Carbonate Sedimentary Rocks PDF

Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Carbonate sedimentary rocks Limestones Carbonate rocks make up about one-fifth to one-quarter of all sedimentary rocks in the stratigraphic record. They occur in many Precambrian assemblages and in all geologic systems from the C...

Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Carbonate sedimentary rocks Limestones Carbonate rocks make up about one-fifth to one-quarter of all sedimentary rocks in the stratigraphic record. They occur in many Precambrian assemblages and in all geologic systems from the Cambrian to the Quaternary. Both limestone and dolomite are well represented in the stratigraphic record. Dolomite is the dominant carbonate rock in Precambrian and Paleozoic sequences, whereas limestone is dominant in carbonate units of Mesozoic and Cenozoic age. carbonate rocks are obviously an important group of rocks. They are important for other reasons as well. They contain much of the fossil record of past life forms, and they are replete with structures and textures that provide invaluable insight into environmental conditions of the past. Aside from their intrinsic value as indicators of Earth history, they also have considerable economic significance. They are used for a variety of agricultural and industrial purposes, they make good building stone, they serve as reservoir rocks for more than one-third of the world’s petroleum reserves, and they are hosts to certain kinds of ore deposits such as epigenetic lead and zinc deposits. The microscopic study of carbonate rocks dates back to the beginning of petrographic analysis. Mineralogy Principal carbonate groups Carbonate rocks are so called because they are composed primarily of carbonate minerals. These minerals, in turn, derive their identity from the carbonate anion (CO32− ), which is a fundamental part of their structure. The CO32− carbonate anion combines with cations such as Ca2+, Mg2+ , Fe2+, Mn2+, and Zn2+ to form the common carbonate minerals. (Table.1). 1 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy The common carbonate minerals fall into three main groups: the calcite group, the dolomite group, and the aragonite group (Table.l). Minerals in the calcite and dolomite group belong to the rhombohedral (trigonal) crystal system, and those in the aragonite group belong to the orthorhombic system. Dolomite-group minerals differ from calcite-group minerals in that they are double carbonates. That is, they contain Mg2+ and/or Fe2+ in addition to Ca2+. Table 1 Common carbonate minerals 2 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy 3 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Major components of limestones The mineralogy of carbonate rocks is almost totally different from that of sandstones, but many limestones resemble sandstones texturally in that they consist of various kinds of sand- and silt-size carbonate grains and various amounts of fine lime mud matrix and carbonate cements. Although limestones commonly contain only one or two dominant minerals, in contrast to sandstones, several distinct kinds of carbonate grains are recognized. Most of these grains are not single crystals but are composite grains made up of large numbers of small calcite or aragonite crystals. Folk (1962) proposed the term allochem to cover all of these organized carbonate aggregates that make up the bulk of many limestones. The principal kinds of carbonate grains are illustrated and briefly described in Fig. 5. They include both non-skeletal grains (e.g. lithoclasts, ooids) and skeletal grains (fossil and fossil fragments Ion substitution (replacement) in carbonate minerals In the calcite structure, disordered cation substitution of Mg2+ for Ca2+ can occur, up to several mole percent MgCO3. Calcite containing more than about 4 mol% MgCO3 (5 mol% according to some authors) is commonly called magnesian calcite or high- magnesian calcite (Mg-calcite). Calcite with less than about 4 mol% MgCO3 is called low-magnesian calcite or simply calcite. Less commonly, Fe2+ can substitute for Ca2+ or Mg2+ in calcite to form ferroan calcite, and minor amounts of Mn2+ can also substitute for Ca2+. High-magnesian calcite is metastable with respect to calcite and may lose its Mg in time and alter to calcite. Alternatively, if exposed to magnesium-rich pore waters, high-magnesian calcite can gain additional Mg and be replaced by dolomite. Magnesium does not commonly replace calcium in aragonite, although the aragonitic skeletons of some organisms incorporate Mg during growth. Small 4 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy amounts of Sr or Pb may substitute for Ca in aragonite. Identification of carbonate minerals Because calcite, aragonite, and dolomite are the dominant minerals in carbonate rocks, it is s important to identify these minerals in petrologic study. Some distinguishing features of the carbonate minerals are listed in Table.1. Nonetheless, it is often difficult to distinguish among these minerals in hand specimens and thin sections. Identification can be greatly aided by staining and etching techniques. For example, aragonite is stained black with Fiegl’s solution (Ag2SO4 + MnSO4), whereas calcite remains unstained. Calcite is stained red in a solution of Alizarin red S and dilute HCl, whereas dolomite remains unstained. Dolomite and high-magnesian calcite can be stained yellow in an alkaline solution of Titan yellow. Mineralogy of carbonate-secreting organisms The skeletal remains of calcium carbonate-secreting organisms are volumetrically important components of many limestones. These skeletal remains may consist of aragonite, calcite, or high-magnesian calcite containing as much as 30 mole percent MgCO3. For example, most molluscs are composed of aragonite, although some (e.g. some gastropods) are composed of low-magnesian calcite. Echinoderms are composed of high-magnesian calcite, and foraminifers are composed of low- or high-magnesian calcite. Some groups of organisms may build skeleton of both aragonite and calcite. A few organisms, e.g. diatoms and radiolarians, secrete skeletons composed of silica. Note that the mineral composition of calcareous organisms may change with burial diagenesis. Aragonite in skeletal grains transforms to calcite with time and, as indicated, high-magnesian calcite may either lose Mg and alter to low-magnesian calcite or gain Mg to form dolomite. 5 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Noncarbonate components: Carbonate rocks commonly contain various amounts of noncarbonate minerals, but generally less than about 5 percent. Noncarbonate minerals may include common silicate minerals such as quartz, chalcedony or microquartz, feldspars, micas, clay minerals, and heavy minerals. Clay minerals are particularly abundant constituents of some carbonates. Other minerals reported in carbonate rocks include fluorite, celestite, zeolites, iron oxides, barite, gypsum, anhydrite, and pyrite. Most noncarbonate minerals in limestones and dolomites are probably of detrital origin; however, some minerals such as chalcedony, pyrite, iron oxides, and anhydrite may form during carbonate diagenesis. Major components of limestones As discussed in befour, sandstones consist dominantly of various kinds of sand and silt-size silicate grains with various amounts of fine, siliciclastic mud matrix and secondary cements, including carbonate cements. The mineralogy of carbonate rocks is almost totally different from that of sandstones, but many limestones resemble sandstones texturally in that they consist of