Abstract

Dr D. H. Kupper writes:In geology we are constantly trying to reconstruct the past from the present, and in the process we use all the help we can get. Thus the ‘balanced cross-section’(Hossack 1979) is a valuable tool, but we must keep firmly in mind the assumptions that are involved.Hossack has pointed up a few of these, but many geologists still hold the end result in too great faith. In essence, the balanced section is just an improvement on the old process of unfolding a geological section as described, for example, by Chamberlin (1910).

When the unfolding process was first used it received much attention; less so in recent years, because it is now realized that most folding is not a simple case of contraction. Carey (1962) showed that a fold can form even if a section is expanding. In the balanced section, the attempt is made to correct for this potential error by measuring the ‘original thickness’ of a bed in the undisturbed foreland. The area of the present, observed geologic cross-section is then ‘balanced’ against this to calculate the ‘original length’, instead of finding it by unfolding (L = A/T).

As pointed out by the father of the geosynclinal theory, James Hall (1859, pp. 67–73), beds in the orogenic belt are commonly10-20 times thicker than those in the foreland. Much more important, Cooper (1957) demonstrated that geosynclinal beds change thickness rapidly fromp lace to place, being many times thicker in the depositional lows (depo-centers), than in the adjacent

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