E-content for B.Sc. Geology Hons. Part 2 Sedimentary Petrology
Lithification and Diagenesis
Presented By- Dr. Akhileshwar Tiwari Associate Professor Dept. Of Geology B.N. College, Patna Patna University Lithification is that complex process which converts a newly deposited sediment into an indurated rock. Lithification may be Partial or may be Incomplete It may occur shortly after deposition or long after deposition Diagenesis
Refers primarily to the REACTIONS which take place within a SEDIMENT between ONE MINERAL and • ANOTHER MINERAL, • SEVERAL MINERALS, • INTERSTITIAL or SUPERNATENT FLUID. The term DIAGENESIS was first introduced by VON GUMBEL (1888) as a designation for processes which act on the sediment after deposition. Diagenesis is in fact the beginning of metamorphism which leads to the modification of – Texture, Structure, and Mineral composition of sediment The sedimentation can be grouped into the following stages 1.Mechanical and Chemical weathering- Disintegration and Decomposition of parent deposits, 2. Erosion, Transportation and deposition- Formation of different types of sediment 3. The diagenetic process- Consolidation and cementation of the deposited sediment STRAKHOV (1953) divided the History of S/R into three distinct stages (i) SEDIMENTOGENESIS -formation of sediments, (ii) DIAGENESIS - transformation of sediment into S/R (iii) CATAGENISIS - long stage of secondary changes in already formed S/R William et al. (1955) included the CATAGENESIS STAGE of Strakhov under the term LATE DIAGENESIS which represents a transition to metamorphism. SUJKOWASKI (1950) coined the term EPIGENESIS which includes all processes at low temperature and pressure that effects the S/R after diagenesis up to metamorphism. According to SHVETSOV (1960) the process of Diagenesis in subaerial environment and shallow stable sea differ from those in subsiding basin and were termed EXODIAGENESIS. The term diagenesis includes all Physical, Biochemical & Physico-chemical processes which modify sediments between deposition and lithification or cementation at low temperature and pressure
P
Thermal
Minimum Ingredient
2000 kg/cm²
Region of Region of Metamorphism Diagenesis
200℃ Temp Diagenesis includes • HALMYROLYSIS – transition between the process of deposition and those of diagenesis, • METHARMOSIS – is the changes taking place after consolidation or uplift Evidence of diagenetic reactions consist of evidence of – 1) Post depositional recrystallisation, 2) Replacement, 3) Overgrowths, 4) Porphyroblastic growth, 5) Segregation of mineral materials, & 6) Intrastratal solutions The diagenetic changes are achieved by ordinary processes of chemical re-organisation such as - a) Solution, b) Precipitation, c) Crystallisation, d) Re- crystallisation, e) Oxidation, & f) Reduction. From geological point of view the principal diagenetic processes are -- i) Cementation, ii) Diagenetic re-organisation -- Authigenesis, iii) Diagenetic differentiation & Segregation, iv) Diagenetic metasomatism, v) Intrastatal solution, vi) Compaction. Cementation It is the process of precipitation of mineral matter in the pores/ voids of clastic sediments, the end result of which is rock induration. Cement constitutes – 25% to 30% of the whole rock. TYPES – i) Siliceous –Silica (Qtz, Opal, Chalcedony), ii) Calcareous – Calcite, Dolomite, iii) Ferruginous - Siderite, limonite, iv) Argillaceous – Anhydrite, Barite, Zeolite. Textures of Cement –
If mineral composition of cement & detrital grains is same, the cement is deposited in crystallographic continuity, the end result is interlocking crystalline aggregates, viz. a Quartzite
If the cement is mineralogically unlike the detrital material it shows various textural relations, Calcite cements forms a crystalline mosaic between the grains. Each pore is filled with single, 2 or 3 crystals. If the crystal of calcite is 1 cm or more in diameter - such sandstone show Luster-mottling. Paragenesis of cementing material – Some sandstone have more than one species of cement- e.g. Qtz, Calcite, clay. The mineral precipitated first will be better formed or more euhedral and will be attached to the walls of the interstices. The last deposited mineral will occupy the remaining unfilled space. Origin of Cement – 1. Extra- stratal origin, 2. Intra- stratal origin. 1. Extra- stratal origin – Cement can be derived from - i) Meteoric water, ii) Artesian water, iii) Connate water, iv) Dissolution in zone of weathering & ppt in zone of cementation Silica content of most of the ground water hardly goes into ground water (1/ 50,000 ppm). Such low content cannot account for the origin of cement because cement constitute 25 to 30% of the total mass 2. Intra- stratal origin - According to Wald Smith (1941), Krynine (1941), Gilbert (1949), Tallore (1950) the silica is dissolved at the point of contact and precipitation takes place in the pores/voids of the grain. The evidences are- i) Interlocking boundary of grains, ii) Different types of grain contact, iii) Concavo-convex contact, iv) Micro- stylolitic contact, v) Suture boundary. De-cementation- If the void filling fluid move out as well as in, or If the ions can diffuse out and in, the materials precipitated in pores might also be dissolved out of them. In other words leaching of the cement or de-cementation might take place. AUTHIGENESIS The Authigenic process attempt to establish an equilibrium assemblage or facies by Elimination of unstable facies, Growth of stable facies, The production of new and stable species by appropriate chemical reactions. An authigenic mineral is a new mineral formed at the place of its occurrence. or A new or regenerated mineral is an ‘Authigenic mineral’. According to BATURIN (1937) the ‘Authigenic mineral’ form in sediment before burial and consolidation. The ‘EPIGENIC minerals’ are those formed in the sediment after burial. The more important of the Authigenic processes are . Reduction, . Dehydration, . Reaction between solid and liquid phase Hydrated opaline silica tends to become dehydrated and crystallised. Gypsum may be dehydrated to form anhydrite. Clay mineral reacts with Fe & Mg to form chlorites and other iron silicates. Carbonate of calcium picks up Mg to form dolomite. DIAGENETIC DIFFERENTIATION and SEGREGATION
The segregation process is a part of diagenetic differentiation. It leads to the formation of Concretion, Nodules and Related bodies. These segregation take in the following forms Large crystals or Symmetrical crystal aggregates (sand crystals, spherulite etc.), Irregular microcrystalline bodies (chert nodules), May replace the matrix of host rock, May thrust aside enclosing rock, May be deposited in open space (pores, fractures, vugs). DIAGENETIC METASOMATISM
It involves introduction of material without appreciable volume change.eg.- .Dolomitisation, .Feldspathisation. These replacements may be in large scale or may be partial or minor. The replacing material may be derived from i. Sea water, ii. Connate water, If sea water- the replacement was early or contemporaneous with accumulation. If connate, artesian or juvenile water- replacement might have taken place long after deposition. INTRASTATAL SOLUTION Intra- stratal solution is that solution which takes place within a sedimentary bed after deposition. It may or may not be accompanied by simultaneous precipitation of material dissolved. The loss of material, without corresponding gain, leads to an increase in porosity (secondary porosity). If accompanied by precipitation of same or unlike materials, there may be a net loss of porosity and resultant expulsion of intergranular fluids. Concurrent solution and re- precipitation may lead to thorough cementation of the rock. COMPACTION Compaction is the reduction of pore spaces and expulsion of interstitial fluids resulting from grain re-arrangement under load. In fine grained sediments compaction is greatest and most important. In general, the oldest rocks lying at great depths show lowest porosity and hence they are most compact