Sedimentary Volumes and their Significance
Sedimentary volumes are of prime interest in many fields of geology: as measures of erosional rates, of geochemical balance, and recently, with the virtual demonstration of continental drift, as measures of movement of the continental and oceanic plates. The Basement Map of the United States, published by the U.S. Geological Survey in 1968, provides a partial basis for an improved estimate of the volume of Phanerozoic rock in the center, minous United States. The map requires correction for this purpose, because all metamorphic rocks of whatever age have been classed as basement. We have, therefore, attempted to allow for the metamorphic rocks of Phanerozoic age. We have made estimates of volumes for areas not controlled by contours on this map and have used such offshore data as we have been able to assemble from the literature in order to extend our estimates to include offshore sediments reasonably attributable to erosion from the area of the contiguous United States. Our results are as follows: We consider this estimate to be within 10 percent of the true volume. Of it, we estimate about 3.2 ×10 6 km 3 to be volcanic rock, not representing erosion of pre-existing rock. The remaining 56.8 × 10 6 km 3 , rounded to 57 × 10 6 km 3 , we consider products of continental denudation. This volume is so large, representing, as it does, only 5.3 percent of the continental surface of the earth and only a sixth of recognizable geologic time, that it appears to invalidate schemes of geochemical balance such as those of Clarke, Goldschmidt, and others. These students assume that the salt in the sea is a measure of the amount of some “average igneous rock” that has been eroded during the whole of geologic time to produce some “average sedimentary rock.” Instead, our result points strongly toward the hypothesis of Livingstone, Gregor, Earth, and others that the oceanic salt is merely the cyclic salt not yet returned to the continents in a continuing cycle. Assuming that this volume was derived from erosion of the contiguous United States—an assumption that we recognize as invalid in detail, though not seriously in error—we obtain an ostensible average rate of Phanerozoic erosion of about 10 m/ m.y., about a sixth of the present rate. But inasmuch as present erosion is attacking a surface that exposes about 76 percent sedimentary rocks and only 24 percent igneous, most of its product is recycled rather than first-cycle sediment. An analysis of the broad features of the paleo-geography of the country indicates that a similar disproportion between first-cycle and recycled sediment has been characteristic of nearly all the Phanerozoic. The ostensible erosion rate is therefore spurious, and it is likely that the average erosion rate durin g the Phanerozoic was more than half that of the present, and perhaps was nearly or quite equal to it. The great disparity in volumes of sediment offshore in the Atlantic and Pacific—in a ratio of more than 5 to 1—is consonant with expectations if the continent has been moving westward and over-riding the Pacific Basin on a Benioff fault system activated at the beginning of the Mesozoic, though now dormant.