Relative-age dating of transported regolith and application to study of landform evolution in the Appalachians

Abstract Surficial deposits in the Appalachians generally are thin, discontinuous, and difficult to date. In the absence of numerical dates, particularly for older deposits, relative-age indices based on degree of weathering and soil development have been used to distinguish and correlate deposits of different ages. A favorite locale of relative-age studies has been stream terraces, where chronosequences have been developed to shed light on the relationship between time and weathering/soil development. The lack of applicable numerical dating techniques has led some researchers to establish “relative” chronosequences in which height of stream or fan terraces above modern river level (ARL) serves as a proxy for time. Approximate incision rates can then be used to estimate numerical ages of the deposits. Relative-age correlations between the weathering indices on the terraces and comparable indices on dated deposits on the Coastal Plain and in nearby glaciated regions suggest that ages so estimated are the right order of magnitude. Relative-age indices show correlations with height ARL that are as high as those with time, perhaps because more data sets are available for relative chronosequences than for true chronosequences. Analogous relative-age sequences can be established for alluvial fans and even for hillslope colluvium. Many weathering indices have been employed, but weathering-rind thickness on mafic-to-intermediate clasts has proved to be a particularly consistent and useful parameter. In addition to showing how weathering and soil development progress over time, relative-age dating has shed light on the manner in which landforms evolve. Along some rivers, for example, the areal extent of old, highly weathered alluvium far exceeds that of younger alluvium, suggesting that formerly floodplains and low terraces were much more extensive than at present. In addition, the manner in which weathering indices on some river terrace sequences vary with height ARL suggests that incision rate has changed through time. Relative-age mapping of alluvial fans produces a map pattern of older and younger fan surfaces that allows the sequence of fan development to be inferred, in one case suggesting a scenario different from those previously described for fans. Mapping also shows that the relative abundance of young, intermediate, and old fan surfaces varies greatly from one area to another. Several studies have demonstrated clustering of relative-age values into a small number of groups, suggesting temporal grouping and thus episodic deposition on fans. Regression of weathering-rind thickness against height above modern drainageways of remnant fan surfaces in two areas suggests that fans are being incised more rapidly in one area than in another. Relative age of hillslope colluvium also aids geomorphic interpretation. Young brown colluvium is much more common than old red colluvium near the glacier border, where red colluvium rarely occurs at the surface but typically is covered by brown colluvium probably mobilized during the late Pleistocene. Distant from the ice margin, however, red colluvium is extensively exposed on interfluves. Relatively stable hillslopes can also be distinguished from active ones in this manner. For example, colluvium on hillslope noses is somewhat more weathered than that on hollow floors and sideslopes.