Abstract The term “diagenesis” refers to essentially everything that happens to sediments and rocks after their deposition but prior to metamorphism. There are a variety of diagenetic processes, biological, chemical and physical, that ultimately convert sediments into sedimentary rocks. The earliest of those events are covered in this chapter on near-surface diagenesis; subsequent chapters cover processes and products that occur primarily during later stages of diagenesis (mainly mesogenesis). Those include mechanical and chemical compaction, cementation, dissolution, replacement and structural deformation. All these processes can profoundly affect the porosity, permeability and hydrocarbon reservoir potential of clastic terrigenous deposits, and most of them are a function of initial sediment composition and the changes in pressure, temperature and water chemistry that accompany progressive burial. Less explicitly covered, but potentially no less important, is diagenesis that can occur during one or more episodes of local or regional uplift and consequent exposure (telogenesis). These events also introduce changes in the pressure/ temperature/water chemistry regime of rocks, and thus can cause major diagenetic changes, especially grain dissolution and cementation. In an attempt to address the impacts of the various diagenetic events that rocks may experience, this book includes both a section on the recognition of porosity types as well as one on paragenesis (i.e., the placement of diagenetic events into a temporal sequence related to the burial/uplift history of rocks). Burial diagenesis is critically important in controlling the porosity of clastic terrigenous rocks and is, in the main, porosity destructive—that is, almost all rocks lose porosity with increased burial depth. Nonetheless, several factors can retard or inhibit porosity loss, including early grain-coating cements, that block later overgrowth cementation, regional overpressuring of basins that reduce effective overburden stresses and, under some circumstances, hydrocarbon entry that can reduce rock-water interactions. In addition, the processes of dissolution and fracturing may, under the right circumstance, lead to actual increases in subsurface porosity. So the discussion of porosity destruction, preservation and creation pervades all chapters in the diagenesis section, and emphasis is placed on recognition of key features associated with anomalous porosity retention or creation.

Abstract The Middle to Upper Jurassic shallow marine carbonate platform (Amran Group) is predominantly limestone to the west and northwest of Sana’a and limestone and dolomite to the east and northeast of Sana’a. Diagenesis of the Amran Group encompasses many processes with conspicuous effects, including cementation, dissolution, neomorphism, and compaction (both physical and chemical), producing secondary microporosity, micritization, and dolomitization. Dolomite cements are common and were precipitated mostly during later diagenesis in cavities and fractures. Replacive dolomitization occurred during shallow burial (small rhombic types) and during burial diagenesis with the formation of saddle dolomite. Integration of field, petrographic, and geochemical analysis (ICP) indicates that lithification of these carbonates occurred during synsedimentary and burial diagenesis, with much of the alteration controlled by eustatic sealevel change and regional tectonism. Four major subenvironments, in which diagenesis of the Amran Group was operative, can be recognized. (1) Synsedimentary diagenesis is characterized by the formation of isopachous and syntaxiai cements, hardgrounds (with associated borings and burrows, and shelter, fenestral, framework, interparticle, and intraparticle porosity), geopetal structures, and intraclasts, indicating deposition under marine conditions. (2) Shallow burial diagenesis shows other specific features such as leaching, recrystallization, and early dolomitization (both replacive and void–filLing) and mold–filling cements. Moldic and vuggy porosity distribution, early compaction, collapse breccia, and silt deposition indicates that the Amran Group continued to receive meteoric water following sediment stabilization, enlarging some molds and vugs by solution. (3) Deep burial diagenesis is characterized by dissolution, blocky calcite cement, late compaction (fractures and sutured grains), and saddle dolomite. (4) Uplift diagenesis is characterized by reopening of stylolites along fractures and development of dolomitization under meteoric conditions. The occurrence of nonferroari calcite and ferric oxides in rhombohedral zones in dolomite indicates that dedolomitization was driven by oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering.

