CHAPTER 3 

 COMMENTS ON SEDIMENTARY ROCKS 



L. W. LeROY 



No attempt is herein made to present a complete synopsis of the 

 major types of sedimentary rocks. However, brief mention of the various 

 methods of study applicable in evaluating each type is made. 



The classification of sedimentary rocks is difficult because of inter- 

 gradations of textures and compositions. Various classification outlines 

 have been proposed and suggested, though none are fully complete. In 

 general these classifications fall into two categories: descriptive and 

 genetic. The former involves classification without knowledge of origin, 

 whereas the latter requires data concerning origin. Neither approach can 

 be completely divorced from the other. Two workable classification charts 

 are given, one by Van Tuyl ^ (fig. 33) and one by Shrock ^ (fig. 34) . 



For a comprehensive and monumental dissertation on sedimentary 

 rocks, Pettijohn's Sedimentary Rocks, published by Harper and Brothers 

 in 1949, should be consulted. 



Types of Sedimentary Rocks 

 Conglomerates and Breccias 



A conglomerate or its unconsolidated equivalent (gravel) is com- 

 posed mainly of rounded granules, pebbles, and boulders exceeding two 

 millimeters in diameter. Roundness, sphericity, and flatness of these com- 

 ponents show considerable variation. An accumulation of fragments ex- 

 hibiting high angularity is commonly termed a "breccia." 



Stratification and degree of sorting shown by the coarse elastics range 

 from poor to excellent. Cross-lamination and imbrication patterns are 

 frequently developed. 



Conglomerates and breccias have a wide range in color which is 

 controlled by the type of finer matrix, the composition of the fragments, 

 and the degree of weathering. 



In some elastics the variety of the pebbles represented is relatively 

 simple (oligomictic) , whereas others contain a complicated and diver- 

 sified pebble suite (polymictic) . Pebble composition is a useful attribute 

 for decipherment of the origin of the deposit and for interpreting condi- 

 tions under which the deposit accumulated. 



Limonite, calcite, silica, clay, and a combination of two or more of 

 tjiese minerals are common bindents. 



' Van Tuyl, F. M., Profesfor and Head of Geology Department, Colorado School of Mines, Golden, 

 Colorado, 



- Shrock, R. R. Associate Professor of Geology, Massachusetts Institute of Technology, Cambridge, 

 Massachusetts. 