various kinds of sand- and silt-size carbonate grains and various amounts of fine lime mud matrix and carbonate cements. (Note: some limestones are texturally similar to siliciclastic mudstones; they are composed dominantly or entirely of lime mud and contain few or no silt- or sand-size carbonate grains.) Most of these grains are not single crystals but are composite grains made up of large numbers of small calcite or aragonite crystals. Folk (1962) proposed the term allochem (Table 2 ) to cover all of these organized carbonate aggregates that make up the bulk of many limestones. The principal kinds of carbonate grains are 6 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy illustrated and briefly described in Fig. (2). They include both nonskeletal grains (e.g. lithoclasts, ooids) and skeletal grains (fossil and fossil fragments). Folk (1962) proposed the term allochemical as (allos: differentiation from the normal) are thase components that have formed by chemical precipitation within the basin of deposition, but which for the most part have suffered some later transport, or, if they have not been transported, they include such organized aggregates sedentary fossils or fecal pellets differentiated from normal chemical precipitation as one usually thinks of them. Only four types of allochem are of importunes : antraclasts, oolites, fossils, and pellets. Carbonate grains 1- Peloids: Peloids are spherical, ovoid, or rod-shaped, mainly silt-sized carbonate grains that commonly lack definite internal structure (Fig. 1). They are generally dark gray to black owing to contained organic material and may or may not have a thin, dark outer rim. The most common size of peloids ranges from about 0.05 to 0.20 mm, although some are much larger. Peloids are composed mainly of fine micrite 2 to 5 microns in size, but larger crystals may be present. They are commonly well sorted and they may occur in clusters. They are produced by a variety of organisms that ingest fine carbonate mud while feeding on organic-rich sediments. Pellets are homogeneous aggregates of microcrystalline calcite well rounded and sorted , overaging 0.03 to 0.2 mm. They probably represent fecal pellets of worms or other invertebrates. It is possibles that some may from in place in place by aform of recrystallization; vaguely defined (pelletes) in ooiz are sometimes termed grumeleuse. 7 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Table 2 Sedimentary rock classification after Folk,1962 Figure(1) Peloids and a few small coated grains cemented by sparry calcite cement. 8 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy 2- Coated grains ooids, oncoids, and cortoids Definition: Coated grain is a general term used for all carbonate grains composed of a nucleus surrounded by an enclosing layer or layers commonly called the cortex. Various kinds of coated grains are recognized, largely on the basis of the structure of the cortex. Coated grains are divided into four broad groups: ooids, oncoids, cortoids, and pisoids. Figure(2) Descriptive terminology of the major kinds of carbonate grains 9 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy A- Ooids Calcareous ooids are small, more or less spherical to oval carbonate particles, which are characterized by the presence of concentric laminae that coat a nucleus. The nucleus may be a skeletal fragment, peloid, smaller ooid, or even a siliciclastic grain such as a quartz grain. Most ooids are sand- to silt-size particles. They range in size from about 0.1 mm to more than 2 mm, with 0.5 mm to 1 mm being most common. Carbonate grains that are structurally similar to ooids but are larger than about 2 mm are called pisoids (to be discussed). Ooids are white to cream in color and commonly have a pearly luster if formed in agitated water. Quiet-water ooids may have a dull luster. Ooids are distinguished especially by the presence of concentric, accretionary layers or laminae. Ooids that consists of numerous laminae are regarded to be mature or normal ooids (Fig.3 A). Those that have only a few laminae are called superficial ooids (Fig. 3B), following the usage of Illing (1954). Carozzi (1960, p. 238) restricts the term superficial ooid to those ooids having only a single accretionary layer; The ooids are distinguished by the following general characteristics: (1) they are formed of a cortex and a nucleus of variable composition and size, (2) the cortex is smoothly laminated, (3) the laminae are either concentric or they are thinner on points of stronger curvature of the nucleus, and vice- versa, thus increasing the sphericity of the ooid during its growth, and (4) constructive biogenic structures are lacking. With respect to structure of the cortex, carbonate workers distinguish three principal kinds of primary ooids (1) ooids in which crystals are arranged tangentially within layers, (2) ooids with radially arranged crystals (Fig 4), and (3) micritic or microsparitic ooids with randomly oriented crystals. In addition, secondary fabrics occur in some ooids. Most modern-Holocene marine ooids are composed of aragonite, although some are composed of Mg-calcite. Most modern nonmarine ooids are 10 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy composed of calcite. Most ancient ooids are also composed of calcite, presumably owing to diagenetic alteration of aragonite or Mg-calcite Water agitation appears to be important to growth of ooids, particularly tangential ooids which commonly have highly spherical forms and polished surfaces. As discussed, however, some ooids can form under quiet-water conditions. Quiet-water ooids are more likely to have a radial structure, less-spherical form, and less-polished surfaces. Ooids occur most commonly on shallow carbonate platforms, but can be deposited in a wide variety of environments from fluvial to deep-marine, owing in some cases to retransport. Ooids may contain organic matter such as the remains of blue-green algae, fungi, and bacteria. A B Figure (3) A- Ooid (center of photograph) made up of numerous thin, concentric layers surrounding an intraclast (nucleus). B- Superficial ooid (arrow) that displays a single thin layer or coat around a large micritic nucleus Figure (4) Radial ooids of marine origin. 11 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy B-Oncoids Coated grains more irregular in shape than ooids, and with more irregular laminae, are called oncoids (Fig. 5). Oncoids are also generally larger than ooids, commonly ranging from < 2 mm to > 10 mm. They form in both nonmarine and marine environments. Many authors restrict the term oncoid to grains of cyanobacterial and bacterial origin. Others include also as oncoids carbonate grains encrusted by red algae and bryozoans. Figur(5 ) Large oncoids exposed on a weathered limestone surface. C- Cortoids Some coated grains consist of fossils, ooids, or peloids coated with a thin envelope of generally dark colored micrite (Fig. 6), commonly consisting of crystals 2 to 5 microns in size. The micrite envelopes may originate by several processes, which can include destructive micritization related to the activities of microboring organisms. 