Soil geomorphology and change over time: A case study from the Catawba River, North Carolina
Shortcut URL: https://serc.carleton.edu/60213
Location
Continent: North America
Country: USA
State/Province:North Carolina
City/Town: Charlotte
UTM coordinates and datum: none
Setting
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy, Chronology
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Figure 4. Depth profile plot of clay content (%) for representative soil profiles on terrace units. Details
Description
Despite their value in Quaternary studies, relatively few soil chronosequences or long-term landscape evolution studies exist for the Piedmont physiographic province of the southeastern United States. Investigating how landscapes have changed in the past is important in order to better understand potential environmental change in the future. The geomorphic properties of soils develop under the influence of several environmental factors including climate, organisms, relief, parent material and time (Jenny, 1994). A soil chronosequence is defined as a sequence of soils for which soil development varies as a function of time. Using this paradigm, soil geomorphology can be employed as a tool for mapping, correlating and assigning relative ages to Quaternary deposits as well as for understanding landscape evolution (e.g. Mills, 2005). Where the ages of deposits and soils are known, soil properties can be used to develop chronofunctions, which quantitatively describe the relationship between time and soil development. This case study presents a chronosequence of soils on five fluvial terraces (Qt1-5; Fig. 1) from the Catawba River near Charlotte, NC. From soils data, chronofunctions were established to investigate the long-term geomorphic history of the Catawba River watershed. The chronofunction results suggest a unique, dramatic change in the sediment source of the Catawba River between 50,000 and 128,000 years ago.
Soil properties were described from ten soil profiles (two descriptions per terrace). Analysis of soil morphology included descriptions of horizon thicknesses and boundaries, color, structure, gravel content, consistence, roots and pores, texture, clay films, as well as sedimentary descriptions (Birkeland, 1999). Terrace ages (Fig. 2) were determined by comparing the height of each terrace above the modern river channel with regional age/elevation curves (Mills, 2000).
Color hue, iron content and clay content recorded positive trends with increasing terrace age (Fig. 3). The rate of development of these soil properties flattens/plateaus after ~128 ka. Color hue, measured with a Munsell color chart, reddens over time because of the continual accumulation of iron oxides in the soil. The rate of iron oxide formation flattens over time because there are fewer fresh mineral surfaces available to weather (e.g. Birkeland, 1999). Clay content in soils typically increases with time because of 1) weathering processes that decrease particle size and 2) illuvial processes (vertical water-assisted transport) that move clay particles down the soil profile. The plateau in clay content likely indicates a threshold, such that the high clay content (~60%) prevents further illuviation.
Changes in iron content are commonly expressed through the iron activity ratio, which is the ratio between amorphous iron oxides and crystalline iron oxides. Generally, this ratio decreases progressively with time as amorphous iron oxides convert to more stable crystalline forms. In the terrace soils, iron activity ratios and clay contents both record a break in soil development at Qt4 time (~50 ka) (Figs. 3 and 4). For example, there are unexpectedly low iron activity ratios for Qt4 and Qt5 soils and unexpectedly high clay contents in Qt4 soils. Low iron activity ratios and high clay contents normally indicate older, well-developed soils. The presence of well-developed soil properties in the youngest two terraces suggests that these properties are likely inherited rather than developed in-situ (see below). These results suggest a unique, dramatic change in the sediment source of the Catawba River (see below). We also observed sedimentological changes over time from cobble gravel facies, which were evident in older deposits (Qt1-2), to sandy facies in younger deposits (Qt3-5).
The apparent increase in soil development at Qt4 time reflected in the chronofunctions of both clay content and iron ratios indicates that sediment deposited at that time largely came from the erosion of older, previously weathered materials in the Catawba watershed. We suggest that between Qt3 and Qt4 time (~128-50 ka), the primary sediment source for the Catawba River switched from relatively unweathered bedrock to relatively well-developed soils stripped from hillslopes and/or from the erosion of older terrace deposits in valley bottoms. Such a switch represents a major change, in terms of sediment source, in the overall landscape evolution of the Catawba River basin at that time. Also, the lack of coarse gravels in Qt3 – Qt5 deposits suggests that the Catawba River also experienced a decrease in stream power during this time period, so that the river could no longer transport coarse cobble gravel clasts.
In conclusion, by investigating soil geomorphology, fluvial landforms and sedimentological changes, we can make some tentative assertions concerning the geomorphic history of the Catawba River. This history can be summarized as one of overall incision punctuated by periods of aggradation that created terrace fill deposits. A change from cobble to sand facies implies a reduction in stream discharge between ~128 ka and 50 ka. The development of certain soil properties in terrace deposits (i.e., iron oxides and clay) record a break in soil development around 50 ka, which is likely caused by the presence of reworked older, previously weathered material. These results suggest there was a significant change in the sediment source of the Catawba River at this time, namely, from the erosion of relatively unweathered bedrock to relatively well-developed soils within the basin. These data represent some of the first insights into the long-term soil and landscape evolution of a major drainage in this region of the Piedmont.
