Comments on Sedimentary Rocks 79 



tified layers in carbonate sections have served many times as dependable 

 correlation criteria. 



2. Ferruginous rocks: The ferruginous sediments may be classified 

 as carbonates (siderite), iron silicates (glauconite), ferric oxides and 

 hydroxides (hematite, limonite), and sulphides (pyrite, marcasite) . 



Iron carbonate (siderite) is commonly associated with argillaceous 

 cherty strata in the form of beds, lentils, and concretions. Siderite also 

 occurs in the carbonate rocks (limestones, dolostones) . 



Chamosite is the principle iron-bearing silicate in sedimentary rocks, 

 principally the finer-grained elastics. Glauconite constitutes an important 

 silicate in many types of marine strata (shales, sandstones, and lime- 

 stones) . This mineral should be recorded in all stratigraphic investiga- 

 tions because of its correlation value and depositional environment index. 

 Glauconite varies in color from pale green to greenish-black. It occurs 

 mainly as rounded to elliptically shaped granules commonly exhibiting 

 shrinkage fractures. 



Limonite and hematite are common oxide bindents of sedimentary 

 elastics. These minerals occur as disseminations or in some cases as oolites. 

 In the Silurian strata of the Appalachian region sedimentary hematites 

 are commercially important. 



Pyrite and marcasite occur in all types of sedimentary rocks in 

 varying percentages. They may be present as nodules, crystal aggregates, 

 and disseminations. Pyrite is particularly common in highly organic, 

 black shales. Occasional beds of pyrite have been observed. 



3. Manganiferous rocks: Manganese occurs in minor amounts in all 

 sedimentary rocks. The oxides, hydroxides, and carbonates are the chief 

 mineral types. 



4. Phosphatic rocks: Phosphate-bearing rocks are commonly re- 

 ferred to as phosphorites. Phosphatic materials are found primarily in 

 shales and limestones, and may be either of primary or secondary origin, 

 or both. Color of phosphates varies from brown to black, although by 

 leaching it may assume lighter hues. The material may be bedded or may 

 occur as concentrically banded oolites and nodules. Nodules range up 

 to several centimeters in diameter. Shell fragments are frequently phos- 

 phatized. The origin of phosphate deposits appears to be related to animal 

 remains (bones, guana) . 



Texture of Sedimentary Rocks 



The texture of a rock refers to the size, shape, and arrangement of 

 the individual particles. Textural values are extremely important in the 

 description of a rock. The size of clastic particles may be expressed by 

 the terms coarse (gravel), medium (sand), and fine (clay) depending on 

 their dimensions as determined by selective screening or by actual meas- 

 urement. The limits of various grain sizes are more or less arbitrary. 