12 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (6) Micrite envelopes (arrows) developed around echinoderms and other fossil fragments Lithoclasts Carbonate lithoclasts are detrital fragments of carbonate rock produced by disintegration of pre-existing carbonate rock or sediment, either within or outside a depositional basin. They are also sometimes called limeclasts. Lithoclasts may range in size from very fine sand to pebbles or even boulders. They tend to be well rounded, but may also be subrounded, subangular, or angular. Very small lithoclasts may be confused with large peloids. Two kinds of lithoclasts are recognized on the basis of origin: intraclasts and extraclasts. Intraclasts Some lithoclasts originate within a depositional basin by fragmentation of commonly weakly cemented, carbonate sediment. Small pieces of this sediment are eroded from the seafloor and redeposited at or near the original area of deposition. Lithoclasts having this origin are called intraclasts. Although most intraclasts are probably produced by physical disruption of sediment by normal waves, storm waves, or currents, some intraclasts may form by other mechanisms. These mechanisms could include organic activity on the surface of 13 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy sediment, burrowing or boring activity within sediment, and local sliding of weakly consolidated sediment (Fig. 7). Figure (7) Well-rounded intraclasts (produced in a high-energy environment), cemented with sparry calcite cement. Extraclasts Lithoclasts generated by erosion of much older, lithified carbonate rock exposed on land (outside the depositional basin in which the clasts accumulate) are called extraclasts. They are simply carbonate rock fragments (Fig. 8). Figure (8) Angular to subrounded lithoclasts in a lime-mud (dark) matrix. 14 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Skeletal grains (bioclasts) Skeletal grains are among the most abundant and important kinds of grains that occur in limestones of Phanerozoic age. Skeletal grains may consist of whole fossil organisms, angular fragments of fossils, or fragments rounded to various degrees by abrasion. Skeletal grains may occur with other kinds of carbonate grains or they may constitute the only kind of carbonate grains in a particular limestone. Some limestones are composed almost entirely of skeletal remains, which are cemented together with a small amount of micrite or sparry calcite cement. The kinds of skeletal grains that occur in limestones encompass essentially the entire spectrum of organisms that secrete hard parts; however, the relative abundance of various kinds of skeletal remains has varied through time. Most skeletal grains are composed of aragonite, calcite, or magnesian calcite. Vertebrate remains, fish scales, conodonts, and the remains of a few invertebrate organisms such as inarticulate brachiopods are composed of calcium phosphate. Also, a few, e.g. diatoms and radiolarians, are composed of silica. The original composition of skeletal grains may be altered during diagenesis. Aragonite skeletons transform to calcite, and high-magnesian calcite grains may alter to calcite or become dolomitized. Carbonate skeletal grains may also undergo replacement by silica. Each kind of organism that lived in the past was adapted to a particular set of ecological conditions. Because this was so, fossils yield vital information about environmental conditions such as water depth, salinity, turbidity, and energy levels. Therefore, it is extremely important in petrologic studies of carbonate rocks to identify each skeletal grain (Fig. 9). high-magnesian calcite grains may alter to calcite 15 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy or become dolomitized. Carbonate skeletal grains may also undergo replacement by silica Figure (9) Fusulinid foraminifers as they appear in thin section Mixed skeletal grains cemented with sparry calcite (white). C, crinoid; B, bryozoan; Br, brachiopod. Microcrystalline carbonate (lime mud) Although many limestones are composed dominantly of carbonate grains (allochems), few if any limestones are made up entirely of sand–silt-size grains. The remaining part of the rock is composed either of sparry calcite cement (described in the next section) or microcrystalline calcite or aragonite, commonly referred to as lime mud. Texturally, lime mud is analogous to the clay-size matrix in siliciclastic sedimentary rocks and to siliciclastic mudstones. In modern carbonate environments, lime muds are composed mainly of aragonite needles about l–5 microns in length. In ancient limestones, they consists of similar sized, but more equant, crystals of calcite. Lime muds may also contain a few percent clay-size, noncarbonate impurities such as clay minerals, quartz, feldspar, and organic matter. Folk (1959) 16 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy proposed the term micrite as a contraction for microcrystalline calcite; this term has been universally adopted to signify very fine-grained carbonate sediment. It is used loosely to include all carbonate mud, including aragonite muds; that is, all microcrystalline carbonate. Micrite has a grayish to brownish, subtranslucent appearance under the microscope (Fig. 10). It is generally easily distinguished from carbonate grains by its finer size and from sparry calcite, which is coarser grained and more translucent. Although micrite typically occurs as a matrix among carbonate grains, some limestones are composed almost entirely of micrite. Such a limestone is texturally analogous to a siliciclastic shale or mudstone. The presence of substantial micrite in a limestone is commonly interpreted to indicate deposition under fairly low-energy conditions. Figure (10) Photomicrograph of a limestone composed dominantly of micrite, with a few skeletal fragments (white) The origin of seafloor micrite (microcrystalline aragonite and calcite mud) presents an interesting problem. Theoretical considerations of carbonate equilibria suggest that CaCO3 should precipitate inorganically under conditions of supersaturation. The equilibrium relationship for CaCO3 in water and dissolved carbon dioxide is 17 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Loss of CO2 owing to heating, pressure decrease, photosynthesis, or other reasons disturbs this equilibrium, causing the reaction to shift to the left. This reaction is easily demonstrated in the laboratory. Nonetheless, it has now been adequately established that calcite does not precipitate freely in the presence of Mg in seawater. Aragonite nucleation is also inhibited, apparently by the presence of organophosphatic compounds Sparry calcite: The third major constituent of limestones, in addition to carbonate grains and micrite, is sparry calcite. Crystals of sparry calcite are large (0.02–0.1 mm) compared to micrite crystals and appear clear or white when viewed in plane light under a polarizing microscope. They are distinguished from micrite by their larger size and clarity and from carbonate grains by their crystalline shapes and lack of internal microstructures. Much of the sparry calcite in limestones occurs as a cement that fills interstitial space among carbonate grains. Sparry calcite cement is particularly common in grain-rich limestones, such as oolites, that were deposited in agitated water that prevented micrite from filling pore spaces. Therefore, as mentioned, the presence of significant amounts of sparry calcite cement in a limestone is commonly interpreted to indicate deposition of the limestone in agitated water. however, because much pore space in limestones can be secondary – produced by dissolution during diagenesis. Sparry calcite cement that fills secondary pores has no relationship to depositional conditions. Sparry calcite can form a variety of cementation fabrics, and several distinctive types of cement are recognized. The most common types are granular or mosaic cement, which is composed of nearly equant crystals; fibrous cement, either coarsely or finely fibrous; bladed cement; and syntaxial cement (overgrowths) – see Fig. (11). 