USING NATURAL TRACERS TO INFER THE GEOMORPHIC EVOLUTION OF THE EASTERN HIGHLAND RIM, TENNESSEE

Well rounded vein quartz pebbles (RQP) derived from the Pennsylvanian conglomerates and sandstones that cap the Cumberland Plateau are common on the Eastern Highland Rim. These pebbles act like tracers in a sense, although they were inserted not at a point in a geomorphic system, as is the case in artificial tracer studies, but at a plane some 285 m above the general elevation of the Highland Rim. The purpose of this study was to determine the variation in abundance of the pebbles over an 11-quadrangle area. We thought that the results might help us to address the question of just how the present surface of the Eastern Highland Rim came about. Was it formed by vertical lowering as the overlying rocks gradually weathered away, or was the surface simply left behind as the Cumberland Plateau Escarpment retreated to the southeast? If the former, RQP should be common in residual and colluvial materials. If the latter, considerably greater lateral transport is implied, and only streams are capable of the required distance of transport, so that pebbles should be associated mainly with alluvium. Deposits in the first case should be more uniform in distribution, and are likely to be somewhat older than those in the second. In the second case, distribution should be nonuniform, associated mainly with stream deposits. Abundance of RQP at a site was determined by taking a sample of surficial sediments and sieving out all clasts between 8 and 16 m in intermediate diameter, washing them, and then sorting them into quartz and nonquartz (mainly chert, with some sandstone), weighing the two groups, and then dividing the weight of quartz clasts by the weight of the total of the two groups. The percentage ranged from 0 to more than 50. Percentages generally were somewhat greater in streams and on high terraces, supporting the idea that the Eastern Highland Rim developed by backwasting.