Sediments have been deposited and destroyed continuously throughout geologic time. The writers constructed simplified models of world sediment distribution as a function of time and compared these models

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with the actual distribution. The results suggest that approximately half of the sediments existing today are younger than 600 million years (m.y.), whereas the remainder is distributed irregularly through a stratigraphic column representing 2,500 to 3,000 m.y. Such a distribution means that the total mass of sediments deposited during geologic time would have to be 4 to 6 times the existing mass and that sedimentary material is rapidly recycled forward in time. Thus, one may think of the half-mass age of all sedimentary rocks as approximately 600 m.y.; however, the half-mass age of carbonate rocks is less, about 300-400 m.y., and that of evaporites even less, about 200-300 m.y., and that of evaporites even less, about 200-300 m.y.

The relatively high percentage of carbonate rocks, and the almost complete restriction of evaporites to the post-Precambrian result from the fact that the components required to make these rocks are cycled forward at a rate 1.5 to 2 times the rock mass as a whole. Geochemical "uniformitarianism"--the concept that the total mass of sediments existing at any one time in the geologic past had about the same composition as observed today--should be considered when geological conclusions are drawn that are based on the proportions of sedimentary rock types in the geologic column.

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