18 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figur(11 ) Some important types of sparry calcite cement fabrics in limestones. B = brachiopod, C = crinoid, I = intraclast. The change from bladed crystals to larger, granular crystals in the lower left corner of the figure illustrates “drusy’ fabric. The most common syntaxial overgrowths are monocrystalline overgrowths around echinoderm fragments, which consist of plates composed of a single calcite crystal. Monocrystalline overgrowths are in optical continuity with these single-crystal echinoderm plates. Syntaxial overgrowths are also known to occur on brachiopod and mollusk fragments, foraminifer tests, and corals. Euhedral to anhedral, bladed or coarsely fibrous crystals that are oriented perpendicular to carbonate grain surfaces may display an increase in grain size toward the center of the pore or cavity and a concomitant change to more equant, granular crystals. This distinctive pore-filling fabric is commonly referred to as drusy cement. Drusy fabric is illustrated in Fig. (11) by the change from small bladed to larger granular crystals in the lower left corner of the figure. Figure (12) also illustrates drusy fabric. Some sparry calcite, referred to 19 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy as neospar, originates through recrystallization of micrite or carbonate grains (Fig. 13). This kind of sparry calcite does not fill pore space; therefore, its presence has no value in interpreting depositional conditions. It is important in petrologic studies to differentiate sparry calcite formed by recrystallization processes from sparry calcite cement. Although recrystallized sparry calcite does not commonly form a drusy fabric, it can mimic most of the other type of cement mentioned above. Therefore, differentiating between sparry calcite cement and recrystallization spar is a major problem in carbonate petrology Figure (12)Sparry calcite cementing rounded (dark) intraclasts. The cement displays drusy texture: small calcite crystals, oriented with their long dimensions perpendicular to the clast surfaces, grade outward from the margins of the clasts into larger, 20 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure(13) Coarse neospar (large, clear crystals) formed by recrystallization of micrite (dark). Note the indistinct boundary between the neospar and the “dirty” incipiently recrystallized micrite. Classification of carbonate rocks Classifications for carbonate rocks have not proliferated quite to the extent of sandstone classifications. Nonetheless, twenty or so carbonate classifications have appeared in print since the early 1960s. Folk’s classification (1962) Folk’s classification is applicable primarily to thin-section analysis. It is not an easy classification to use in the field. Use of the classification requires a definite knowledge of the kinds and abundances of carbonate grains (allochems) and the relative abundance of micrite and sparry calcite cement. Classification is made by first determining the relative abundance of carbonate grains (allochems) vs. micrite plus sparry calcite cement (Table 2). Further subdivision is then made on the basis of the relative abundance of the various types of carbonate grains and the relative abundance of micrite plus sparry calcite cement. Note, as mentioned, that the classification is hierarchal (e.g. > 25% intraclasts, < 25% intraclasts; > 25% ooids, < 25% ooids)…. For example, an oosparite is an ooid- rich rock cemented with sparry calcite cement that contains little micrite, whereas an oomicrite is an ooid-rich limestone in which micrite is abundant and sparry calcite is subordinate(Table2). 21 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Table 2 Classification of carbonate rocks according to Folk (1962) Additional textural information can be added by use of the textural maturity terms shown in Fig. (14). Thus, a packed oomicrite indicates a grain-supported oolitic limestone, and a sparse oomicrite is an oolitic rock with a mudsupported fabric. Note that Folk’s classification can also be used to classify dolomite rock, if “ghosts” of the original allochems are still identifiable in the dolomite. 22 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (14) Textural classification of carbonate sediments on the basis of relative abundance of limemud matrix and sparry calcite cement and on the abundance and sorting of carbonate grains (allochems). [After Folk, R. L., 1962, Dunham’s classification (1962 Dunham classification is focusing upon depositional limestone textures rather than upon the identity of specific kinds of carbonate grains. He considers two aspects of texture: (1) grain packing and the relative abundance of grains and micrite and (2) depositional binding of grains. To use this classification 23 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy the user must first determine if the original constituents of the limestone were or were not bound together at the time of deposition. For rocks composed of components not bound together during deposition (i.e. components deposited as discrete grains or crystals), the rocks are further divided into those that contain lime mud (micrite) and those that lack mud Rocks that contain lime mud are either mud-supported or grain- supported (Table 3).. Table 3; Dunham classification of carbonate rock Mud-supported limestones are mudstone (i.e. lime mudstones) if they contain less than 10 percent carbonate grains and wackestone if they contain more than 10 percent grains. Grainsupported limestones that contain some micrite mud matrix are packstone. Grain-supported limestones that lack mud 24 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy matrix are grainstone. Dunham uses the term boundstone for limestones composed of components bound together at the time of deposition. Embry and Klovan (1972) modified Dunham’s classification by subdividing limestones composed of originally unbound constituents into two groups on the basis of carbonate grain size. This modified classification scheme places more emphasis on limestone conglomerates (Table4).. Table 4 Classification of limestone according to depositional textures after Embry and Klovan (1972) Nonmarine carbonates Carbonate rocks also form in a variety of nonmarine settings, including lakes, streams, marshes, springs, caves, soils, and dune environments. The volume of these nonmarine or terrestrial carbonates is small, but they are an interesting addition to the overall carbonate record. Also, when they can be identified in ancient deposits, they make useful paleoenvironmental indicators. 25 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy 1- Lacustrine carbonates: Carbonate sediments occur in some freshwater lakes. The principal carbonate mineral formed in freshwater lakes is low- Mg calcite. The deposits of saline lakes may include low-Mg calcite, high-Mg calcite. Lacustrine carbonate sediments may include abiotic precipitates, algal carbonates, and carbonate shell accumulations. These carbonate materials may be mixed to various degrees with organic matter; biogenic silica (mainly diatom frustules); fine, detrital siliciclastics; and evaporite minerals. Abiotic precipitation is probably important mainly in saline lakes in areas where evaporation rates are high. Under these conditions, both loss of water and loss of CO2 can trigger precipitation of calcite and Mg-calcite. 