Crust of the Earth: A Symposium
Theories as to time relations of orogeny and epeirogeny are only as strong as the available evidence; this evidence leaves much to be desired. In many regions total effects of orogeny and epeirogeny are apparent but not the stages by which they were built up; in others, superimposed structures demonstrate the episodic nature of the process without indicating a correlation of the episodes with the standard time scale. Chief evidence for orogenic and epeirogenic episodes therefore remains the sedimentary record—the occurrence of unconformities and of various types of clastic sediments. In many places this evidence is available only in the marginal zones of mobile belts (as in the Southern Appalachians) and may not reflect details of orogenic history in the interior of the belt. Where evidence is available in the interior of the belt (as in the Maritime Provinces) it is less complete for the main orogeny than for the minor succeeding episodes. A further difficulty is lack of precision in correlation of the sedimentary record between different regions. Block faulting in the Rio Grande area of New Mexico, epeirogenic uplift of the Rocky Mountains in Colorado, and compressional deformation of the Coast Ranges of California all seem to have occurred near the Pliocene-Pleistocene boundary, but whether they are simultaneous or closely spaced separate episodes cannot be determined.
Much of the evidence regarding time relations of orogeny and epeirogeny is negative and can be interpreted in various ways according to the theories or predilections of the geologist. The writer’s predilections are that orogeny and epeirogeny were episodic rather than continuous; that episodes affected fractions of continents rather than whole continents or all continents; and that episodes may be expressed in one place by compression of the crust, in another by tension, at one place by orogeny, at another by epeirogeny.