Arid region alluvial fan development: A soil-geomorphology investigation

Amy Brock
Western Illinois University

Description

Arid and semi-arid regions cover approximately 36% of Earth's surface and are becoming increasingly significant as world populations grow. Landforms found in these regions directly affect and in turn are affected by urbanization and agriculture. Amplified urbanization of these regions has prompted an increase in building of homes, businesses and industrial sites on alluvial fans, bajadas, and other arid and semi-arid region landforms. Improper management and development may adversely influence soil stability and fertility, water quality, biological habitats and air quality. Poor planning of these surfaces may also promote flooding and increase seismic risks associated with urban development on present or future faults. Although their importance is great, our knowledge and understanding of how these landforms develop is limited relative to landforms of humid regions.

Since soil forming processes begin once a landform is stable, geomorphologists can use surface and soil characteristics to compare the development and relative age of individual alluvial fan surfaces. Differentiation between landforms is attained by comparing soil development, degree of dissection, degree of desert varnish, desert pavement development, vegetation differences, curvature, surface color and topographical position. Soil characteristics are observed by describing and sampling pedons. Soil characteristics include secondary mineral accumulation, color, boundaries between horizons, structure, and calcium carbonate morphology (Figure X). Laboratory analyses are also performed for analysis of degree of weathering and mineral identification. These analyses include texture, electrical conductivity, pH, and investigation with macro- and microscopic techniques, scanning electron microscopy and x-ray diffraction.

Two alluvial fans present in the northern Pahranagat Valley provide an ideal setting to study the effect of time and lithology on soil development of indivudual alluvial fan lobes. Two drainages, are present on two fan complexes that exhibit distinct and separate parent lithologies (Figure B). The northernmost wash drains Tertiary volcanic rocks that have provide material for the fan. This fan has volcanic surface clast percentages ranging from 59-94%. The southernmost fan has dominantly dolomite lithologies ranging from 99-100%. Each landform displays a set of surface characteristics that can be compared through time (chronosequence) and with lithology (lithosequence). Soil profiles of surfaces of the same age but with different parent lithologies can give insight into the development of soils and soil features in varying lithologies. The type of parent material that a soil forms in can greatly influence its pedogenic development.

Carbonate-rich soils found in the northern portion of the Pahranagat Valley provide an opportunity to contribute to our understanding of arid soil and landform development (Figure A[make location figure]). The Pahranagat Valley is ideal for a chronosequence and lithosequence study because a number of soil forming properties are constant. Biota, climate, and topography are consistent between landforms with local changes in lithology and temporal development. Because of this, specific changes in soil development can be attributed to lithology and/or time.

In order to compare soil development on landforms, several factors must be evaluated. These factors are topography, biota, climate, parent material and time. Depending on environment, each factor can dominate the characteristics of a soil. When all but one factor is constant, soil characteristics can be attributed to be the result of that one factor. Therefore, changes in soil characteristics are attributed to time of soil formation. In arid and semi-arid regions, calcium carbonate accumulates due to low effective precipitation. Stages of morphology of calcic soils have been used to identify the relative age of landforms. These stages range from slight coatings on pebble bottoms and filaments to massive horizons that are completely indurated with secondary carbonate with overlying laminar caps. These stages directly relate to the age of the associated landform. This provides a standard for carbonate development through time that can be correlated across arid regions of the world.

Five Quaternary landforms in the northern Pahranagat Valley range in age from Early-Pleistocene to modern (Q1-Q5) and are derived from two different lithologies of parent material. Landforms in dolomite material exhibit similar development according to surface characteristics and soil evolution through time as the equivalent aged landforms that were derived from volcanic material. The oldest landforms (Q1) are topographically highest located adjacent to bedrock mountain fronts on the eastern and northern-most parts of the study area followed by Q2, Q3, Q4 and the lowest, modern, active Q5 channels. Bar and swale is a common feature of the Q3-Q5 landforms in contrast to Q1 and Q2 that contains significant Av horizons and well developed desert pavement. Q1 landforms have undergone significant soil erosion since time of deposition. Soil development on each landform increases with increasing age with a stage IV laminar cap reached in the Q1 and Q2 surfaces. Surfaces Q3 and Q4 have stages II to I respectively and Q5 contains no significant soil development. Soil pendants also increase in thickness with increasing age. An increase with age of silica accumulation is seen on the bottoms of clasts and pendants in the Q1-Q4 landforms. Silica accumulations are not as well developed in the landforms derived from volcanic materials as the dolostone-derived landforms. The soil characteristics of these landforms vary with age and exhibit increased pedogenic development corresponding to increased age (Figure C). Complexity in carbonate, silica and clay accumulation also increase with increasing age.

Associated References:

• Brock, A.L. , and Buck, Brenda J., 2005, A New Formation Process for Calcic Pendants from Pahranagat Valley, Nevada: Implications for Dating Quaternary Landforms, Quaternary Research, v. 63, p. 359-367
• Bull, W.B., 1991, Geomorphic response to climate change: Oxford University Press, New York, NY., 326 p.
• Gile, L.H., Peterson, F.F., Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in desert soils: Soil Science v. 101, p. 347-360.

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