Carbonate Diagenesis
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Lithification
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Diagenesis refers to those changes which take place in sediments after sedimentation but before lithification (the conversion to solid rock), while metamorphism is defined as the process whereby alterations to the composition, texture or mineralogy take place in consolidated rocks. However, the boundary between diagenesis and metamorphism is gradual/transitional and somewhat arbitrary; it cannot be sharply defined. A consequence of this is that studies of rocks affected by alteration at low temperatures and pressures have been neglected. Within the last decade, however, more attention has been focused on this type of alteration and significant advances have been made in the field of low‐grade metamorphism, particularly over the last five years or so. The Caledonide rocks of Wales have been at the forefront of such investigations.
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In this chapter we discuss the role of groundwater and other fluids in metamorphism. Metamorphism is a broad term that encompasses all adjustments of solid rocks to changes in physical and chemical conditions. It thus includes diagenesis, a more narrowly defined term that is specific to changes undergone by sediments after initial deposition and during and after lithification. We consider diagenesis elsewhere: compaction and associated diagenesis in sedimentary basins is treated in Chapter 11 and the diagenesis of carbonate platforms in Section 13.8. Further, diagenetic processes within accretionary prisms are mentioned in Section 13.7. In this book, then, we use diagenesis to refer to chemical, physical, and biological changes undergone by sediments in a typical sedimentary environment (P generally <100 MPa, T generally <100 ◦C). Though there is no hard and fast distinction between diagenesis and metamorphism, we have reserved the term metamorphism to refer to changes in the solid state at greater pressures and/or temperatures. There is a further distinction between contact metamorphism, which takes place near an igneous intrusion, and the regional metamorphism that occurs in orogenic belts at a wide range of pressures and temperatures. In both types of metamorphism there is generally a gain, loss, and/or exchange of chemical constituents via a fluid phase, a process sometimes termed metasomatism.
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Investigation of the processes involved in lithifying sands made up of grains of silicate minerals indicates that crystallization of mineral matter in intergranular pores is only one mechanism which can be classified as cementation. Together with other methods binding grains, there appear to intergradational relations that the term involves mineral authigenesis, recrystallization, and intergranular welding. In effect, cementation and lithification appear to be synonymous terms. Variation in the aspect of cementation is dependent strongly upon the framework of the sand mixture which ranges from sand-grain supported (arenite) to clay-silt matrix supported (wacke). Frameworks intergradational between the two are classified as subwacke. A cement is recognized as compatible if its composition is similar to that of the detrital grain to which it becomes welded. An incompatible cement differs in composition from the grain to which it becomes attached, and adhesion is achieved through surface effects or local grain replacement. A different variety of cementation involves clay minerals attached principally to quartz grains. The attachment is accomplished through coalescence of lattices along a common junction. As diagenetic processes are intensified, each variety of cementation is believed to progress through a series of textural modifications which also may involve compositional changes. Such alterations are part of a sequence for each rock and develop textures which attain culminating stages characteristic of the sandstone framework. Arenite frameworks tend to induce grain overgrowth, which may be followed by recrystallization to produce granular interlocks of bilateral and triple-grain junctions. Wacke frameworks pass through stages of cementation involving reconstitution of clay minerals and recrystallization with authigenic chert into an equilibrium-mineral assemblage. Such assemblages show culminating textures characterized by neoformation of micas and intergrowths with detrital monoc ystalline quartz along margins of such grains. Subwacke frameworks are affected by intense diagenetic processes to develop cementation textures distributed locally in the rock. There may be culminating textures of both arenites and wackes in the same thin section.
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The Miocene Zia Formation consists of sands and muds deposited in fluvial, aeolian and playa lake environments. Although much of the formation is poorly consolidated, resistant zones of calcite cementation are common. These range in size from isolated nodules to tabular cemented zones several metres thick that extend for over 2 km laterally. The calcite cemented zones are highly complex, exhibiting a wide range of macroscopic and microscopic textures and geometries. After considering a combination of microscopic, macroscopic and geochemical characteristics, we have inferred the environment of precipitation (i.e. pedogenic, vadose non-pedogenic, phreatic) of the principal types of cementation. Nodules and rhizocretions with micritic fabrics and alveolar structures are inferred to be vadose carbonates. Ovoid or elongate concretions, characterized by blocky spar cements and preservation of primary sedimentary structures, are inferred to be phreatic carbonates. Most cemented units in the Zia Formation reflect characteristics of both phreatic and vadose zone cementation (e.g. preservation of sedimentary structures plus rhizocretions and alveolar microtextures). δ13C values for vadose cement tend to be heavier and δ18O values tend to be similar or slightly lighter than phreatic cements. δ13C and δ18O values for units with mixed features tend to have intermediate values. Most cementation types that exhibit a mixture of features may reflect past fluctuations of the water table, where vadose cements were moved into the phreatic zone. Vadose zone cementation occurred principally in association with soil development, whereas phreatic zone cementation occurred preferentially in zones of high primary permeability. In many cases early vadose cements provided nucleation sites for later phreatic cementation. Tabular units in the Zia Formation are often laterally extensive, decreasing potential reservoir/aquifer quality by forming significant barriers to vertical fluid flow. These barriers could result in compartmentalization of the reservoir/aquifer, and extensively reduce production if wells were screened on only one side of a cemented layer.
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Abstract Early‐diagenetic cementation of tropical carbonates results from the combination of numerous physico‐chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High‐Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low‐Mg calcite (0.4 to 1.0 Mol% Mg) and mini‐micritic Low‐Mg calcite (3.2 to 3.3 Mol% Mg). The co‐existence and consecutive development of fibrous aragonite and equant High‐Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High‐Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low‐Mg calcite cements on top of etched substrates of equant High‐Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini‐micritic Low‐Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini‐micrite Low‐Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High‐Mg calcite takes place in about 100 years. Collectively, this study shows that early‐diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.
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Freshwater-phreatic calcite cementation is an active process on 700 and 2700 yr-old ooid-sand islands in the Schooner Cays, Bahamas. Cement fabrics and textures indicate a general, four-stage model of pore infilling. (1) The precipitation of isolated, decimicron-sized, rhombohedrons of calcite on grain surfaces forms an incipient circumgranular cement. (2) Continued precipitation enlarges crystal sizes and forms new rhombohedral crystals, resulting in a continuous circumgranular rim of cement. (3) Additional cementation quickly masks the circumgranular fabric, producing a partial pore-filling mosaic. (4) The remaining pore space is occluded with a mosaic of calcite cement. Petrographic evidence for the earlier circumgranular rim of cement is not necessarily apparent after the last stage of cementation. Empty pores and all four stages of phreatic-zone cementation were observed in the diagenetically immature 700 yr-old rocks, but only stages 2 through 4 were observed in the diagenetically more mature 2700 yr-old phreatic zone samples. Cements are distributed homogeneously within each pore at every stage, yet because each pore may proceed through the four stages at different rates, each pore can be at a different stage of infilling. This results in an inhomogeneous distribution of cement between pores during the initial stages of cementation. Recognition of amore » cement stratigraphy similar to that described here should aid in the identification of freshwater-phreatic diagenesis in ancient carbonate rock sequences. Variability in the amount of freshwater-phreatic cement between pores should be expected and not interpreted as the product of different paragenetic sequences.« less
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Ooid
Phreatic eruption
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