Stratigraphic, Structural, and Correlation Considerations 15 



Petroleum Geologists, the Geological Survey of Canada, the Geological 

 Society of America, and the United States Geological Survey met at Chi- 

 cago under the chairmanship of R. C. Moore to discuss reorganization 

 and improvement of the Ashley report. As a result of this meeting the 

 American Commission on Stratigraphic Nomenclature was founded. The 

 purposes of the commission 



. . . are to develop a statement of stratigraphic principles, to recommend 

 procedures applicable to classification and nomenclature of stratigraphic units, 

 to review problems in classifying and naming stratigraphic units, and to for- 

 mulate expression of judgment.^ 



Distinction between time, time-rock, and rock units must be recognized 

 by geologists before satisfactory stratigraphic concepts can be harmon- 

 iously discussed. Renz '^ ably clarifies this necessity by saying : 



During the last decade, a number of geologists and stratigraphers in the 

 United States have strongly advocated adopting uniformity in stratigraphic 

 nomenclature and following more closely the original definitions of the terms 

 to be used. Incorrect application of terms in stratigraphic geology causes con- 

 fusion and misunderstanding, thereby impeding or even nullifying a clear con- 

 ception of the stratigraphic conditions of areas to be studied from available 

 publications. ... In setting up the stratigraphy of a given area, a clear dis- 

 tinction has to be made between the classification of rocks into lithogenetic 

 units of various magnitudes, such as groups, formations, and members, and the 

 classification of the same rock sequences into time-stratigraphic units delimited 

 by the vertical ranges of fossil life; such time-stratigraphic units are termed 

 "series," "stages," "zones," etc. The corresponding time units, such as epoch, 

 age, and secule (moment), express the interval of time during which these 

 stratigraphic units were deposited. 



The establishment of lithogenetic units is the domain of the field geologist 

 who maps them in the field according to the physical expression of the rocks 

 only, without special reference to the stratigraphic range of the fossils they 

 may contain. The paleontologist and biostratigrapher, on the other hand, build 

 up their classification into time-stratigraphic units by studying the vertical 

 range of fossil life. The classifications arrived at independently by the field 

 geologist and by the biostratigrapher may, but often do not, coincide. In gen- 

 eral, lithogenetic units have a rather limited geographic extent and are useful 

 for correlation over comparatively small areas only. On the other hand, time- 

 stratigraphic units are prone to exceed the geographic extent of lithologic 

 units and, therefore, are more useful for regional or even interregional cor- 

 relations. 



To promote better understanding of these stratigraphic terms, the 

 American Commission on Stratigraphic Nomenclature has recommended 

 the "following three classes of stratigraphic units: (1) "time units" for 

 divisions of geologic time, (2) "time-rock units" for divisions of rocks 

 segregated on the basis of their relation to determined segments of geologic 

 time, and (3) "rock units" for divisions of rocks segregated on the basis 



^Organization, "nd Objectives of the Stratigraphic Commission: Am. Assoc. Petroleum Geologists 

 Bull., vol. 31, no. 3, pp. 513-518, Mar. 1947. Prepared by R. C. Moore. 



' Renz, H. H., Stratigraphy and Fauna of the Agua Salada Group, State of Falcon, Venezuela: 

 Geol. Soc. America Mem. 32, pp. 1-219, 1948. 