2- Carbonates in rivers, streams, and springs: Only a very small volume of carbonate sediment forms in the flowing water of rivers, streams, and springs. Commonly, such water is too undersaturated with calcium carbonate to precipitate carbonate minerals; however, saturation may occur in waters that drain regions underlain by carbonate rocks. Carbonate precipitated from streams and cold-water springs commonly consists of low-Mg calcite, whereas precipitates from hot springs may also include aragonite. Travertine, is aterminology of carbonate sediments formed in these freshwater environments. The name travertine refer to the more massive, dense, finely crystalline varieties of these deposits. These carbonates range in color from tan to white or cream (Fig. 14). The more porous, spongy, or cellular varieties of these freshwater carbonates (Fig. 15), which commonly form as encrustations on plant remains, are called tufa. 26 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (14) travertine Figure (15) tufa Precipitation of travertine occurs predominantly in springs and spring-fed lakes and at waterfalls or cascades. Precipitation of travertine requires that ground waters or streams be supersaturated with calcium carbonate with respect to calcite and supersaturated in CO2 with respect to air. Precipitation can occur in cold-water springs owing to loss of CO2 resulting from higher temperatures at the mouth of the spring and exposure of spring water to the atmosphere (decrease in pressure). Tufas apparently form as a result of precipitation of calcite onto plants such as mosses and algae. In addition the bacteria are also important agents in precipitation of travertines. 27 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy 3- Speleothem (cave) carbonates: Carbon dioxide-rich groundwaters that migrate through carbonate formations dissolve away parts of the formations and create solution pipes, sinks, and caves. The geomorphological features resulting from such solution activity are referred to as karst features(Fig. 16). When carbonate- and CO2-saturated groundwater enters air- filled caves, carbonate precipitation occurs on a massive scale owing to loss of CO2, probably as a result of decreased pressure and evaporation. Therefore, caves in carbonate terrains are the sites of extensive carbonate deposits, which are referred to collectively as speleothems. These deposits may take the form of stalactites (conical projection hanging from the roof, formed by dripping water), stalagmites (conical dripstone projecting upward from the floor), laminated flowstones (formed by flowing water). 28 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (16) Features of a mature karst profile. The vadose zone is the groundwater zone of aeration, and the phreatic zone is the zone of saturation where pore space is filled with water. Note that speleothems occur especially in the vadose zone, whereas collapse breccias and other cave sediments are common in the phreatic zone. 29 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy 4- Caliche (calcrete) carbonates Soils in arid to semiarid regions, especially those developed on underlying carbonate rocks, may become so enriched in calcium carbonate that they form a caliche or calcrete deposit. Genetically, caliche is defined as a fine-grained, chalky to well-cemented, low-magnesian calcite deposit that formed as a soil in or on pre-existing sediments, soils, or rocks. however, as a more useful descriptive definition that “caliche is a vertically zoned, subhorizontal to horizontal carbonate deposit, developed normally with four rock types: (1) massive-chalky, (2) nodular-crumbly, (3) platy or sheet-like, and (4) compact crust or hardpan.” An idealized caliche profile is shown in Fig. 9.44. Esteban and Klappa stress that many variations of this profile exist and that the only consistent relation is that the massive-chalky rock grades downward into the original rock or sediment 30 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Dolomites Introduction Carbonate rocks range in age from Holocene to Precambrian. The mass of Precambrian carbonate rocks is much smaller than that of Phanerozoic carbonates, which are particularly abundant in stratigraphic sequences of Paleozoic age. Carbonate rocks less than about 100 million years old are dominantly calcium carbonates with a low Mg/Ca ratio consistent with the ratio that would be expected if the rocks formed mainly by accumulation of carbonate skeletal debris. The Mg/Ca ratio rises sharply, but irregularly, with increasing age in carbonate rocks older than about 100 million years. Thus, it has been a commonly accepted tenet that dolomites make up an increasing proportion of carbonate rocks with increasing age and that the average composition of Precambrian carbonate rocks approaches that of the mineral dolomite. This long- accepted view that dolomites increase in abundance relative to other carbonates with increasing age was challenged by Given and Wilkinson (1987). These authors maintain that whereas dolomites do change in relative abundance through time, Most ancient dolomites are relatively thick, coarse-crystalline, porous to nonporous, massive rocks, many of which appear to have formed through pervasive dolomitization (replacement and recrystallization) of precursor limestones. On the other hand, dolomites that form in modern environments, as well as some ancient dolomites, tend to be thinner and finer-crystalline and otherwise lack distinctive evidence of massive dolomitization. The origin of these fine-grained dolomites, as well as the mechanisms responsible for large-scale, pervasive dolomitization of ancient limestones, has been hotly debated by geologists for well over half a century. Dolomitic refers to (a) a dolomite-bearing or dolomite- containing rock that contains different amounts of the mineral dolomite. Dolomitic limestones yield a conspicuous amount of the mineral dolomite, but calcite is more important. The opposite is a calcareous dolomite. The ill-defined term 31 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy magnesium limestone designates a rock composed of mixtures of calcite and dolomite or a limestone with some MgO but no dolomite. Dolomite Kinds Two kinds of dolomite are recognized on the basis of the timing and nature of dolomite formation: (1) syndepositional (penecontemporaneous) and (2) postdepositional. Penecontemporaneous dolomites: form while the host sediments are in their original depositional setting; that is, they form under the geochemical conditions of the depositional environment. They form (in small amounts) primarily in shallow-marine to supratidal environments and mainly by direct precipitation from normal or evaporated seawater. They occur as thin layers and lenses in sabkhas, salinas, and evaporative lagoons/lakes. Postdepositional dolomites: form after deposition has ceased and the host carbonates have been removed from the zone of active sedimentation. Removal from the sedimentation zone may occur by sediment progradation, burial, uplift, eustatic sea-level change or any combination of these factors. Postdepositional dolomites form at various burial depths, ranging from a few meters to thousands of meters, where pore- water chemistries differ significantly from those of the depositional environment. Other authors classifieds dolostones as genetically into (1) syngenetic dolostone (penecontemporaneously formed within the depositional environment), (2) diagenetic dolostone (formed by replacement of carbonate sediments or limestones during of following consolidation), and (3) epigenetic dolostone (formed by localized. Mineralogy of dolomites As shown in Table (1), only two minerals belong to the dolomite group: dolomite [CaMg (CO3)2] and ankerite [Ca(Mg, 32 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Fe, Mn)(CO3)2]. Ideal dolomite is rare in the geologic record. Many natural dolomites, particularly modern or Holocene dolomites, are poorly ordered protodolomite with an excess of calcium. Although less common, magnesium-rich dolomite is known also. Dolomite and calcite commonly occur together in many carbonate rocks. they may be difficult to distinguish optically but the following criteria that may be used to help make the distinction : 1- Dolomite can be readily identified by X-ray diffraction techniques. 