Stratigraphic, Structural, and Correlation Considerations 13 



occur disseminated throughout Jurassic and Triassic sandstones. Possible 

 minor sedimentary copper deposits are widely distributed in Texas, New 

 Mexico, Colorado, Utah, and Arizona. Manganese minerals (sulphides, 

 oxides, carbonates, and silicates) are widespread in sedimentary sections 

 west of the Mississippi Valley. Rich deposits of phosphate occur in cer- 

 tain Permian strata in Idaho and Wyoming, in the Silurian and Devonian 

 of Tennessee, and in the Tertiary of the Carolinas and Florida. Vast re- 

 serves of potash (polyhalite and sylvite) are associated with Permian 

 strata in New Mexico and west Texas. Other stratigraphically controlled 

 deposits of nonmetallics include clay, borates, sulphur, gypsum, magnesite, 

 barite, celestite, strontianite, diatomite, limestone, dolomite, slate, and 

 marble. Lead and zinc deposits occur in limestone, dolomite, and calcar- 

 eous shale in various parts of the world. The distribution of the Clinton 

 iron ores (Silurian) of the Appalachian States and the hematite deposits 

 of the Lake Superior region are governed mainly by stratigraphic fabric. 



The distribution, degree of localization, and value of many pyro- 

 metasomatic ore deposits are largely dependent upon the type of sedimen- 

 tary section intruded. Rarely are these deposits found in argillaceous 

 strata, sandstones, and shales, whereas limestones, dolomites, and calcar- 

 eous shales are more reactive and thus are most adaptable for mineral 

 concentrations. 



Many deposits formed under mesothermal conditions (mineralization 

 at 200°— 300° C.) occur in sedimentary rocks. Examples of mesothermal 

 replacement deposits involving sedimentary strata are known in the Cordil- 

 leran region of the United States and elsewhere in the world. 



Hypothermal or deep-seated (mineralization at 300°— 500° C.) ore 

 accumulations are commonly associated with highly metamorphosed sedi- 

 ments. 



Unconformities and variations in porosity, permeability, competency, 

 composition planes, texture, and chemical composition of the host rock 

 are some of the controlling factors governing the development and loca- 

 tion of ore bodies. 



From the foregoing it is obvious that the stratigraphy of a mineral- 

 ized area should be carefully evaluated during prospecting and develop- 

 ment stages. The structural aspects of a region are equally important. 

 Structural irregularities cannot be satisfactorily evaluated without knowl- 

 edge of stratigraphic relationships. Similarly, stratigraphic values may 

 be erroneously recorded if structural conditions are inadequately known. 



Subdivisions of Stratigraphic Geology 



Stratigraphic geology may be subdivided into two major categories, 

 macrostratigraphy and microstratigraphy. The former involves field ob- 

 servation and interpretation of exposed stratigraphic sequences, whereas 

 the latter implies laboratory approach and routine and detailed evaluation 