Soil properties were described from ten soil profiles (two descriptions per terrace). Analysis of soil morphology included descriptions of horizon thicknesses and boundaries, color, structure, gravel content, consistence, roots and pores, texture, clay films, as well as sedimentary descriptions (Birkeland, 1999). Terrace ages (Fig. 2) were determined by comparing the height of each terrace above the modern river channel with regional age/elevation curves (Mills, 2000).
Color hue, iron content and clay content recorded positive trends with increasing terrace age (Fig. 3). The rate of development of these soil properties flattens/plateaus after ~128 ka. Color hue, measured with a Munsell color chart, reddens over time because of the continual accumulation of iron oxides in the soil. The rate of iron oxide formation flattens over time because there are fewer fresh mineral surfaces available to weather (e.g. Birkeland, 1999). Clay content in soils typically increases with time because of 1) weathering processes that decrease particle size and 2) illuvial processes (vertical water-assisted transport) that move clay particles down the soil profile. The plateau in clay content likely indicates a threshold, such that the high clay content (~60%) prevents further illuviation.
Changes in iron content are commonly expressed through the iron activity ratio, which is the ratio between amorphous iron oxides and crystalline iron oxides. Generally, this ratio decreases progressively with time as amorphous iron oxides convert to more stable crystalline forms. In the terrace soils, iron activity ratios and clay contents both record a break in soil development at Qt4 time (~50 ka) (Figs. 3 and 4). For example, there are unexpectedly low iron activity ratios for Qt4 and Qt5 soils and unexpectedly high clay contents in Qt4 soils. Low iron activity ratios and high clay contents normally indicate older, well-developed soils. The presence of well-developed soil properties in the youngest two terraces suggests that these properties are likely inherited rather than developed in-situ (see below). These results suggest a unique, dramatic change in the sediment source of the Catawba River (see below). We also observed sedimentological changes over time from cobble gravel facies, which were evident in older deposits (Qt1-2), to sandy facies in younger deposits (Qt3-5).
The apparent increase in soil development at Qt4 time reflected in the chronofunctions of both clay content and iron ratios indicates that sediment deposited at that time largely came from the erosion of older, previously weathered materials in the Catawba watershed. We suggest that between Qt3 and Qt4 time (~128-50 ka), the primary sediment source for the Catawba River switched from relatively unweathered bedrock to relatively well-developed soils stripped from hillslopes and/or from the erosion of older terrace deposits in valley bottoms. Such a switch represents a major change, in terms of sediment source, in the overall landscape evolution of the Catawba River basin at that time. Also, the lack of coarse gravels in Qt3 – Qt5 deposits suggests that the Catawba River also experienced a decrease in stream power during this time period, so that the river could no longer transport coarse cobble gravel clasts.
In conclusion, by investigating soil geomorphology, fluvial landforms and sedimentological changes, we can make some tentative assertions concerning the geomorphic history of the Catawba River. This history can be summarized as one of overall incision punctuated by periods of aggradation that created terrace fill deposits. A change from cobble to sand facies implies a reduction in stream discharge between ~128 ka and 50 ka. The development of certain soil properties in terrace deposits (i.e., iron oxides and clay) record a break in soil development around 50 ka, which is likely caused by the presence of reworked older, previously weathered material. These results suggest there was a significant change in the sediment source of the Catawba River at this time, namely, from the erosion of relatively unweathered bedrock to relatively well-developed soils within the basin. These data represent some of the first insights into the long-term soil and landscape evolution of a major drainage in this region of the Piedmont.
Associated References
- Birkeland, P.W., 1999. Soils and Geomorphology, 3rd edition. Oxford, New York, p. 430
- Jenny, H., 1994. Factors of Soil Formation: A System of Quantitative Pedology. Dover, New York, p.191.
- Layzell, A.L., Eppes, M.C., Lewis, R.Q., 2011. A soil chronosequence study on terraces of the Catawba River near Charlotte, NC: Insights into the long-term evolution of major Atlantic Piedmont drainage basin. Southeastern Geology (in press).
- Mills, H.H., 2005. Relative-age dating of transported regolith and application to study of landform evolution in the Appalachians. Geomorphology, 67, 63-96.
- Mills, H.H., 2000. Apparent increasing rates of stream incision in the eastern United States during the late Cenozoic. Geology, 28 (10), 955-957.