Ch. 1] DIAGENESIS 29 



only a few types of organisms, particularly certain anaerobic bacteria, 

 can live. A heavy storm or a rare cold spell may cause some of the 

 deep water to rise to the surface, killing off much of the life. 



This influence of the temperature on the density of water influences 

 materially the type of organisms that grow in the lake and hence the 

 chemical equilibrium of the dissolved materials. Lakes that are 

 periodically ventilated by overturning of the water are in a relatively 

 high state of oxidation compared with those in which the water does 

 not turn over. The depth of water, however, is a factor, because, the 

 shallower the lake is, the more likely the bottom water is to be stirred 

 up during a storm. 



In some lakes algae develop luxuriantly, giving rise to deposits of 

 organic matter, which in the course of time may form deposits of oil 

 shale (Bradley, 1931). The organic matter, when first deposited, has 

 a very high water content, but oil shale generally is found in very thin 

 layers, which indicates a high degree of consolidation or compaction 

 during the course of geologic time. As Bradley has shown, layers of 

 oil shale commonly are of a dual nature, one part representing summer 

 conditions, and the other winter conditions. Thus, oil shale, at least 

 the Green River oil shale, resembles varves formed in glacial lakes. 



The deposits in lakes are likely to be cross-bedded and highly vari- 

 able near shore, particularly on the deltas where streams flow into 

 lakes. The larger lakes have bars, spits, and beaches, which have 

 essentially the same characteristics as corresponding features in the 

 ocean. The sediments laid down in the central parts of the large lakes, 

 like sediments in the open sea, are relatively fine in texture, and are 

 comparatively well stratified; however, lacustrine deposits commonly 

 change in texture and character from one layer to another. 



DIAGENESIS 



Sediments change after they have been deposited. The water is 

 squeezed out of pore spaces owing to the effect of superimposed loads, 

 thus causing a reduction in thickness. This compaction of the de- 

 posits presses the constituents more firmly together and gives the 

 sediments greater strength. Materials are deposited in the pore spaces 

 and cement the particles together. New minerals form and old min- 

 erals grow by the addition of new material. Ultimately the sediment 

 is consolidated into rock. The sum total of these processes is called 

 diagenesis. Diagenesis, however, is to be distinguished from meta- 

 morphism, which results in the more or less extensive formation of new 



