Classification and chemical differential of sedimentary rocks

Date: 2015-10-07; view: 582.

Fig. 37 Continental, shoreline and marine sedimentary environments

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.

Sediments are described on the basis of their grain size, chemical and mineralogical composition and modes of accumulation. Classification by grain size is most readily applied to clastic sediments because particle size is related to the origin of the rock.Among the clastic rocks, we recognize four main grain-size groups:

1. conglomerates containing particles greater than 2 millimeters in diameter;

2. sandstones with grains of 1/16 to 2 millimeters size range;

3. siltstones contain grains between 1/256 to 1/16 millimeter in diameter;

4. claystones carry particles finer than 1/256 millimeter in diameter.

Where planar fractures or parts along bedding surfaces are developed, claystones and fine-grained siltstones collectively are termed shale. If fractures parallel to bedding layering are not present in a deposit containing both clay and silt, the massive rock is termed as mudstone.

Limestones produced principally through the accumulation of calcareous fossil materials include coquina, chalk and reef limestones. The low-temperature process where source rock is converted to secondary accumulations of contrasting compositions is termed sedimentary differentiation.Weathering of preexisting rocks at or near the Earth's surface results in the solution of readily dissolved constituents and in the hydration and oxidation of others. Materials transported as clastic grains are separated on the basis of mechanical properties; relatively soft minerals possessing good cleavages are more rapidly and more finely abraded than fragments lacking fractures and perfect cleavage. Therefore, grain size and mineralogical composition are related in sediment.

The average compositions of igneous source rock of the continental crust and of typical sedimentary lithologies, sandstone, shale and limestone are markedly different. The average bulk composition of continental sedimentary rock is approximately 60 to 80% shale, 15 to 20% sandstone and conglomerate and approximately 10 to 15% carbonate.

(Barret E , Hunt A. And Milner B.” Earth and Atmosphere”, 1993, Longman)