Fig. 36 Sedimentary stages in the rock cycle

The nature of sediment depends in part on the source from which it is derived, and on the attendant weathering occurring by the altering of the rock surface; involves mechanical and chemical breakdown of the original materials, usually by interaction with the atmosphere or with water. High temperatures, elevated temperature weathering accelerates chemical sediments. The nature of sediment depends chiefly on the mode and length of transportation from the source area to the depositional site.

The manner of deposition strongly influences the nature of the resultant sediment. Rapid accumulation of ill-sorted clastic debris of several size ranges and high rainfall characteristic of the tropics promote chemical weathering, generally, result in weakly laminated or massive deposit – coarse sand; on the other hand, slow building of fine-grained particulate matter, or chemical precipitation alternating with the setting of fine clastic debris, tends to produce fine-bedded sediments such as laminated muds. The amount and rate of deposition from the transporting medium is a function of the flow velocity and the nature of the transported sediment.

Two principal types of sediment are distinguished based on the site of deposition: continental (or terrestrial) and marine. Terrestrial sediments include the products of mass movements, stream gravels, lakebeds, sand dunes and glacial deposits. Marine sediments tend to be more widespread and continuous. To this group belong shallow-water deposits of deltas, beaches, continental shelves and slopes, as well as deep-sea sediments of the ocean basin. Both chemical precipitates and clastic materials are abundant in such sediments.

The process of lithfication, whereby unconsolidated sediment is converted into a coherent rock, involves several distinct but intergradational phenomena. Compaction of a sediment due to the weight of the overlying rocks decreases pore space and results in deformation of soft mineral grains, clast rotation, flattening and squeezing out of water from the pore spaces and also produces interlocking grain boundaries. Iron oxide, silica and calcium carbonate minerals precipitate from pore salutations and cement the particles together. Reaction among the original, chemically dissimilar fragments produces new minerals that transect former grain boundaries and increase rock coherence: this recrystallization is known as diagenesis.