Geology topics

Leslie D. McFadden

Publications and source records attributed to Leslie D. McFadden.

6 recordsLinked to original sources

USGS2022

Landscape change, fire and erosion

New Mexico has a dynamic landscape, which will become even more so in response to climate change over the next 50 years, in part because of increasing incidence of wildfire. As the climate changes to warmer conditions, less rainfall will infiltrate into aquifers, leading to increased overland runoff. Landform processes can be complex, but in general, the predicted changes in climate and precipitation will lead to increased flooding, increased upland erosion caused by runoff, and increased downstream sediment deposition. Canyons and mesas, small basins or valleys filled with sediment will be particularly affected. Rapid rearrangement of sediments by water is disruptive and potentially hazardous to ecosystems and societies. Dramatic examples of accelerated erosion following the Whitewater-Baldy, Las Conchas and other wildfires here in New Mexico illustrate the types of hazards created when forested landscapes are severely burned. Post-wildfire erosion is typically initiated by intense rainfall events. Given that both the number of wildfires, and rainfall intensities are likely to increase as the climate warms, New Mexico can expect to see increases in widespread erosion and sedimentation across and downstream from upland forested areas in the state. The large volume of sediment predicted to be on the move will be of concern for many reasons including filling reservoirs, choking channels, blocking or destroying infrastructure and positive feedback loops that lead to further reductions in slope stability.

New Mexico↗

USGS2022

Soils

Soils play a strong role in determining how New Mexico’s diverse landscapes will respond to climate change. Soil cover acts like a sponge, holding in water that falls as rain or snow. The presence of soil supports vegetation, and substantially reduces runoff and erosion. Soil enhances other processes such as infiltration of water and aquifer recharge. Soils can be damaged by a warming climate. Loss of vegetation in the Northwest High Desert and Eastern Plains, where soils are not well developed and easily damaged, will lead to dustier conditions in much of the state. On mountain hillslopes, the loss of vegetation cover in response to ongoing climate change will increase soil erosion, which then increases hillslope runoff. This, in turn, causes additional increases in soil erosion and bedrock exposure, which can largely prevent widespread recolonization by most plants, including trees. Soils on mountain hillslopes that face south, which are typically hotter and drier, will be damaged sooner by a warming climate than those on generally north-facing hillslopes that are slightly cooler and moister. Soils take many thousands of years to form, so these hillslopes will increasingly support sparse forests, or, in some circumstances, be entirely deforested. These changes are already well underway in some mountains in New Mexico.

New Mexico↗

USGS2017

Comparison and contrast of processes of soil formation in the San Timoteo Badlands with chronosequences in California

The degree of soil development associated with geomorphic surfaces in the San Timoteo Badlands area (STB), California allows correlation of the surfaces. Soil development indices, based on field descriptions and laboratory analysis, provide a basis for comparison of these soils to each other and to dated soil chronosequences at Cajon Pass, Merced, and Anza, California. The soils in this study record a complex tectonic and climatic history, include preserved surfaces intermediate in soil development to those at Cajon Pass, and do not preserve a record of the major late Pleistocene to early Holocene aggradational event observed elsewhere in the region. Pedogenesis is similar to that of regional soils in southern California. With time, the profiles develop progressively thicker argillic horizons, and they increase in redness, clay, and secondary iron oxide content, and amount and thickness of clay films. With increasing soil age the ratio of dithionite-extractable Fe to total Fe increases, while the ratios of Fe 2+ to Fe 3+ and of oxalate- to dithionite-extractable Fe decrease. These trends are more subdued in the STB than elsewhere in the region, perhaps because the slightly higher temperature and lower precipitation of the STB may slow the rate of transformation of ferrihydrite to hematite. We use weighted means of soil development indices and rubification index values to compare the soils of this study to those at Cajon Pass, Merced, and Anza. Wide-range age estimates using the maximum limits of these values result in 27,500–305,000 yr for surface Q1, 43,000–570,000 yr for surface Q2, and 300,000–700,000 yr for surfaces Q3 and Q4. Weighing most heavily the nearest dated chronosequences, Anza and Cajon Pass, yields best estimates of age. This method constrains the time of formation of the Q1 surface to between 27,500 and 67,000 yr and the Q2 surface to 43,000–67,000 yr, and does not change the age estimates of the older surfaces.