2- Dolomite is more likely to form euhedral crystals than is calcite. 3- Calcite is more likely to be twinned than is dolomite. 4- Dolomite may be colorless, cloudy, or stained by iron oxides, whereas calcite is commonly colorless Dolomite textures Dolomites may be composed of crystals of nearly uniform size (unimodal size distribution) or crystals of various sizes (polymodal size distribution). Dolomite can occur either as rhomb-shaped euhedral to subhedral crystals or as nonrhombic, commonly anhedral crystals,see figer 17 to explain the dolomite textures after (Greeg and Siply,1984) and (Siply and Greeg, 1987 ) 33 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure(17)Classification of dolomite textures. 34 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (18) Photomicrograph of dolomite crystals: A. Planar dolomite exhibiting euhedral crystals with planar faces (arrow),. B. Nonplanar dolomite, with curved or irregular faces. Origin of dolomite Theoretically, dolomites can form in three different ways: (1) by dolomitization, which is the replacement of CaCO3 by Ca Mg(CO3)2, (2) by dolomite cementation, which is precipitation of dolomite from aqueous solution in primary or secondary pore spaces, and (3) by precipitation from aqueous solution to form sedimentary deposits (“primary dolomite”). The volume of dolomite cement is small compared to the total volume of dolomite, and primary dolomite appears to be rare and restricted to some evaporitic lagoonal and/or lacustrine settings. Thus, the great bulk of dolomite in the geologic record apparently formed by dolomitization (replacement). Dolomites have been studied for two hundred years, but their origin is still enigmatic and controversial. The so-called “dolomite problem” arises in part from the fact that geochemists have not yet successfully precipitated well-ordered, stoichiometric dolomite in the laboratory at the normal 35 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy temperatures (∼ 25 o C) and pressure (∼ l atm) that occur at Earth s surface. The reluctance of such dolomite to precipitate under normal surface conditions has given rise to numerous theories to explain the occurrence of dolomite in the rock record. These dolomites clearly formed by replacement processes, with the mineral dolomite replacing calcite, Mg- calcite, or aragonite. Presumably, replacement occurred as Mg- bearing subsurface waters migrated through the carbonate sediments, possibly over periods of tens to hundreds of millions of years. Determining how extensive, thick sequences of ancient dolomites formed is also part of the dolomite problem. The requirements for dolomite formation, along with possible mechanisms of dolomitization, have been reviewed by numerous workers: The formation of dolomite has to be considered in terms of both thermodynamics and kinetics. Thermodynamic considerations include the Ca2+/Mg2+ ratio, Ca2+/CO3 2− ratio, and temperature, which define the CaCO3 and Ca Mg(CO3)2 thermodynamic stability fields in the system calcite–dolomite–water. Dolomite formation is thermodynamically favored in solutions of (1) low Ca2+/Mg2+ ratios, (2) low Ca2+/CO3 2− ratios (high carbonate alkalinity), (3) high temperatures, (4) salinities higher or lower than that of seawater. Several models to explain the origin of dolomites have been proposed; see,Figer 19 to axplain the dolomite models. 36 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure (19)Dolomitization models, after Folk,1962 Mixing-zone model Some modern/Holocene dolomites and most ancient dolomites are not directly associated with evaporites. Therefore, the hypersaline (reflux and sabkha) models do not appear to be appropriate for these dolomites. Hanshaw and others (1971) proposed that dolomitization could occur from brackish groundwaters that were produced through mixing of seawater derived brines and freshwater (e.g. Fig. 19 F). Such low-salinity groundwaters could be saturated with respect to dolomite at Mg/Ca ratios as low as 1:1. Subsequently, this concept was 37 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy further developed by Badiozamani (1973), Land (1973), and Folk and Land (1975). Mixing of meteoric waters with seawater causes undersaturation with respect to calcite, whereas dolomite saturation increases, resulting in replacement of CaCO3 by dolomite. Folk and Land (1975) maintain (in their schizohaline model) that in solutions of low salinity and low ionic strength, dolomite can apparently form at Mg/Ca ratios as low as 1:1. When seawater or evaporated brine with high Mg/Ca ratios is diluted by mixing with freshwater (schizohaline environment), the mixture will retain the high Mg/Ca ratio (low Ca/Mg ratio) but not the high salinity of the saline water. Thus, these mixed waters putatively become special waters capable of forming ordered dolomite. According to Folk and Land, dolomite formed from dilute solutions is perfectly clear with plane, mirror-like faces, (so called limpid dolomite) and is more resistant to solution than ordinary dolomite. In spite of initial acceptance of the mixing-zone model by many workers, and its application in some cases to explain pervasive dolomitization of entire carbonate platforms, the model appears to have fallen into disrepute. Machel (2004) suggests that the mixing-zone model has been highly overrated and that not a single location in the world has been shown to be extensively dolomitized in a freshwater–seawater mixing zone Summary, dolomites form by precipitation of CaMg(CO3)2 from solution (primary dolomite), by dolomite cementation in pore spaces of sediment, and by replacement (dolomitization) of precursor carbonate sediment (CaCO3) with CaMg(CO3)2. Primary precipitation and dolomite cementation account for only a minor amount of dolomite; replacement apparently generated most of the dolomite in the geologic record. Dolomites can form penecontemporaneously, while the host sediments are still in their original depositional setting, or postdepositionally after the host carbonate sediments have been removed from the zone of active sedimentation. Most dolomite is postdepositional. 38 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Diagenesis of carbonate rocks Introduction Carbonate minerals are more or less in chemical equilibrium with the waters of their depositional environment. By contrast,1 siliciclastic sediments are brought into the depositional basin from outside. Further,2 carbonate sediments are composed of only a very few major minerals (aragonite, calcite, dolomite) in contrast to a much larger variety of minerals and rock fragments that may be present in siliciclastic sedimentary rocks. 