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Active Tectonics: Studies in Geophysics
Obsidian hydration has been used to date the last two glaciations in the Rocky Mountain region (Pierce et al., 1976). If temperature and chemical composition are constant, hydration thickness increases proportional to the square root of time. Figure 13.8 shows the increase in hydration with time as determined by the hydration thicknesses of two K-Ar-dated rhyolite flows and carbon-14-dated recessional deposits. The deposits of the next to last, or Bull Lake, glaciation date at about 140 ka (Pierce et al., 1976). This age for the Bull Lake glaciation is about 5 times older than the age considered correct 20 yr ago. The importance of the dating at West Yellowstone to studies of active tectonism is that these ages can be inferred for many deposits throughout the western United States if they can be correlated with
FIGURE 13.8 Obsidian-hydration dating of the Pinedale and Bull Lake Glaciations near West Yellowstone, Montana (from Pierce et al., 1976). Dashed line shows increase in hydration thickness with age based on hydration thickness measured on deposits dated by K-Ar or 14C methods. Above histograms, short lines with dots are means and standard deviations of hydration-thickness measurements, some of which are offset (small arrows) to account for small temperature differences between localities.
those at West Yellowstone on the basis of criteria such as soil development, morphologic changes, and weathering rinds (see Figure 13.9).
Amino Acid Racemization
Amino acid racemization (and epimerization) has provided important age information for deciphering late Quaternary deformation along the West Coast of the United States (Lajoie, Chapter 6, this volume). Because racemization rates for a given species depend on temperature and a kinetic model (Wehmiller, 1982), the method works best if calibrated by numerical methods. On the California coast, uranium-series dating of corals has provided a few calibration points, but even with this calibration the amino acid ratios on mollusk samples did not allow distinction between three global sea-level culminations known from elsewhere to date at about 80, 95, and 125 ka. The problem of distinguishing these three high-sea stands has been resolved by combined studies using amino acid and uranium-series dating, temperature gradients along the coast, and paleontological identification of cool (oxygen isotope substages 5a or b) and warm (substage 5e) faunas (Lajoie, Chapter 6, this volume).
RELATIVE-DATING METHODS, COMPLEX PROCESSES
This group of dating methods includes some of the most widely applicable methods (Table 13.1, column 5). Numerical ages can be empirically estimated by these methods. Rigorous evaluation of these complex methods would require modeling of each process and quantification of their relative effects. Nevertheless empirical quantification has been done, and some age estimates based on these methods (Table 13.1, column 4) may be more reliable than, if not so precise as, some carbon-14 ages.
Rock and Mineral Weathering
Rock and mineral weathering (Table 13.1, column 5) includes such relative-dating techniques as mineral grain etching, seismic velocities in weathered stones, pitting on stone surfaces, and weathering rinds.
Weathering rinds on basaltic and andesitic stones from the B horizons of soils have yielded age information on middle and late Quaternary deposits at seven different areas in the western United States (Figure 13.9; Colman and Pierce, 1981). Multiple measurements of rind thicknesses from a given stratigraphic unit are consistent; and, for a succession of deposits, rind