Diagenesis of Siliciclastics What is Diagenesis? Diagenesis is the physical and chemical processes that convert sediment into sedimentary rock. This conversion is due to an increase in temperature, an increase in pressure, and changes in pore-water composition. Initially, siliciclastic rocks form as unconsolidated deposits of gravel, sand or mud. After diagenesis gravel is lithified to conglomerates, sand is lithified to sandstone, and mud into shale. When Does Diagenesis Occur? Diagenesis normally occurs at a temperature below ~300oC. Some controlling factors of diagenesis are: composition, pressure, temperature, grain size, porosity/permeability, and the amount of fluid flow. After deposition has taken place, diagenesis can begin almost immediately. Three Stages of Diagenesis – earliest stage which takes place at very shallow depths Mesodiagenesis – occurs during deep burial Telodiagenesis – uplift of buried sediment into the system of meteoric waters. Eodiagenesis Eodiagenesis Eodiagenesis occurs mainly under the conditions of a depositional environment. The principal changes that occur are bioturbation, mineralogical changes, and compaction. A common result of eodiagenesis is the formation of mottled bedding due to bioturbation. Bioturbation Burrows dug by organisms in the sediment Mesodiagenesis The processes that take place in mesodiagenesis are: physical and chemical compaction, cementation, dissolution by pore fluids, mineral replacement, and clay mineral authigenesis. Physical compaction – results in porosity reduction, bed thinning, and tighter grain packing. Compaction Due to Pressure Factors that control compaction are: grain shape, sorting, original porosity, and pore fluid Continued Mesodiagenesis Chemical compaction – results in partial dissolution of silicate grains Cementation – results in a reduction of porosity thus bringing about lithification. Carbonate and silica are the most common cements. Dissolution – almost the opposite of cementation; leads to an increase in porosity Diagenesis: cementation Cementation is the process in which chemical precipitates (in the form of new crystals) form in the pores of a sediment or rock, binding the grains together. Common cements: quartz, calcite and hematite Less Common: aragonite, gypsum, and dolomite. uncrossed polars ©K. Simpson Pressure solution produces locally derived cement. Many cements consist of new minerals previously in solution in the fluid phase. Cementation reduces porosity by filling in the pore spaces between the grains. crossed polars ©K. Simpson Photomicrograph of a dolomite-cemented siltstone in crossed and uncrossed polars. The cement between the grains can be easily seen http://www.science.ubc.ca/~geol202/sed/sili/cement.html Mesodiagenesis continued Mineral replacement – results in partial to complete replacement of some silicate grains and clay matrix by new minerals Clay mineral authigenesis – alteration of one kind of clay mineral to another (i.e.) smectite clays will transform to illite at temperatures ranging from 55-200oC. Diagenesis: replacement Opalized wood Although there are many replacement phases, dolomite, opal, quartz, and illite are some of the most important phases Replacement occurs when a newly formed mineral replaces a preexisting one in situ. Replacement may be: •neomorphic: where the new grain is the same phase as the old grain, or is a polymorph of it (i.e. albitization; replacing a grain with a more Na-rich plagioclase grain). •pseudomorphic: where the old grain is replaced with a new mineral but the relict crystal form is retained, •allomorphic: an old phase is replaced with a new phase with a new crystal form http://www.science.ubc.ca/~geol202/sed/sili/replacement.html Telodiagenesis Rocks that have gone through deep burial may be uplifted due to erosion or mountain building activities. Sedimentary rocks will experience dissolution, replacement, and oxidation. The results include: solution of carbonate cements, alteration of feldspars to clay minerals, oxidation of iron carbonate minerals to iron oxides and pyrite to gypsum, and solution of less stable minerals.