California↗

USGS2015

Impacts of climate change on the formation and stability of late Quaternary sand sheets and falling dunes, Black Mesa region, southern Colorado Plateau, USA

Detailed geomorphic mapping and analysis of soil-stratigraphy and optically stimulated luminescence (OSL) dating of eolian sand dunes on Black Mesa, Arizona, reveal eolian sediment deposition occurred from 30 to 16 ka, followed by a period of widespread dune stabilization from 12 to 8 ka. Localized reactivation of the previously stabilized dune forms or local changes in sediment supply have occurred in the middle to late Holocene in this region. Cooler, wetter, and more variable climatic conditions during MIS 3 and 2 led to increased channel and floodplain sediment supply. Eolian sediment derived from these sources was transported up to 60 km. Deposition of this material has reduced regional topographic roughness by filling tributary canyon ‘traps’ oriented perpendicular to the dominant wind and sediment transport direction. Topographically controlled falling dunes and sand ramps in this region are preserved because of their geomorphic position and provide evidence of the paleoenvironmental state of the fluvial and eolian systems before, during, and immediately after the last glacial maximum on the southern Colorado Plateau. Widely used predictive models of eolian system dynamics are typically based entirely on climatic variables and do not account for landscape complexity and geomorphic history. Climate-only assumptions fail to give accurate predictions of the dynamics of this and many other dune fields. A growing body of work suggests that eolian deposits in wind-driven semiarid climates may be more strongly related to increases in sediment supply than to increases in aridity.

Arizona↗

USGS1989

Development rates of Late Quaternary soils, Silver Lake playa, California

Soils formed on alluvial fan deposits that range in age from about 35 000 to 200 yr BP near Silver Lake playa in the Mojave Desert permit study of the rates of soil development in an arid, hyperthermic climate. Field-described properties of soils were quantified and analyzed using a soil development index that combines properties and horizon thicknesses. We compared the variability in index values from five independent descriptions of the same soil profile with the variability in values from four soils of the same age described by different people. The variability due to description accounts for between 30 and 80% of the total variability (that due to both the soils and description) for the properties described, which emphasizes the need for consistent soil descriptions. Pedogenic CaCO 3 (as indicated by color), pH increase, and dry consistence appear to change with age at linear rates, whereas rubification appears to change at a logarithmic rate. The linear rates are best attributed to the progressive accumulation of CaCO 3 - and salt-rich eolian dust derived from the playa and other more distant sources. The total-texture values of soils on fans older than 10 000 yr BP are similar, which suggests that playas in this area may have been wet enough to restrict the availability of fines from these sources for many thousands of years prior to 10 000 yr BP. Index values for most properties of soils on the lower fans adjacent to the playa increase at faster rates than those of soils of the same age on the upper fans farther from the playa, probably due to the proximity to this local dust source. Equations derived from regressions of soil age and properties can be used to estimate ages of undated, lithologically similar deposits in similar climates and geomorphic settings.

California↗

USGS1985

Rate and depth of pedogenic-carbonate accumulation in soils: Formation and testing of a compartment model.

The rate and depth of pedogenic carbonate accumulation in soils formed in Quaternary alluvium may be viewed as a theoretical problem that involves the mutual interaction of several independent and dependent soil-forming variables. We propose a model for carbonate accumulation in which the soil column is defined by a vertical sequence of 1-cm 2 -area compartments, each with a specified texture, bulk density, water-holding content, lithologic and mineralogic composition, soil-air pCO 2 , ionic strength, and temperature. On the basis of these data, rates of carbonate solubility and dissolution within a given compartment are determined. In arkosic to lithic arkosic sandy parent materials, high carbonate solubility (0.137 to 0.212 mg/ml) and the large reactive surface area of eolian calcareous dust result in rapid carbonate dissolution (0.79 to 9.92 × 10 −10 g/cm 2 /sec) that promotes rapid translocation of carbonate by infiltrating water. We derive a group of equations and use them to calculate net carbonate depletion or accumulation in a soil compartment over an interval of time as a function of the independent variables temperature and precipitation. These two variables largely determine or strongly influence soil-water balance, the external carbonate influx rate, and carbonate solubility. The carbonate distribution that our model predicts closely resembles the observed carbonate distribution in soils associated with Holocene deposits forming in arid, hyperthermic to xeric, thermic moisture-temperature regimes in southern California. This modeling indicates that with a mean carbonate influx rate of 1 × 10 −4 g/cm 2 /yr and in a semiarid, thermic climate, the maximum depression of the top of the Cca horizon is attained within only a few thousand years. In contrast, given the same influx rate, our model predicts that a noncalcareous B horizon cannot form in an arid, hyperthermic climate, a conclusion supported by field and laboratory studies of calcic soils in this climate. The influence of glacial-to-interglacial climatic changes on carbonate accumulation can be modeled by calculating latest Pleistocene soil-water balance with the aid of published estimates of full-glacial temperature and precipitation. On the basis of these modeling results, we propose that either of two types of glacial-to-interglacial climatic changes may account for the strongly bimodal, apparently polygenetic carbonate distribution that is observed in late Pleistocene soils of the eastern Mojave Desert of southern California. Such results of compartment-strategy modeling are encouraging and indicate the great potential of combined theoretical and empirical methods for considering pedological problems of interest to Quaternary geologists.

GSA Special Papers↗