Comments on Sedimentary Rocks 75 



Arkose usually is coarser-grained, lighter in color (pink or light gray). 

 It may be strongly cross-bedded, is cleaner sorted, i.e. is without interstitial clay 

 or silt and consequently, is bound by introduced mineral cement (usually car- 

 bonate). Arkose is closely associated with coarse conglomerates that contain 

 much granite debris, and it is usually terrestrial in origin. Graywacke is the 

 opposite in nearly all particulars. It is finer-grained, dark in color, rarely cross- 

 bedded, but with marked graded bedding if any bedding is visible, with an 

 interstitial paste having the composition of a slate, and hence without mineral 

 cement. Graywacke is interbedded with black, pyrite slates and associated with 

 pillow lavas and chert. It is usually marine. 



Sandstones grade in texture from very coarse (2 to 1 mm.) to very 

 fine (i to yg mm-)- Sorting may be in certain instances exceptionally 

 high, whereas in others it is extremely low. The coefficient of sorting 

 may be determined by construction of a cumulative-frequency curve. 



The colors of sandstones have a wide range (white, gray, red, green, 

 to black) depending on composition of the grains, type and amount of 

 cement, and degree of weathering. Cementing materials commonly con- 

 sist of limonite, hematite, carbonate silica, organic material, anhydrite 

 and gypsum, and clay. Pure quartz sandstones are indicative of stable 

 shelf-depositional conditions. 



Bedding characteristics of sandstones may be massive to thinly lamin- 

 ated. Bedding surfaces may be sharp to gradational, parallel or con- 

 verging. 



Accessory components comprising the heavy-mineral fraction of 

 arenaceous rocks consist of such minerals as pyrite, hornblende, pyroxene, 

 olivine, magnetite, leucoxene, garnet, tourmaline, zircon, mica, staurolite, 

 anatase, and glauconite. 



Sandstone varieties should be designated by a qualifying adjective 

 whenever possible; i.e., glauconitic, micaceous, garnetiferous, pyritic, and 

 hornblendic. 



Studies of arenaceous rock types should treat: textural aspect, 

 grain fabric, type and distribution of cement, character of grains (sur- 

 face features, alteration, shape, inclusions), grain composition, and var- 

 iations in porosity and permeability. These values may be determined by 

 thin sections, screen analysis, heavy-liquid separation, polished surfaces, 

 and chemical analysis. In subsurface investigations fluid and gas con- 

 tents may be evaluated by core analysis and by data obtained from var- 

 ious well-logging methods. 



SiltstoTie 



Siltstones, the indurated equivalent of silts, are fine-grained elastics 

 represented by particles ranging in size from 1/16 to 1/256 millimeter. 

 These rocks vary considerably in color and structure. They frequently 

 contain various compounds of organic matter and as a rule yield normal 

 suites of heavy accessory minerals. In many instances rocks of this cate- 

 gory are classified as sandy (arenaceous) or silty shales. 



