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Diagenesis Francis, 2014 pore. Diagenesis is the conversion of unconsolidated sediments into rock. The transition from diagenesis to metamorphism is somewhat.

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Presentation on theme: "Diagenesis Francis, 2014 pore. Diagenesis is the conversion of unconsolidated sediments into rock. The transition from diagenesis to metamorphism is somewhat."— Presentation transcript:

1 Diagenesis Francis, 2014 pore

2 Diagenesis is the conversion of unconsolidated sediments into rock. The transition from diagenesis to metamorphism is somewhat arbitrary, but is typically taken to be in the range 250 o to 300 o C at which point green phyllosillicates with definite compositions, such as muscovite and chlorite, become stable at the expense of mixed-layered clay minerals. This is also approximately the minimum temperature required for mineral assemblages to represent reaction equilibria, as opposed to the kinetic effects which dominate the diagenesis regime. The physical differences between unconsolidated sediments and rocks are those of porosity, volume, and cohesion. The changes during diagenesis are dominantly compaction, dewatering, and cementation. Two idealized conceptual end-member models for diagenesis can be postulated: The rock represents an closed-system, except for the expulsion of water, in which porosity reduction is achieved by compaction and cementation by pressure solution. The rock represents an open-system through which fluids moved, porosity reduction is a result of infilling by cement precipitated from the fluids. No compaction occurs. Diagenesis

3 Diagenetic processes are also sensitive to local geothermal gradients and the difference between hydrostatic and lithostatic pressure gradients. For example, sandstone sequences with impermeable mudstone horizons can develop over pressured states in which hydrostatic pressure exceeds lithostatic pressure and impedes compaction. To make matters more complicated, many sedimentary rocks show evidence for the development of secondary porosity due to dissolution during diagenesis. The development of secondary porosity is an important factor in the preparation of oil reservoirs. Compaction clearly occurs in the real world and the amount of the crystalline cement in most sedimentary rocks is small (< 10 %). The solubility of the main cementing agents, quartz and calcite, in interstitial pore waters are so low, however, that they are not capable of supplying even 1% cement. Clearly fluids have moved through rocks and the end results of diagenesis represent the accumulated interaction of pore fluids with the sediments. The results of diagenesis are thus sensitive to fluid / rock ratios and position with respect to fluid pathways. Many dolomites, for example, represent rocks whose composition have experienced a regional scale Mg enrichment due to the passage of large volumes of fluids during diagenesis.

4 Eodiagenesis (shallow burial, ~ 0 - km) Biogenic Activity: Reworking of sediment by burrowing organisms and conversion of shell and other fossil fragments to micrite by boring organisms such as algae. Weathering: The breakdown of feldspars to kaolinite and ferromagnesian silicates to smectites (montmorillonte) that occurs during weathering continues during early diagenesis. The fine clay minerals can migrate to intergrain voids, while the fine silica produced can lead to silica saturation in the pore fluids. albite + water > kaolinite + silica + K NaAlSi 3 O 8 + H 2 O > Al 2 Si 2 O 5 (OH) 4 + 4SiO 2 + 2K + Physical Compaction and Dewatering: Reduction of porosity due to compaction by grain realignment and migration of fine clay minerals and micritic carbonate to grain interstices.

5 Cementation Early cementation occurs in some open systems where conditions favour chemical precipitation. In warm water areas, CaCO 3 may locally become sufficiently supersaturated such that aragonite or high-Mg calcite will precipitate in the pore spaces to produce "hard ground‘‘and "beach rock". On the other hand, warm fluids rising from depth or volcanic zones become supersaturated in silica as they cool, precipitating a silica cement in siliciclastic sediments. Sandstones rich in volcanic glass clasts commonly develop early silica cement due to the release of silica during weathering of these thermodynamically unstable glass fragments, which yields excess silica.

6 Cementation Changes in oxidation fugacity can result in the precipitation of minor Fe cement, as pyrite in reducing environments (high organic matter) or goethite and hematite under oxidizing conditions (atmospheric exposure). Overall, however, the amount of cementation in the eodiagenetic environment is probably small, and tends to be locally developed when it occurs. Aragonite or high-Mg calcite cements, as opposed to calcite, are indicative of the eodiagenetic environment, as is opaline silica rather than quartz cement.

