
Lithofacies:	Conglomerate	Conglomerate and sandstone	Sandstone in mudstone	Mudstone

5.) Braided Streams: (many branching, interwoven small channels and channels overloaded with sediment).

a. No sharp bends in channel systems.
b. No cohesive banks, episodic flow
c. Constantly changing flow patterns around islands and bars

i.) Cross bedding dip directions fairly consistent down stream

d. Coarse sand-gravel system.

i.) Bar, dune, ripple migration fills channels; the sand bodies produced = Laterally extensive and sheet-like sand bodies (See Fig. 8.11 in Boggs).

e. Relatively little suspension deposition **Little or no shale!

f. Characteristics Stratigraphic packaging

 

6.) Meandering Streams:

a. Stream confined to one main sinuous channel with cohesive banks

b. Maximum flow velocity thread (thalweg) corresponds to the outside of bends and cuts diagonally across straight reaches (see Fig. 8.9 in Boggs).

c. In the meander bends maximum turbulence (greatest erosion) occurs near the base of the channel against the outer bank = cut bank.

d. Therefore, maximum erosion of the outer bank (cut bank) results in stream widening and lateral migration. Sediment eroded from one cut bank is carried downstream and deposited on the inside of the bends (Point bars).

e. Point bar growth results in lateral and vertical accretion (aggradation and pro graduation) and produces cross bedding with many dip directions.

f. Because they aggrade and prograde point bar deposits become finer-grained through time.

g. Lower gradient, lower velocity, finer sediment load than braided streams.

h. During flood stage mud deposited on flood plain and Levees build upward.

i. Form ribbon-like sand bodies (See Fig. 8.12 in Boggs).

j. Meandering streams often break-out of original channels = Avulsion.

Next: C. Eolian Systems.