3 Carbonate minerals are more susceptible in general to diagenetic changes such as dissolution, recrystallization, and replacement than are most silicate minerals. Also, 4 they are generally more easily broken down by physical processes, and they are much more susceptible to attack by organisms that may crush or shatter shells or that may bore into carbonate grains or shells. That is, carbonate sediments go through early (shallowburial), middle (deep-burial), and possibly late (uplift and unroofing) stages of diagenesis. In terms of time and burial depth, these stages are similar to the eodiagenetic, mesodiagenetic, and telodiagenetic stages of siliciclastic diagenesis. Most carbonate sediments originate in marine environments. Therefore, this discussion of carbonate diagenesis focuses on the diagenesis of marine carbonates. Nonmarine carbonates also undergo diagenesis; however, diagenetic effects are generally less severe in nonmarine carbonates because they are composed of more stable carbonate minerals (mainly low-magnesian calcite) than are marine carbonates. Figure 20 illustrates schematically the principal environments of carbonate diagenesis. We recognize three major regimes of diagenesis (James and Choquette, 1983a): 39 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure 20 Major diagenetic environments. A -Simplified scheme B - Major processes occurring in different diagenetic environments 1. The seafloor and shallow-marine subsurface regime includes the seafloor and the very near-surface environment, It is characterized mainly by marine waters of normal salinity, although hypersaline waters are present in evaporative environments. Mixed marine–meteoric waters may be present also at the strandline and in the shallow subsurface at the mixing interface between the marine realm and the meteoric realm (Fig. 20) 2. The meteoric regime is distinguished by the presence of freshwater. It includes the unsaturated, vadose zone above the water table and the phreatic 40 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy zone, or saturated zone, below the water table. As mentioned, a zone of mixed marine–meteoric water exists between the meteoric and marinerealms. Nonmarine carbonates originate in the meteoric regime. Marine carbonates may be brought into this regime, in three ways: (a) by falling sea level, (b) by progressive sediment filling of a shallow carbonate basin (which produces a shallowing-upward sequence) until the sediment interface is at or above sea level, and (c) by late-stage uplift and unroofing of a deeply buried carbonate complex 3. The deep subsurface is referred to as the marine phreatic or deep phreatic zone by some authors. In the deep subsurface, sediment pores are filled with waters that were either marine or meteoric waters in the beginning. The composition of deep pore waters is commonly different, however, from either marine or meteoric waters owing to burial modifications Diagenetic Processes and Controls Major diagenetic processes affecting carbonate sediments and rocks are micritization ,dissolution and cementation, compaction, neomorphism, dolomitization, and the replacement of carbonate grains and matrix by non-carbonate rnineralogies A - Biogenic Alteration Organisms in carbonate depositional environments rework sediment by boring, burrowing, and sediment-ingesting activities, just as they do in siliciclastic environments. These activities may destroy primary sedimentary structures in carbonate sediment and leave behind mottled bedding and various kinds of organic traces. In addition, many kinds of small organisms, such as fungi, bacteria, and algae, create microborings in skeletal fragments and other carbonate grains. Fine-grained (micritic) aragonite or high-magnesian calcite may then precipitate into these holes. This boring and micrite- precipitation process may be so intensive in some warm-water 41 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy environments that carbonate grains are reduced almost completely to micrite, a process called micritization. If boring is less intensive, only a thin micrite rim, or micrite envelope, may be produced around the grain (Fig. 6.14A). Larger organisms, such as sponges and molluscs, create macroborings in skeletal grains and carbonate substrate, and other organisms, such as fish, sea cucumbers, and gastropods may break down carbonate grains in various ways to smaller pieces B -Cementation Cementation is an important process in all diagenetic realms. On the ocean floor, cementation takes place mainly in warm- water areas within the pore spaces of grain-rich sediments or in cavities. Reefs, carbonate sand shoals on the margins of platforms, and carbonate beach sands are favored areas for early cementation. Areas of the seafloor along the platform margin where sediments become well cemented are referred to as hardgrounds. Cemented carbonate beach sand is called beachrock. Seafloor cement is commonly aragonite, less commonly highmagnesian calcite. Seafloor cement can take several textural forms, as shown in Figure 6.15. Beachrock may contain meniscus cements that form where water is held by capillary forces as interstitial water drains from beaches during low tide. Because beach sediments are not constantly bathed in water, pendant cements may also form in beachrock along the bottoms of grains where drops of water are held. Isopachous rinds, which completely surround grains, form under subaqueous conditions where grains are constantly surrounded by water. Aragonite cements may also occur as a mesh of needles or as fibrous radial crystals that have a botryoidal form. In the meteoric realm, dissolution is a more important process than cementation; however, cementation does occur. The cement is almost exclusively calcite. As mentioned, the calcium carbonate that forms this cement is derived by dissolution of less stable aragonite and high-magnesian calcite. 42 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy In the (water-) unsaturated vadose zone, calcite cements are commonly meniscus and pendant cements. In the water- saturated phreatic zone, they are isopachous, blocky, or syntaxial rim cements. Syntaxial rims form by precipitation of optically continuous calcite around single-crystal fossil echinoderm fragments, in much the same way that cement overgrowths form around quartz grains. Calcite cementation may also take place during deep burial,. Factors that have been cited to favor carbonate cementation during deep burial include1 unstable mineralogy (aragonite and high-magnesian calcite favors solution and reprecrpitation);2 pore waters highly oversaturated in calcium carbonate; 3 high porosity and permeability (which enable high rates of fluid flow);4 increase in temperature; and 5 decrease in carbon dioxide partial pressure.. Coarse mosaic calcite and bladed prismatic calcite (Fig. 6.15) are common kinds of deep-burial cements. The combination of bladed prismatic and coarse mosaic cement shown in Figure (21) is called drusy cement. These calcite cements are commonly coarse grained and clear or white in appearance. They are usually referred to as sparry calcite cement 43 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy Figure 21: Principal kinds of cements that form in carbonate rocks during diagenesis C -Dissolution: Dissolution of carbonate minerals requires conditions essentially opposite to those that lead to cementation. Dissolution is favored by1 unstable mineralogy (presence of aragonite or high-magnesian calcite),2 cool temperatures, and 3 low pH (acidic) pore waters that are 4 undersaturated with calcium carbonate. Dissolution takes place particularly in chemically aggressive pore waters highly charged with C02 and/or organic acids. Dissolution is relatively unimportant on the seafloor but is particularly prevalent in the meteoric realm where chemically