Comments on Sedimentary Rocks 83 



occur prior to lithification, whereas others are not evident until long after 

 burial. 



Compaction, cementation, and metasomatism are largely responsible 

 for the rearrangement and replacement of sedimentary constituents. 



Compaction is most obvious in the fine-grained elastics (shale, mud- 

 stone, claystone) . Porosity is substantially minimized and original fabric 

 drastically modified. New minerals may be formed by closer packing and 

 by increased pressures and temperatures produced as a result of weight 

 of overburden. Compaction generally has little or no diagenetic effect on 

 the coarse elastics. 



Cementation is responsible for many modifications of a sediment. 

 Porosity and permeability are reduced, new minerals formed or the or- 

 iginal minerals replaced in whole or in part. Secondary silica deposited 

 around quartz grains and giving rise to secondary facets is a common 

 phenomenon. 



Metasomatism, involving replacement and alteration, is a common 

 process in sedimentary rocks, particularly in the carbonates. Calcite is fre- 

 quently replaced by dolomite and silica. Several stages of replacement 

 may be involved. 



Questions 



1. In evaluating a conglomerate, what features should be considered? 



2. What is the difference between a graywacke, an orthoquartzite, and 

 an arkose? 



3. Define shale and mudstone. 



4. What methods of study may be followed in analyzing a shale or mud- 

 stone? Limestone or dolostone? 



5. What are the most common evaporites? 



6. State the difference between humus, peat, and sapropel. 



7. What is porcellanite, glauconite, and chert? 



8. Define texture. 



9. What is a petrofabric diagram? 



10. Give an example of a metasomatic change in sedimentary rocks. 



