Transcript McFadden and Weldron (1987) paper synopsis

Synopsis of
McFadden, Leslie D., and Ray J. Weldon, II, 1987: Rates and processes of soil
development on Quaternary terraces in Cajon Pass, California. Geological Society
of America Bulletin, v. 98, p. 280-293.
Cajon Pass, the San Andreas Fault, and I-15.
The Terraces of Cajon Pass
Clays in youngest terrace soils are
almost entirely kaolinite.
Vermiculite becomes more
abundant in the older soils. This
latter is the normal clay to form
in the current climatic regime.
Iron oxides in the youngest terraces
are also hematite-dominated, but
ferrihydrite [Fe5O3(OH)9] is more
common in older soils.
Mineralogy of the silts in the soils does
not faithfully reflect the mineralogy of
the plutonic and metamorphic rocks
exposed in the drainage basin.
Silt content in the soils increases
dramatically with age, and is always
highest near the surface.
The east-to-west Santa
Ana winds blow dust
from the Mojave Desert
to the west, bringing
the silt, kaolinite, and
hematite to the Cajon
Pass area (as well as
elsewhere).
November, 2014, dust storm blowing out of the Mojave Desert.
The gravels of the terraces, initially too porous to retain water, gradually
accumulate enough eolian "fines"(the silts) that they can retain water,
that then allows chemical weathering to get underway, which yields the
vermiculite and ferrihydrite as secondary minerals. The pedogenic
threshold for this is about 8000 years. In other words, that's how long it
takes for enough silt to accumulate to have a significant effect.