Compaction of Sediments

ABSTRACT An attempt is made to explore the theoretical relations between compaction of sediments and depth of burial. On the basis of available data and assumptions that seem to be reasonable, a porosity-depth curve for mud and shale is constructed and overburden pressure is calculated. From these data, other relations of interest in stratigraphy are derived. Mud and shale are compacted in several stages that involve: (1) squeezing out of interstitial water until the sedimentary grains come in contact with each other; (2) rearrangement of grains and development of closer packing; (3) soft clay minerals are forced into the interstices between the more resistant minerals; (4) the latter are deformed until all porosity is eliminated. The colloidal properties of clay are very important in this process whose results appear to be predictable for sediments with different proportions of clay minerals. Much less compaction is possible in sand and sandstone because initial porosity is much lower. Colloidal effects are negligible. Most compaction appears to result from intergranular solution but the results are unpredictable because intergranular cement strengthens structure. No consistent relations exist between porosity and depth of burial. The consolidation of limestone is not understood. The initial porosity of most calcareous sediments is greater than that of sand but most limestones show little evidence of compaction. Their consolidation appears to have been accomplished by the deposition of intergranular cement at an early stage before the sediment was subjected to the pressure of much overburden. The source of the cementing calcium carbonate is not known. Organic material, of which coal is presented as an example, is reduced in volume by (1) compression and elimination of pore space, and (2) the loss of substance resulting from, first, biochemical decay and, later, dynamochemical metamorphism. Coal is essentially non-porous and its volume is reduced also by dehydration and increase in specific density. The degradation of organic matter follows one or the other of two paths: (1) carbonaceous (coal), leading to development of larger and more complex molecules (polymerization), or (2) bituminous (oil and natural gas), leading to the development of smaller and simpler molecules (cracking). The difference probably is related to the type of decay that occurs very soon after organic material is deposited as a sediment. Compaction data probably can be applied to the solution of some stratigraphic and structural problems but serious difficulties will remain until the lithification of limestone is more adequately understood. The study of diagenesis has been greatly neglected and it deserves much increased attention from geologists.