Abstract Synsedimentary diagenesis in the marine realm is relatively uncomplicated (by comparison with meteoric and burial diagenesis) because it generally operates over short time spans (only years to thousands of years, in most cases) and involves a restricted range of pore fluid chemistries. Nevertheless, through a combination of physical, chemical and biological processes, coupled with access to a nearly unlimited supply of dissolved materials in seawater, marine diagenesis can often bring about remarkable change in carbonate sediments and produce some very complex fabrics. Furthermore, the subsequent overlay of meteoric or burial diagenetic alterations can greatly complicate the recognition of marine diagenetic fabrics in ancient carbonate rocks. That is especially true because the aragonitic or Mg-calcitic cements that result from marine diagenesis are essentially just as unstable in meteoric or burial-stage pore fluids as primary grains of those compositions. The intensity or extent of marine cementation is a function of the supply of solutes from seawater. Solute supply, in turn, depends on sedimentation rates and the effectiveness of water transport from the surface into the interior of a sediment pile. Mechanisms of water movement include, among others, wave forcing, tidal pumping, thermal convection, and diffusive transport. Areas of very slow sedimentation (e.g., hiatus surfaces, low-sedimentation-rate platform interiors, or low-productivity deep sea settings) can have substantial marine cementation (including hardgrounds) because they all have long times of contact between seawater and a thin package of sediment, even with no special mechanism for water pumping. In high-sedimentation rate areas, on the other hand, substantial marine cementation occurs mainly in reef front or coastal settings where wave or tidal action can force seawater through the sediments to a considerable depth. Likewise, atoll margins and steep carbonate platform flanks are sites of extensive marine cementation because of convective water input coupled, in some cases, with low sediment accumulation rates. Hot or cold seeps on the sea floor also represent sites of exceptional water throughput and extensive cementation. Grain and matrix dissolution are widespread in certain marine environments, particularly in cold- and deep-water areas. Modern oceanic waters have an aragonite compensation depth or ACD at roughly 1,500 m (the ACD is the depth below which aragonite does not accumulate because the rate of dissolution exceeds the rate of aragonite supply). Aragonite also is extensively dissolved in cool and cold-water shelf areas. The modern calcite compensation depth (CCD) lies at roughly 4,500 m (but that depth, as well as that of the ACD, varies with latitude, productivity, and other factors, and undoubtedly has varied significantly with geologic time). Bored (biodegraded) grains with cement infill of borings and generation of micrite envelopes (also discussed in the sections on pellets/peloids and sedimentary structures-borings). Isopachous crusts of fibrous to bladed, peloidal, or aphanocrystalline high-Mg calcite cement. The aphanocrystalline crusts consist of equant, less than 4 μ m-sized rhombs that look much like micrite. Isopachous crusts of fibrous aragonite cement within grain cavities and as intergranular cements (predominantly found in warm-water, slightly hypersaline settings and tropical beachrock deposits). Marine-cemented hardground formation in selected areas (see above) — associated, in many cases, with phosphate and glauconite cementation, boring and faunal encrustation, and intraclast formation. Large botryoids of cavity-filling aragonite and high-Mg calcite cement. Internal sediment fills of primary cavities or neptunian dikes in framework-supported sediments. Coastal beachrock and spray-zone cements. Microbe/cement associations in marine methane and thermal seeps. Modern marine cements in warm-water settings consist mainly of high-Mg calcite (~12-18 mol% Mg), but with extensive aragonite as well. In colder-water areas (temperate, polar and deep marine), high-Mg calcite cements predominate, but become scarcer and less Mg-rich at higher latitudes. Many ancient carbonate deposits certainly had aragonite and high-Mg calcite cements, perhaps with secular variations in their abundance (e.g., Wilkinson and Given, 1986 ), but low-Mg calcite marine cements may also have formed at some times. In older limestones, original aragonite and high-Mg calcite cements generally have been converted diagenetically to low-Mg calcite and must be recognized by micro-inclusions, geochemical analysis (especially Mg and Sr contents), relict morphologies or crystal outlines, or, as a last resort, characteristic patterns of preservation or alteration (former aragonitic cements, for example, typically have poor primary fabric preservation.) Characteristic morphologies of marine cements A diagrammatic depiction of some common types of modern marine high-Mg calcite and aragonite cements. Most of these morphologies will be illustrated in this section. Adapted from James and Choquette (1983) .

