Stratigraphic, Structural, and Correlation Considerations 69 



rock units. Some indicators applicable for local or even regional corre- 

 lation work include coprolites; fish teeth and scales; pollen; spores; 

 ooliths; chert phases; glauconite, bentonite, and ash layers; detrital min- 

 erals; limestone, coal, and anhydrite beds; cyclothems; and lithic and 

 paleontologic sequences. For interregional and intercontinental correla- 

 tions pelagic Foraminifera, fusulinids, orbitoids, and ammonites have been 

 used with varying success. Evolutionary trends in certain species and 

 genera are notably applicable in certain long-range correlation problems. 



Correlation Difficulties 



Some of the more commonly encountered difficulties in correlation 

 work are (1) the discontinuity of outcrops; (2) lateral variations in thick- 

 ness and lithology; (3) the interval variation between key strata; (4) the 

 presence of unrecognized unconformities and faults; (5) the lack of lith- 

 ologically and paleontologically controlled sections; (6) the multiplicity 

 of time-rock and rock nomenclature; and (7) erroneously compiled and 

 interpreted data obtained from the literature. 



Upon entering a new area, these factors should be carefully analyzed. 

 All possible lithologic, paleontologic, and structural data should be 

 screened and coordinated with the intention of defining intervals and sur- 

 faces which will foster improved correlations. 



Questions 



1. Define stratigraphy. 



2. Distinguish between microstratigraphy and macrostratigraphy. 



3. What are the purposes of the American Commission on Strati- 



graphic Nomenclature? 



4. Define "time unit," "time-rock unit," and "rock unit." 



5. Carefully read Hedberg's statement pertaining to stratigraphic 

 units. 



6. On what basis may "time surfaces" be defined? 



7. What considerations are necessary for naming a sedimentary for- 

 mation? 



8. What recommendations are made by the American Commission on 

 Stratigraphic Nomenclature for naming and defining subsurface 

 units? 



9. Define "sedimentary facies" and "lithofacies." 



10. What is the importance of evaluating sedimentary-facies changes? 



11. What are the two major types of unconformities and what im- 

 portance is attached to these features in stratigraphic geology? 



12. Give ten criteria for recognition of unconformities. 



13. Faults may be recognized on what criteria? 



14. What is meant by marine onlap and off lap? 



15. Study carefully the schematic diagrams illustrating the various 

 types of oil and gas traps. 



