30 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



minerals as a result of dynamic stress, comparatively high tempera- 

 ture, and mineralizing solutions. 



At present the processes of diagenesis are not well understood. Com- 

 paction of sediments, with resulting decrease in porosity, is clearly re- 

 lated to the imposed load and starts as soon as the sediments have 

 been deposited. Compaction is used by geologists in essentially the 

 same sense as the word consolidation is used by soil mechanics engi- 

 neers. The water content of the sediments, or, more appropriately, 

 the porosity, is directly related to the texture of the sediments. Clays 

 have a high initial water content compared with silts or sand and thus 

 compact more than silt or sand. For sediments to compact, the water 

 must be squeezed from the deposits. The permeability, therefore, is a 

 big factor in the rate of consolidation. Sands, being permeable, have 

 a high initial rate of compaction compared with clays, which are rela- 

 tively impermeable. In fact, sands ordinarily compact very quickly 

 to the point where each grain is in contact with one or more other 

 grains; after this they compact very slowly as grains are deformed or 

 rotated so that adjoining grains can nestle more effectively into avail- 

 able pore space. Clays likewise compact more rapidly at first, but, 

 after the initial period of rapid forced removal of pore water, they 

 compact very slowly for a long time, as the pseudo-anticlinal structure 

 of sediments over buried hills indicates (Athy, 1930; Hedberg, 1936). 

 Essential factors in this compaction are load, grain size, permeability, 

 and time. 



During consolidation of the sediment into rock, that is, while water 

 is being squeezed out, the strength or ability of the sediment to sup- 

 port a load is affected to a considerable extent by deformation of min- 

 eral constituents, which causes the individual constituents to be more 

 firmly locked together. While this process is going on, but particu- 

 larly after the initial stage of settlement, material is precipitated 

 from the pore water and thus binds the constituents together. The 

 concentration of dissolved substances, the rate of movement of the 

 water, the solubility relationships of the dissolved materials, base ex- 

 change, rock pressure, activity of microorganisms, size and shape of 

 the pore spaces, and mineral composition of the rock particles all in- 

 fluence the precipitation of cementing materials. Commonly these 

 materials are laid down in pore spaces between grains, but in places 

 zones of cement are deposited around some focus of precipitation un- 

 til a nodule or concretion is formed. The causes of localization of pre- 

 cipitated material in the pore spaces of sediments is at present poorly 

 understood, but it is of great importance in interpreting the migration 

 of oil and also the origin of ore deposits in permeable rocks. (See 