Abstract In terms of zinc, lead, and silver metal endowment, the Proterozoic sedimentary basins of northern Australia rank number one in the world. The Mt. Isa-McArthur basin system hosts five supergiant, stratiform, sedimentary rock-hosted Zn-Pb-Ag deposits (McArthur River, Century, Mt. Isa, Hilton, and George Fisher) and one supergiant strata-bound Ag-Pb-Zn deposit (Cannington). These superbasins consist of units deposited during three nested cycles of deposition and exhumation that occurred in the period from 1800 to 1580 Ma. The cycles took place in response to far -field extension and subsidence associated with a major northward-dipping subduction zone in central Australia. All major stratiform zinc-dominant deposits occur within rocks of the sag phase of the youngest Isa superbasin, which was deposited between 1670 and 1580 Ma. The strata-bound silver- and lead-rich Cannington deposit is hosted by highgrade metamorphosed clastic sedimentary rocks that are temporal correlatives of the basal extensional phase of the Isa superbasin. It exhibits distinct differences from the stratiform zinc-dominant deposits but shows similarities with Broken Hill-type deposits. The major stratiform Zn-Pb-Ag deposits exhibit many similar geological and geochemical features that include: (1) location close to regionally extensive normal and strike-slip synsedimentary faults, (2) organic-rich black shale and siltstone host rocks, (3) laminated, bedding-parallel synsedimentary sulfide minerals, (4) stacked ore lenses separated by pyritic and Fe-Mn carbonate-bearing siltstones, (5) lateral zonation exhibiting an increasing Zn/Pb ratio away from the feeder fault, (6) vertical zonation exhibiting decreasing Zn/Pb ratio upstratigraphy, (7) an extensive strata-bound halo of iron- and manganese-rich alteration in the sedimentary rocks surrounding and along strike from ore, (8) a broad range of δ 34 S values for sulfide minerals, from about 0 to 20 per mil, with pyrite exhibiting a greater spread than base metal sulfides, and (9) lead isotope ratios that indicate derivation of lead from intrabasinal sources with interpreted lead model ages being similar to the measured zircon U-Pb ages of the host rocks. These common features demonstrate that the stratiform Zn-Pb-Ag ores formed approximately contemporaneously with sedimentation and/or diagenesis. The exact timing of mineralization relative to these processes varies from deposit to deposit. However metamorphic overprints in some deposits (e.g., Mt. Isa, Hilton, Dugald River, Lady Loretta) have lead to recrystallization of sulfide minerals, making it difficult to interpret primary paragenetic relationships and absolute timing of mineralization. Mount Isa is the only northern Australian stratiform Zn-Pb-Ag deposit that has spatially associated highgrade copper mineralization. Textural and isotopic data for the stratiform Zn-Pb-Ag deposits suggest there is a spread of ore depositional processes from synsedimentary exhalative to syndiagenetic replacement. At McArthur River, for example, the highgrade laminated ores principally formed by synsedimentary exhalative processes. However, there is good evidence that the lower grade ores at the margins of the deposit formed at shallow depth in the organic-rich muds by syndiagenetic replacement and open-space fill. At Century, on the other hand, the textual and lead isotope evidence indicate the major mineralization probably formed by syndiagenetic replacement about 20 m.y. after sedimentation. At Mt. Isa, Hilton, and George Fisher, overprinting metamorphism precludes determination of the precise timing of ore deposition relative to sedimentation and diagenesis, but recent studies at the least metamorphosed George Fisher deposit suggest that syndiagenetic replacement was likely dominant. The lack of footwall stringer zones or hydrothermal vent complexes in the Zn-Pb-Ag deposits, coupled with the lateral and vertical Pb-Zn metal zonation, suggest the ores are of the vent-distal type, forming at some lateral distance from the hydrothermal vent or feeder fault. The laterally extensive strata-bound Fe-Mn car bonate halos indicate significant hydrothermal fluid volumes that have interacted with the sea-floor and sub-sea-floor sediments. These halos provide an important vector for exploration. Basin-scale, fluid-flow modeling has emphasized the importance of (1) early rift phase volcanic and volcaniclastic rocks as potential deep sources for metals, (2) clastic units at the top of the rift package that act as aquifers for basin-wide hy drothermal fluid flow, (3) evaporitic units that lead to high fluid salinity, which enhances metal transport, (4) thick packages of fine-grained dolomites and siltstones in the overlying sag phase sequence, which act as a seal over the fluid-rich reservoir rocks (rift clastics), and (5) deeply penetrating faults that provide the fluid conduit from the fluid reservoir and metal source area, located deep in the sedimentary basin, to the organic-rich trap rocks at the top of the section. Fluids were oxidized, low- to moderate-temperature (100°–250°C), near-neutral pH brines, with sulfate reduction in organic-bearing trap sites being the principal cause of zinc- and lead-bearing sulfide deposition.

