Which process causes many sedimentary rocks to have visible layers?

Answer 1

There is no single process that causes the layering of sedimentary rocks and the layers can be very thin laminae up to very thick beds. Usually the reason why one sees layering is either because there are colour-changes or compositional changes in the sedimentary rocks. I give some examples below.

Very thin layering can be formed by annually varying sediment input. In some lakes a summer- and a winter-layer form a couplet of layers that can be recognized by their differing colour. People use these so-called warves for absolute age dating of sediments.

Layering can also be due to changes in the input of sediment from coarser grained material to finer grained material or the other way round. There are also packages of sediments of sub-marine sediment flows called turbidites, which show a sorting of the grain sizes in each package and that might be stacked to very thick successions.

There are also bedding patterns that are believed to be driven by changes in the Earth's climate via changes in the orbital parameters of our planet, so-called Milankovich-cyclicity. In some marine sedimentary rocks it is possible to pick these cycles up as changes in colour or as changes in the fraction of carbonate present in the rock.

Another type of layering exists in "chemical sediments", these being the products of evaporation of seawater. In restricted basins that periodically fall dry there is a well-defined succession of different minerals precipitating as the water evaporates and one can fined these cycles as stacked layers of carbonate, gypsum, anhydrite, halite (rock salt) and sometimes other highly soluble salts.