aggressive meteoric waters percolate or flow down through the vadose zone into the phreatic zone. Extensive dissolution of aragonite and high-magnesian calcite takes place in this environment and even calcite may be dissolved if pore waters are sufficiently aggressive. Dissolution tends to be concentrated particularly along the water table (the boundary between the vadose and phreatic zones), which accounts for the common presence of caves in carbonate rocks at the level of the water table. Dissolution is less intensive in the deep burial (subsurface) realm than in the meteoric realm for two reasons. First, most aragonite and high-magnesian calcite may already have been converted to more stable calcite in the meteoric realm. Second, increasing temperature at depth decreases the solubility of all carbonate minerals. Dissolution may occur at depth if enough C02 is added to pore waters as a result of burial decay of organic matter to overcome the decrease in solubility resulting from increased temperature Primary and secondary porosity: The porosity of sedimentary rocks falls into two major groups: Primary porosity forms during the predepositional stage 44 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy ( e.g. intragranular pores in foraminifers, corals, or ooids) and during the depositional stage (depositional porosity), e.g. intergranular porosity, framework growth porosity. Secondary porosity is formed during diagenesis at any time after deposition. The time involved in the formation of secondary porosity may be tremendously long, and can be subdivided into three stages called eogenetic, mesogenetic, and telogenetic by Choquette and Pray (1970) Figure (22). Important processes generating secondary porosity are dissolution, dolomitization/dedolomitization, fracturing and brecciation. Secondary porosity formation by dissolution occurs at any point in the burial history and can substantially enhance reservoir properties. Prime prerequisites for the formation of secondary porosity are the existence of a solution undersaturated with respect to carbonate, and fluids that transport the solutions. Figure 22: Major kinds of porosity after Choquette and Pray (1970). 45 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy D -Neomorphism Neomorphism is a term used by Folk (1965) to cover the combined processes of inversion (e.g., transformation of aragonite to calcite) and recrystallization. Inversion refers to the change of one mineral to its polymorph, such as aragonite to calcite. Strictly speaking, inversion takes place only in the solid (dry) state. When the transformation of aragonite to calcite takes place in the presence of water, it occurs by means of dissolution of the less stable aragonite and nearly simultaneous precipitation (replacement) by more stable calcite. Many geologists refer to this process as calcitization, as mentioned. During diagenesis, most aragonite is eventually calcitized. Recrystallization indicates a change in size or shape of a crystal, with little or no change in chemical composition or mineralogy. Calcitization and recrystallization commonly go hand in hand. Neomorphism may occur in all three diagenetic realms but is particularly important in the meteoric and subsurface diagenetic environments. Neomorphism may affect both carbonate grains and micrite and commonly increases crystal size. This process destroys original textures and fabrics and, when pervasive, may cause the entire rock to become recrystallized. Thus, a fine- grained (micritic) limestone can be converted into a coarse- grained sparry rock. On a smaller scale, recrystallization results in the formation of large, dear crystals of calcite that closely resemble sparry calcite cement. In fact, one of the most difficult problems in the microscopic study of carbonate rocks is to differentiate between sparry calcite cement and neomorphic spar E -Replacement: Replacement of calcium carbonate minerals by other minerals is a common diagenetic process. Dolomitization of CaC03 sediment is one kind of replacement process. In addition, many other kinds of noncarbonate minerals may replace 46 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy carbonate minerals during diagenesis, including microcrystalline quartz (chert), pyrite (iron sulfide), hematite (iron oxide), apatite (calcium phosphate), and anhydrite (calcium sulfate). Replacement can occur in all diagenetic envirorunents. We have already discussed the replacement of CaC03 by dolomite in seafloor and burial environments. Replacement of carbonates by microcrystalline quartz (chert) is also common in the meteoric and deep-burial environments. F -Physical and Chemical Compaction Newly deposited, watery carbonate sediments have initial porosities ranging from 40 to 80 percent. As burial into the subsurface proceeds, the pressure of overlying sediments brings about grain reorientation and tighter packing. As with siliciclastic sediments, compaction results in loss of porosity and thinning of beds at fairly shallow burial depth. At deeper burial to depths of about 1000 ft (305m) and at progressively higher overburden pressures, grains may also deform by brittle fracturing and breaking and by plastic or ductile squeezing. Even at burial depths as shallow as 100 m, compaction can reduce the depositional thickness of carbonate sediments by as much as one-half, with accompanying porosity losses of 50 to 60 percent of original pore volumes. At burial depths ranging from about 200 to 1500 m, chemical compaction of carbonate sediments is also initiated. On a larger scale, pressure-solution seams called stylolites develop. Stylolites are particularly common in carbonate rocks. The stylolite seams are marked by the presence of clay minerals and other fine-size noncarbonate minerals (commonly referred to as an i soluble residue) that accumulate as carbonate minerals dissolve. Stylolites range in size from microstylolites between grains, in which the amplitude of the interpenetrating contacts between grains is less than 0.25 mm, to stylolites with amplitudes exceeding 1 em (e.g., Fig. 23). Pressure solution, with accompanying stylolite formation, causes significant loss of porosity (perhaps as much as 30 47 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy percent of original pore volume) and thinning of beds Figure 23: Well-developed sutured stylolites in Cretaceous limestones, G- Dedolomitization: Dedolomitization is the diagenetic replacement of dolomite by calcite, particularly under the influence of meteoric water and porewaters of different composition, and often resulting in the formation of secondary porosity. Tlhis process affects marine, lacustrine, and terrestrial carbonates and occurs in meteoric and burial diagenetic environments Late diagenetic dedolomitization controlled by salinity variations of burial pore waters. Staining with Alizarine Red S shows the distributio of calcite and dolomite within the dolomite rhombs very clearly Summary Major controls on carbonate diagenesis are mineralogy and crystal chemistry, the chemistry of pore waters, water movement, dissolution and precipitation rates, grain size, and the interaction with organic substances. The extent and path of 48 Carbonate sedimentary rocks Dr.Rafe' I. Al-humaidy diagenetic reactions are determined by the thermodynamic stability of carbonate minerals being dissolved or precipitated, the saturation state of the diagenetic fluid and the available surface area for reaction. The effect of the parameters summarized above depends on the saturation stage and flow rates of the diagenetic fluids 49