Abstract The Troya Zn-Pb deposit is located in the Basque-Cantabrian basin (northern Spain) hosted by carbonate and terrigenous units of Lower Aptian age. The mineral association is very simple with sphalerite and galena as ore minerals, accompanied by pyrite-marcasite, and minor chalcopyrite. Barite, quartz and iron-bearing carbonates are included as gangue minerals. The sulfides occur as lens-shaped bodies parallel to the bedding in association with sideritized and silicified limestones. The Troya deposit occurs close to where synsedimentary faults intersect the limestone unit. Three exhalative pulses of mineralization are recognized indicating different episodes of fluid expulsion during basinal dewatering. The deposit consists of five tabular, stratiform lenses of massive sphalerite, pyrite-marcasite with variable amounts of siderite, barite and quartz. The thickness and distribution of the ore lenses and their sedimentary structures are controlled by local tectonism. Metal-zoning distribution outline the positions of the feeder zones, normally parallel to NW-SE and NS synsedimentary faults. Detailed petrography has revealed the presence of four mineralization stages which characterize early and late diagenesis conditions, produced by brines generated within the sedimentary sequence. The first stage probably represents deposition processes developed prior to burial diagenesis near the sediment-water interface. Consequently, this mineralization is regarded as the result of an exhalative sedimentary process (SEDEX deposit). Nevertheless, microtextural aspects of ores indicate successive stages of mineral accumulation, involving overgrowths, replacements, breccias, banded structures and redeposition during continuous burial diagenesis. These textures may be explained by the interaction of hydrothermal fluids with early sedimented sulfides and carbonate host rocks. Fluid inclusion studies indicate salinities of 14-26 wt%NaCl equivalent and temperatures in the range of 100°-175°C, consistent with those calculated according to the sulfur isotope fractionation between mineral pairs. Because sulfur has a bulk δ 34 S composition of about 7.5%o, and carbonate gangue shows both 18 O and 13 C depletions from normal regional values, a homogeneous source involving a warm deep brine is indicated. Lead isotope analyses of galena are also quite homogeneous and show that lead has an upper crustal provenance. In short, the Troya deposit is a typical carbonate hosted lead-zinc deposit, formed from basinal brines onto the seafloor (carbonate SEDEX), that exhibits a wide spectrum of syngenetic and replacement features.

Abstract: A one-meter-thick marine red bed (Cretaceous oceanic red beds, CORBs) is reported from Early Turanian sediments associated with the Levant carbonate platform in central Jordan. These CORBs are of regional significance, in that deposits similar in facies and age are present in various sections of the Levant carbonate platform farther southwest in the Sinai. The red bed represents a rare shallow marine counterpart to the widely known deeper marine CORBs. The onset of sedimentation of these brick-red marls in a shallow sea (shallow subtidal) of the southern Tethys margin is shown to be synchronous with the Tethys-wide onset of marine red beds in oceanic settings in the latest Early Turonian. The transition into red marls marks a significant change in sedimentation from marly, gypsum-rich clay, representing lowstand deposits below, into a sequence including massive platform limestone beds forming a transgressive systems tract above the red bed. The sedimentary conditions on the Levant platform during red-bed deposition show some similarities to its deeper marine counterparts on the Northern Tethys margin: they are related to strongly fluctuating sedimentation rates, and they follow periods of high marine productivity, which occurred in the aftermath of OAE2. It is obvious that both strong synsedimentary fluctuations in water depth and accumulation rate and significant early and late evaporite diagenesis influenced the investigated section, so the cause of the red coloring is likely to be not solely a synsedimentary feature. The time-equivalent onset of shallow marine red beds and deep marine red beds in the Early Turonian indicates that both share common global prerequisites regardless of the paleobathymetry. Key Words: Cretaceous oceanic red beds, diagenesis, shallow marine, Levant carbonate platform, Turonian