Part IV: Clastic Depositional Environments.

A. Alluvial Fans (p. 211-215 in Boggs, Chp. 8):

1.) Minor part of the stratigraphic record, but provide information about:

a. Paleotectonics
b. Paleoclimate
c. Source area composition

Diagram showing general relationship between distance from a mountainous source and stream competency. Alluvial fans form where the stream(s) meet the mountain front and there is a corresponding competency drop.

4.) Lithofacies:

Diagram showing general terminology (proximal Vs. distal facies) on an alluvial fan.

 

a. Proximal settings: i.) Texturally immature, very coarse-grained, angular-subrounded clasts. ii.) Conglomerates, matrix supported clasts-supported fabrics. iii.) Unstable minerals present [f (source rock)] = compositionally immature (usually) iv.) Debris flow deposits: massive, un-bedded, very poorly sorted, mud matrix. v.) Usually there is a confined permanent channel that may contain an intermittent stream.

b. Distal settings on alluvial fans. i.) Better sorting (sands can be well sorted), fine-grained. -Sandstones dominate with thin gravel layers in channels. -Minor mudstones possible. ii.) Unstable minerals still present, but amount depended on climate and source composition. iii.) Main channel branches into many side channels (braided channels). -Periodic rainfall produces sheet flows over the flat surface and mudflows can occur.

 

Diagram above shows general depositional model for ancient alluvial fans developed based on Devonian examples of Norway. From Stell and Gloppen (1980).

5.) Recognizing ancient Alluvial Fan deposits.

a. Shape not easily defined
b. Rapid lateral change in grain size
c. Poor internal stratification
d. Debris flow deposits:

i.) Mud matrix-supported conglomerates
ii.) Angular to subrounded clasts up to boulder size
iii.) No bedding

e. Paleosoil horizons:

i.) Red horizons with calcite concretions
ii.) Root traces
iii.) Mud cracks

f. Radiating pattern of paleocurrents

g. Stratigraphic “packaging” = coarsening upward

Diagram showing general stratigraphic style associated with alluvial fans.

Here are some great photos of modern alluvial fans taken by NASA astronauts:

• One from the Zagros Mountains in Iran.
• Some in Egypt.
• One in China.
• Some examples in Death Valley, California.
  
  

Next: B. Fluvial Systems.