Geotimes
Highlights
Mapping
Kevin M. Schmidt

Geologic maps of Earth's surface can be templates in forecasting landscape change. Surficial geologic maps are vital to understanding and predicting the effects of climate and associated hydrologic changes, monitoring human impacts on landscapes, understanding how ecosystems function, and mitigating the effects of geologic hazards. Surficial geologic maps are multipurpose and are necessary to evaluate diverse landscape attributes, particularly when the maps are combined with databases of physical properties associated with the deposits. As landscape responses are tightly coupled to climatic and hydrologic stresses, sedimentary sequences provide invaluable records of past climate, which in turn can be used to infer possible consequences of future climate variability or anthropogenic influences, such as changes in the land cover.

The Kingston Range debris-flow fans. See text for description. Figure courtesy of Kevin Schmidt.


Recent efforts to forge interdisciplinary ties to forecast landscape change with respect to anticipated droughts (e.g., Brent Newman et al., 2003; Kevin Schmidt and Robert Webb, 2001), have determined that a vital foundation for these predictions is knowing the distribution of surficial materials. At the same time, despite their importance, detailed surficial geologic maps are lacking for much of the United States. Historically geologic mapping was focused on bedrock geology. Much of the East Coast has good coverage of surficial geologic maps but the Central and Western U.S. has only sporadic coverage.

Landscapes and their associated ecosystems are interwoven physical, hydrological and biological systems that are affected by the presence and type of surficial materials. Current projects within the U.S. Geological Survey (e.g., deserts, climate change, and Mojave Desert Ecosystem) aim to document ecosystem processes, climate and hydrologic history, hazard susceptibility and surface process information at a variety of scales by preparing a series of multipurpose surficial geologic map databases. The maps and associated databases contain information on the temporal and spatial patterns of surface processes and hazards, including material properties and vegetation cover, with which the databases can be used to model specific landscape responses.

Our interdisciplinary understanding of the links between the magnitude and duration of climate fluctuations and geomorphic and biotic change are rapidly improving. Documenting the physical properties of deposits through mapping provides critical baseline information on the first order availability of water and nutrients for biota. For example, the spatial distribution of surficial deposits influences vegetation distribution, ecosystem function and relative susceptibility to fires. Constraining baseline information provides opportunities to forecast ecosystem response to climate variability, but the responses are driven by such diverse influences as topography, lithology, soil thickness, ecosystem type, sediment-transport process, nutrient availability, and degree of recovery from the last disturbance. Locally, vegetation responds to climate change via soil moisture that is largely regulated by the distribution of surficial materials. Unfortunately, assessing the rate and magnitude of a landscape's response to a disturbance remains relatively elusive and qualitative because a response occurs over longer time periods than the duration of historic records. Additional surficial geologic mapping supported by robust dating can extend historical records, allowing quantitative evaluation of forcing functions (such as climate variability) and of the corresponding erosion and deposition of material.

Mapping of surficial geology can document the spatial distribution and timing of deposits related to specific sediment-transport processes. Along with information on deposit thickness, the geologic record can be used to estimate deposit volumes and relative activity of transport processes over time, and thereby used to predict future deposition rates (see the sidebar). For example, even under the presently arid climate, alluvial fans in the southwestern United States episodically flood and deposit sediment. Recent research in the Kingston Range of the Mojave Desert, California has combined geologic mapping, surveying, and geographic information system (GIS) modeling to distinguish debris-flow deposits and their relative ages, to estimate volumes of age-stratified deposits, and to infer minimum watershed-scale erosion rates. The Kingston Range (Fig. 1a) has numerous debris-flow fans located near the outlets of steep watersheds (Fig. 1b). Mapping of debris-flow deposits (Fig. 1c) provides a means to determine the amount of material transported during time intervals representative of different climatic cycles. Debris flows are similar in age to nearby fluvial deposits suggesting that floods and debris flows were active simultaneously. Field mapping identified historic debris-flow deposits containing asphalt and an automobile (roughly 1930's vintage), underscoring the relevance of the study. Distances traveled by debris flows can also be inferred from mapping; one massive debris flow lies 25 km from the source rock! Pleistocene debris-flow deposits are more voluminous than Holocene deposits, but when time-averaged, the Holocene rates exceed Pleistocene rates because they are averaged over shorter time periods. By comparing the geologic record of deposits and contemporary estimates of landslide susceptibility from GIS modeling, it is possible to constrain forecasts of debris flow activity in response to climatic or land use disturbances.

Although landslide rates are low in most areas, landsliding within shallow soil on steep hillslopes is highly sensitive to vegetation changes. In mountainous areas of managed land, the impacts of vegetation die-off arising from mega-drought (Stephen Gray et al., 2003), timber harvest or fire may weaken root strength within granular soils (Joshua Roering et al., 2003). Decreased root reinforcement can heighten regional landslide susceptibility, particularly during large storms. Forecasts of landsliding benefit from mapping of both source material and deposits through geologic time and from dating to constrain deposit ages. They also benefit from time-series analyses of remote-sensing imagery to document the distribution of regolith and vegetation type and health.

The investigation of surface processes to evaluate interdisciplinary landscape dynamics diversifies the use of geologic maps. A comprehensive geologic map can have many unique applications, and through the use of GIS techniques geologic maps are no longer static documents (e.g., Geotimes 2002). Rather, derivative maps, created for specific purposes, can be generated from an original geologic map database and tailored to any particular application. Ongoing research efforts aim to develop remote sensing techniques for extending mapped surficial geology and physical properties into unmapped areas and produce derivative maps of paleohydrology (shallow groundwater, spring activity, lakes), geologic hazards (surface rupture via faulting, deep-seated landsliding, debris flows), and sediment transport by surface water and wind through space and time.

Learn about U.S. Geological Survey mapping programs aimed at understanding ecosystem changes.

Visit:

Our Dynamic Desert Web site
Climate change and land use
Where desert meets city


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Schmidt is a research geologist with the U.S. Geological Survey in Menlo Park. E-mail: kschmidt@usgs.gov.

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