Stratigraphic, Structural, and Correlation Considerations 17 



of objective characteristics deemed to have significance in classification, a 

 differentiation not based on time relations. (See figs. 5 and 6.) 



Time units, involving eras, periods, epochs, and ages, are defined 

 indirectly and somewhat indefinitely on the basis of time-rock units. They 

 represent time spans. 



Time-rock (time-stratigraphic) units, including systems, series, and 

 stages, have time boundaries only and are represented by sediments de- 

 posited during time intervals. Stratal thickness is not involved. The 

 boundaries of time-rock units are essentially established on the basis of 

 paleontology. Hedberg ^ comments: 



Fossils, of course, constitute one of the best means of both correlating and 

 dating rocks; because of the more or less orderly evolutionary sequence of 

 life forms on the earth (worked out, however, only through relation of fossil 

 occurrences to the succession and superposition of strata), they constitute by 

 far the most effective means of setting up a chronological system of time- 

 stratigraphic divisions. However, there are limits to the resolving power of 

 fossils as chronological indicators. While sediments differing in age by twenty 

 million years, for example, may be readily placed in their correct sequence by 

 an experienced paleontologist, smaller differences in time become progressively 

 more difficult to place correctly, and a limit is finally reached where facies 

 variations and otlier factors completely mask the changes in fossil record due 

 to difference in age. 



Numerous other features besides order of superposition and paleontology 

 can contribute evidence of relative age. Among these are radioactive measure- 

 ments, relations to diastrophic events, evidences of volcanic activity, climatic 

 changes, unconformities, sedimentary cycles, transgressions, and regressions. 

 Many of these may, in special cases, become of outstanding importance and 

 exceed in value all other means. However, only fossils (and perhaps radio- 

 active measurements) are of much service in determining complete and world- 

 wide geochronological sequences. . . . In short, it is desirable to be able to ex- 

 press as a time-stratigraphic unit the sediments equivalent in age to the time 

 scope of any recognizable features of sedimentary rocks which may be useful 

 as a stratigraphic measuring stick. 



Relationships between time-rock and rock boundaries are difficult to 

 establish, and many are impossible to evaluate accurately. 



Time surfaces may be defined by (1) careful study of stratigraphic 

 sections containing lithologies and faunas common to two or more con- 

 trolled stratal sequences; (2) "walking out" of key beds such as ash, ben- 

 tonite, and limestone; (3) correlation of benthonic faunas possessing wide 

 ecologic valence; (4) application of pelagic faunas and floras; (5) widely 

 dispersed detrital minerals; (6) vertical limits of faunal sequences; and 

 (7) biologic evolutionary changes. 



Rock units ( lithogenetic units), including the group, formation, 

 member, lentil, tongue, stratum, and layer, are defined on the basis 

 expressing structural conditions, and deciphering the geologic history of 

 of lithology. These units are essential in geologic mapping, description, 



* Hedberg, H. D., Time-Stratigraphic Classification of Sedimentary Rocks: Geol. Soc. America Bull., 

 vol. 59, pp. 447-462, May 1948. 



