76 Subsurface Geologic Methods 



Shale and Mudstone 



Shales and mudstones constitute the finest of the clastic materials. 

 Particle sizes range below 1/256 millimeter. The primary constituents of 

 these rocks are represented by the complex clay minerals which are ex- 

 tremely difficult to determine, especially without reverting to advanced 

 petrographic procedure. Silica is the dominant element in shales and 

 mudstones. It is present either as free silica (quartz) or in the form of 

 silicates. Alumina is next in importance to silica. Other elements in- 

 clude titanium, iron, manganese, calcium, sodium, potassium, and phos- 

 phorous. The common clay minerals include kaolinite, montmorillonite, 

 and illite. 



Many lithologists arbitrarily designate all argillaceous elastics as 

 shales. This terminology should be applied to only those rocks exhibiting 

 fissility or lamination. Rocks failing to show these structural features 

 should be termed mudstone or claystone depending on the plasticity value. 



The porosity of argillaceous rock types may range up to 50 percent, 

 although the average value, according to Pettijohn, is 13 percent. 



Hybrid types of argillaceous rocks include marlstones (50 to 80 per- 

 cent carbonate), clay ironstone (rich in siderite), porcellanite (high in 

 opaline silica), and black shale (exceptionally rich in disseminated or- 

 ganic matter) . 



Important accessory minerals in shales and mudstones include mica, 

 glauconite, pyrite, silt, and sand. Cementing materials may be of a silic- 

 eous, ferruginous, calcareous, carbonaceous, or bituminous nature. 



In descriptive work these sediments should be qualified by proper 

 adjectives; i.e., glauconitic, carbonaceous, dark gray shale. 



During the past few years considerable attention has been given to 

 clay mineralogy. This study involves such methods as elutriation, thin- 

 section, rate of settling. X-ray, specific gravity, spectroscopy, differential 

 thermal analysis, electron microscope, staining and chemical analysis. 



Limestone and Dolostone 



Limestones and dolostones may be of clastic or chemical origin or 

 may be developed as a result of both processes of deposition. The chem- 

 ical composition of these rock types varies considerably. Rarely does 

 there occur a pure limestone, CaCOs, or dolomite, CaMg(C03)2, because 

 of the inclusion of detrital materials. These two carbonate types invari- 

 ably grade into each other. In certain instances limestones and dolomites 

 contain an abundance of organic remains, whereas in other cases the re- 

 mains of organisms are absent or nearly so. The dominant organic re- 

 mains within carbonate rocks include those of algae, mollusks, corals, 

 echinoids, Bryozoa, ostracodes, and Foraminifera. 



The partial or total alteration and replacement (metasomatism) of 

 limestone strata have been recognized and have resulted in dolomitization, 

 chertification, and phosphatization. Diagenetic changes frequently oblit- 



