FIGURE 13.9 Use of weathering rinds to date and correlate glacial deposits in seven areas in the western United States. As shown by decreasing curvature of lines, rind thickness is assumed to increase logarithmically with time (from Colman and Pierce, 1984). Open circles are for deposits independently dated at West Yellowstone and for deposits correlated on basis of soils and other criteria with 140,000-yr-old deposits at West Yellowstone. Solid circles are for other glacial deposits plotted on the appropriate curve according to their rind thickness.

thickness increases with stratigraphic age. Local calibration by numerical dating indicates that rind thicknesses increase logarithmically with time. Based on a logarithmic increase in rind thickness and on the assumption that deposits with a certain relative moraine form and degree of soil development correlate with deposits of the Bull Lake glaciation at West Yellowstone (oxygen-isotope stage 6), ages can be estimated for all the deposits in seven glacial successions in the western United States. Deposits representing isotope stages 2 and 6 apparently are present in all areas sampled, but moraines representing stages 3 and 4 were apparently obliterated in many areas by glaciation during stage 2 (Figure 13.9).

Recent developments in the study of desert varnish suggest that systematic changes in varnish properties, such as decreasing ratios of leachable cations to manganese, occur with time (Dorn, 1983). With local calibration, age estimates on faults in desert environments may be obtained by this method.

Soil Development

On land, soil development is nearly always pertinent to estimating the age of deformation. Soil development is a function of climate, parent material, organisms, topography, and time. If all the factors other than time can be held constant, the effect of time on soil development can be isolated and used to calibrate soil development with time at other sites.

Recently, Harden (1982) devised the soil “Profile Development Index” based on quantification of standard field descriptions of soils, including such features as color, clay content, texture, and soil-horizon thickness. Each of 10 or so soil properties is objectively quantified for each soil horizon on a scale that goes from zero to the maximum observed development. For the Merced, California, area, an individual soil property such as rubification (reddening and brightening of soil colors) shows a progressive increase with time from 100 yr to more than 1 m.y. (rubification, Figure 13.10A). The Profile Development Index (Figure 13.10B) combines several soil properties such as texture, pH, dry consistence, and soil structure and shows the cumulative effect of the development of many soil properties with time. Dating by this soil Profile Development Index is improved by using only soil properties that show the highest correlation with age (see Index of four best properties, Harden and Taylor, 1983).

Although calibration of soil Profile Development Index with age is best restricted to local areas where climate and parent material are the same, soils from four different areas of the United States appear to show similar Profile Development Index values with increasing age (Figure 13.10C). The soil Profile Development Index should prove useful in estimating ages of deformation, for it is based on readily describable field properties, provides an objective numerical basis for comparison between soils, and eliminates the need for subjective estimates of a soil’s “development.”



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