32 Subsurface Geologic Methods 



Adams ^^ admirably discusses the facies changes within the upper- 

 most Permian deposits of southwestern United States. 



Dunbar -^ reports that the Kazanian series about Perm (Russia) 



is represented by a bright-red, unfossiliferous sandstone and siltstone some 500 

 feet thick. This is the nonmarine Ufimian facies, so called for the city of 

 Ufa, which is on the strike south of Perm. In conspicuous contrast, the equiv- 

 alent beds in the environs of Kazan are richly fossiliferous white limestone. 



The Upper Cretaceous of the Rocky Mountain region of Colorado and 

 Wyoming offers excellent examples of facies changes wherein the section 

 becomes more arenaceous and less marine from east to west. 



Improving Interpretation of Stratigraphic Sequences 



Confusion and disagreement in stratigraphic studies often arise as a 

 result of one or more of the following: (1) projection of stratigraphic 

 nomenclature of one area into another without sufficient intervening con- 

 trol; (2) failure to distinguish between rock and rock-time units; (3) 

 publication of stratigraphic material by those who are inexperienced or 

 who have drawn conclusions based on inadequate data; and (4) failure 

 to recognize the importance of structural and depositional idiosyncrasies. 



The evaluation of stratigraphic sequences has been greatly improved 

 by application and coordination of the following studies: (1) detailing 

 of surface and subsurface sections; (2) subsurface logging techniques; 

 (3) detailed paleontologic and mineralogic analysis; (4) improved lab- 

 oratory and field procedures; and (5) better understanding of sedi- 

 mentational principles. 



Projects devoted to the study of clay minerals, bacteria, organic and 

 carbonate sediments, formation waters, recent and ancient sedimentary 

 processes, diagenesis, and well logging, as now being sponsored by the 

 American Association of Petroleum Geologists, will contribute greatly 

 in more accurately evaluating stratigraphic sequences. 



Unconformities 

 W. ALAN STEWART 



Unconformities are of prime importance in problems of strati- 

 graphic, sedimentational, structural, and historic nature. They are the 

 natural basis for separating both rock units and units of geologic time. 

 The economic importance of unconformities is constantly increasing be- 

 cause of their relationship to oil accumulation and to ore deposition. 

 The occurrence, development, extent, type, and relationships of uncon- 

 formities are therefore of major concern in petroleum and mining de- 

 velopment programs. 



Since the detection of unconformities in the subsurface is of para- 

 mount value to the stratigrapher and economic geologist, much emphasis 



^' Adams, J. E., Upper Permian Ochoa Series of Delaware Basin, West Texas and Southeastern New 

 Mexico: Am. Assoc. Petroleum Geologists Bull., vol. 28, no. 11, pp. 1596-1625, Nov. 1944. 



^"Dunbar, C. O., Permian faunas: A Study In Facies: Gaol. Soc. America BulL, vol. 52, p. 313-32, 

 1941. 



