Page 200 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 175

Aeolean desert style denudation is characterised by extreme fluctuations in
temperature and wind. Rainfall is sporadic and low with periods of flash
flooding during which ephemeral streams are reactivated and both small-scale
and major mass movements may occur. Individual fans are small and may
pass downward into desert floor environments with internal drainage,
including playa lake environments (Boggs, 1987). Fans in arid to semi-arid
regions may join together laterally to form bajadas along mountain fronts.
Aeolean desert style deposition is characterised by extreme fluctuations in
temperature and wind. Rainfall is sporadic and low with periods of flash
flooding during which ephemeral streams are reactivated and both small-scale
and major mass movements may occur. Slope angle and length, the manner in
which individual mass flows split or converge, and the differential fluidity that
follows an uneven distribution of seepage and run-off direct the movement of
materials along the lines of least resistance and govern sorting relationships on
the fans.
Hillslope profiles show a great number of shapes of which two contrasting
models, one for humid climates the other for arid climates are described
schematically in Fig. 3.20. The weathering profile for the humid climate
commences typically on a rounded summit on which the effects of rain-
splash and soil creep are dominant. Barren material is removed both as
sediment and as dissolved solids. With the passage of time the rounded
summit will increase in area while the lower part of the profile flattens.
Colluvium built up at the base of the slope will further reduce the relief;
ultimately the topography will be lowered to a level of peneplanation.
The initial profile in the arid environment is assumed to be steep and straight.
Throughout all stages of denudation the steep angle of the slope profile element
remains constant but retreats toward the divide in successive parallel profiles.
The pediment widens as the base of the steep element recedes until a pediplain

3.20 Hillslope evolution with decreasing relief in humid and arid climates (from
Strahler and Strahler, 1992).