7 Mesodiagenesis (deep burial, 4 – 10 Km) Compaction / Cementation Compaction and decrease in porosity continue with increasing depth in sediments resulting in the explusion of large volumes of water that must move upwards and outwards through the sedimentary pile. The compaction is achieved by both physical and chemical means, and the final thickness of sedimentary rock beds is typically 0.5 to 0.75 that of the thickness of the unconsolidated sediment layer. The expelled water moves upwards and outwards through the sedimentary pile, reacting with their host sediments during their passage. Some horizons of the sedimentary pile will be preferential fluid pathways (aquifers) and experience extensive alteration, while other more impermeable horizons will remain relatively unaffected by the effects of fluid through put, but act as important aquatards. Physical Compaction Deformation and/or crushing of weak grains such volcanic lithic fragments, micas, and altered feldspar, combined with the movement of fine-grained material into interstices

8 Pressure Solution Under conditions in which the pore fluid pressure is less than the lithostatic pressure there will be a preferential dissolution of quartz or calcite grains at high stress points where they touch and a re- precipitation of quartz or calcite cement in the adjacent interstices. Opal and aragonite cements are absent at this stage, replaced by quartz and calcite. This re-crystallization results in intergrowths between grains and is probably the dominant cementation process in sedimentary rocks.

9 In carbonates, the extent of recrystallization, or ‘neomorphism‘ can be extreme, especially in the case of dolomites, resulting in a rock in which little evidence of the original fragments remain. Stylolites are contorted sutures or seams of insoluble material; such as clay minerals, Fe oxides, and organic matter; thought to be produced by extensive pressure solution and recrystallization. Stylolites in Recrystallized Limestones and Dolomites

10 Stylolites Produced by Pressure Solution in Dolomite

11 SapropelicHumic kerogen typealgalamorphousherbaceouswoodycoaly Type IType II Type III H / C O / C Environmentmarine, delta, lacustrinedelta, lacustrineterrestrial Fossil fueloil, sapropelsoil, gasgas, tarhumic coal At depths of 1500 to 4000m (T = o C), solid kerogen (organic matter, < 10% of black shales) becomes converted into mobile hydrocarbon fluids that migrate into the pore spaces. Catagenesis is a distillation process in which high H/C fluids are released leaving behind a refractory low H/C solid residue (vitrinite). The progress of these reactions in a rock can be measured by measuring the spectral reflectance of the residual vitrinite. There is a range of kerogen types, reflecting the type of original organic matter and the environment of deposition. Catagenesis Classification of Organic Matter (kerogen) in Sedimentary Rocks

12 Clay Mineral Maturation As temperature rises in the accumulating sedimentary pile, a point is reached at which the weathering reactions reverse and the progressive recrystallization of the clay minerals stable at the surface begins with increasing depth. kaolinite + silica + K albite + water Al 2 Si 2 O 5 (OH) 4 + 4SiO 2 + 2K + NaAlSi 3 O 8 + H 2 O 100 o C 200 o C 300 o C kaolinite mixed layers with pyrophyllite / illite muscovite / paragonite smectite mixed layers with chlorite, illite chlorite/ muscovite This is a gradual process with intermediate members characterized by randomly mixed layers of the end-member phyllosilicates. During this progression, the recrystallizing phyllosilicates react and become intergrown with the original clastic grains, tending to blur their margins and acting as a cementing agent (typical in Paleozoic greywackes).

13 I = illite S = smectite Chl = chlorite Q = quartz K = kaolinite Clay Minerals 300 o C kaolinite/smectiteIllite

14 Replacement / Dissolution Open systems with high fluid through put, and thus high fluid / rock ratios, can result in the replacement of original minerals, such as carbonate by dolomite, resulting in a 5% increase in porosity. Furthermore, porosity may actually increase with depth in carbonates at temperatures above 100 o C, the point at which the maturation of organic matter (catagenesis) gives off CO 2, resulting in increased solubility of CaCO 3 Telodiagenesis (uplift) Late stage diagenetic processes occur in rock sequences that have been uplifted above sea level. These sequences are infiltrated with meteoric water and can be divided into two important zones: Vadose - above the water table, water undersaturated. - oxidation & weathering Phreatic - below the water table, pore spaces completely filled with fluid. - dissolution resulting in secondary porosity The contact between these two zones, the water table, is typically a zone of intensive carbonate dissolution because of the mixing between meteoric and ground waters, this is the horizon along which caves form in karst terranes. 0.5 mm pore dolomite porosity


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