Mk. Eghbal et Rj. Southard, STRATIGRAPHY AND GENESIS OF DURORTHIDS AND HAPLARGIDS ON DISSECTED ALLUVIAL FANS, WESTERN MOJAVE DESERT, CALIFORNIA, Geoderma, 59(1-4), 1993, pp. 151-174
Presumed slow weathering rates in arid-region soils have given rise to
the hypotheses that (1) the major carbonate source in most Aridisols
is from eolian deposition, and (2) soil horizons cemented by carbonate
s and opaline silica are indicators of geomorphic surfaces of mid-Plei
stocene age. To test these hypotheses we studied the stratigraphy and
genesis of four soils, a Typic Durorthid (Alko series), two Typic Hapl
argids (Neuralia and Garlock series and a Typic Torripsamment (Cajon s
eries), found in close association on a dissected alluvial fan in the
western Mojave Desert. The study area has a rolling topography with th
e calcareous Alko soil occurring on narrow summits and on the edges of
broad summits, Neuralia (calcareous) on broad summits and on sideslop
es, Garlock (noncalcareous) on sideslopes, and Cajon in the drainage w
ays. Soil stratigraphy in an excavated trench indicates that these soi
ls are forming on deposits of differing ages. Paleomagnetic data indic
ate that at least four aggradational events have occurred in the past
783 ka (thousand years). The sequence of alluvial units in the area is
similar to other sequences observed in the Mojave Desert and eastern
San Joaquin Valley. The Garlock soil, with 5YR hues and a distinct car
bonate-free argillic horizon, occurs on an older geomorphic surface, b
ut younger deposit, than the Alko and Neuralia soils. Rodents are acti
ve in the profiles of Alko and Neuralia soils. Bioturbation of the upp
er soil mantle brings calcareous soil and duripan fragments to the sur
face, thus creating a pedogenically rejuvenated geomorphic surface on
which profile development has been retarded. The occurrence of calcare
ous Haplargids adjacent to Haplargids that are noncalcareous to a dept
h of 65 cm, indicates that the atmospheric input of CaCO3 in the study
area has been minimal during the Holocene. The rate of CaCO3 accumula
tion in soils of the study area is estimated to be 0.03 to 0.05 g cm(-
2) 1000 yr(-1) over the past 200 ka. A rate of accumulation of 0.27 g
cm(-2) 1000 yr(-1) is required to form the 2-cm-thick laminar cap of t
he upper Alko duripan during the Holocene. This amount could easily be
added to the surface by bioturbation from calcareous duripan fragment
s in the profile. Thus, much of the laminar part of the duripan is pro
bably a Holocene phenomenon.