Part IV: Clastic Depositional Environments.

Photo of Chugater Fm, Wyoming

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

2.) Fan-shaped landform.

a. Base of mountains (maybe associated with retreat of valley glaciers) where steams emerge onto a plain.

3.) Alluvial fans form where there is a sharp decrease in stream energy and thus a dramatic drop in competency occurs.

photo of modern alluvial fan, Colorado

photo close up of alluvial fan

competency diagram

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 facies 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.

Photo of proximal facies

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.

photo of distal facies


diagram showing alluvial fan model

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

succession diagram Diagram showing general stratigraphic style associated with alluvial fans.

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

Next: B. Fluvial Systems.

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