78 Subsurface Geologic Methods 



Anhydrite and gypsum beds frequently serve as dependable correla- 

 tion markers in red-bed sections. In certain instances they are quite con- 

 tinuous and uniform. The Blaine anhydrite in the Permian section of 

 southeastern Colorado is an example. 



The study of evaporite rocks should include chemical analysis; tex- 

 tural, structural, and spectrophemical investigations; thin-section analysis; 

 and rhythmic characteristics. 



Carbonaceous Rocks 



Carbonaceous rocks are represented by three types of residue : humus, 

 peat, and sapropel. Humus is produced within the upper part of the soil 

 phase. Peat originates from partial decay of plant material under fresh- 

 water swamp conditions. Sapropel (high in fatty and protein substances) 

 results from concentration of complex organic compounds which accumu- 

 late on the bottoms of lakes, lagoons, and quiet-water embayments. 



Coal is by far the most common variety of carbonaceous deposit and 

 is classified according to degree of coalification and physical character- 

 istics. Lignite is the lowest grade and anthracite the highest. Some of the 

 more common ingredients of coal include vitrain, fusain, clarian, and 

 durain. The chemical composition of coal is extremely variable. The main 

 constituents in coal are carbon, hydrogen and oxygen, nitrogen, sulphur, 

 and water. 



Sections containing coal beds generally present many complex prob- 

 lems to the stratigrapher and structural geologist owing to their extreme 

 diversity of lithologies and depositional irregularities. 



Considerable attention in recent years has been given to cyclothemic 

 sequences in coal sections. In the Midcontinent region, in the Illinois 

 basin, and in Kansas, coal cyclothems and megacyclothems have been 

 used advantageously in establishing correlations and decipering structure. 



The investigation of coals includes such analyses as thin section, 

 moisture, volatile matter, fixed carbon, and ash. 



Sapropelic deposits have been considered by some geologists to be 

 responsible for the formation of petroleum; however, there is no agree- 

 ment as to the physico-chemical processes of convfersion. 



To evaluate sapropelic rocks properly, studies involving thin-section, 

 heat analysis, and ether solubility should be made. 



Miscellaneous Rock Types 



1. Siliceous rocks: The most common siliceous rocks are repre- 

 sented by the microcrystalline cherts and flints. Colors are extremely 

 variable : white, green, red, brown, and black. Chert and flint are common 

 constituents in the carbonate rocks and occurs as lenses, nodules, thin 

 beds, and fracture fillings. The saturation of argillaceous strata with 

 opaline silica produces siliceous shales and porcellanites. The presence of 

 abundant diatoms and volcanic ash produces this type of deposits. Cher- 



