Part IV: Clastic Depositional Environments.

A. Alluvial Fans.

B. Fluvial systems (Rivers and streams).

C. Eolian (Aeolian) depositional systems (chp. 8 in Boggs, p. 220-228):

1.) Arid climate (usually)

a. Also form when retreat of glaciers exposed silt and sand to wind (deposit = loess)

b. Alluvial fan, braided stream and glacial association

2.) Diagnostic features:

a. Nature of cross bedding:

i.) Large scale!
ii.) Cross bedding dips steeply: up to 35°, but 20-25° angles typical for ancient deposits.
iii.) Cross bed dip directions are highly variable.

b. Nature of the grains

i.) Very fine to medium sand usually (silt = loess and can have medium coarse sand)
ii.) Excellent sorting well to very well sorted
iii.) Each layer within the dune (= foreset) may exhibit Reverse Grading

c. Ripple marks that are oriented parallel to cross bed dip direction

d. Ventifacts: pebbles, cobbles, etc… carved by wind and sand

e. Dunes: type depends on consistency of wind direction and strength, and the supply of sand (see Fig. 8.16)

i.) Constant direction

-Abundant sand leads to Transverse dunes
- Lower sand supply leads to Barchan dunes

ii.) Shifting wind direction:

• Longitudinal, star dunes.

f. Ripple Lamination and inversely graded strata (see Fig. 8.15).

Next: D and E. Deltas and Marine Systems.