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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.

Introduction

The Middle–Upper Jurassic carbonate sediments of the Amran Group extend from outcrops in the Sana’a region to the east into the subsurface in the central part of Yemen in the Wadi Al–Jawf–Marib–Shabwa’a Basin (Fig. 1). Full details of the sedimentology and stratigraphy of the measured sections, together with a complete review of previous works, are given in Al–Thour (1988), El–Anbaawy and Al–Thour (1989), Al–Ganad (1991), Al–Thour (1992b), Simmons and Al–Thour (1994), and Al–Thour (1997). These sediments either unconformably overlie the Precambrian basement rocks or disconformably overlie the Kohlan Formation and unconformably underlie the Tawilah Group and/or the Sabatayn Group. The thickness of the Amran Group is variable, ranging from 300 to 520 m. Through correlation between the different sections of each traverse, a composite lithostratigraphic log of the Amran Group was drawn up, and has a total thickness of 900 m (Fig. 2). Correlation between traverses is based on the specific sedimentary characteristics of the key horizons, as well as fossils (e.g., algae, foraminifera, corals, stromatoporoids, and palynomorphs), in different traverses (Al–Thour, 1988; Al–Ganad, 1991; A–Thour, 1992b; Simmons and Al–Thour, 1994).

Studies of the diagenetic history of ancient carbonate platforms suggest a number of basic controls on the nature and degree of diagenesis, including the original facies and mineralogy, climatic conditions, sea–level changes, and burial history. Periods of emergence and flooding of a platform brought about by eustatic and/or tectonically induced changes in relative sea level produce significant variations in the patterns of diagenesis. Climate has been shown to exert a major control on early diagenesis and diagenetic history (Hird and Tucker, 1988; Wright, 1988; Moss and Tucker, 1995).

Fig. 1.

—Simplified geological map of the central and western part of Yemen showing the location of the studied areas and measured traverses (1, A1 Khothally; 2, Hamdah; 3, Wadi Nager; 4, A1 Ayein; 5, Wadi Ahjur; 6, A1 Gedaan; 7, Jabal Salab). Major tectono sedimentary units in the Arabian Peninsula (after Alsharhan and Kendall, 1986) are shown on the left corner of the map.

Fig. 1.

—Simplified geological map of the central and western part of Yemen showing the location of the studied areas and measured traverses (1, A1 Khothally; 2, Hamdah; 3, Wadi Nager; 4, A1 Ayein; 5, Wadi Ahjur; 6, A1 Gedaan; 7, Jabal Salab). Major tectono sedimentary units in the Arabian Peninsula (after Alsharhan and Kendall, 1986) are shown on the left corner of the map.

Fig. 2.

—Composite lithostratigraphic log of the Jurassic Araran Group.

Fig. 2.

—Composite lithostratigraphic log of the Jurassic Araran Group.

The carbonate sediments of the Amran Group exhibit evidence of great variety of diagenetic processes that caused porosity reduction through physical compaction, pressure solution, and mineral precipitation. These processes reflect the initial cycle of burial of the sediment, which then endured multiple episodes of burial and uplift. Despite the economic importance of these Jurassic carbonates, their diagenesis remains poorly understood. By detailing the overall diagenetic processes of the Middle–Upper Jurassic carbonate platform of the Amran Group, this paper aims to provide a diagenetic synthesis of this platform as a whole, through integration of field, petrographic, and geochemical analysis (ICP). It deals with the description and interpretation of the postdepositional processes and their sequences. Field observation and petrographic examination allow for more definitive statements about the extent of diagenesis.

Regional Geology

The sedimentary cover of the Arabian Peninsula is divided into three tectonic provinces (Powers et al., 1966). These are: (a) the Arabian Shield, where Precambrian (igneous and metamorphic) rocks are exposed, (b) the Arabian Shelf, including basins where the basement rocks are covered by a wedge of Phanerozoic sediments, and (c) the Mobile Belt, the unstable northern and eastern margins of the era ton where the Taurus–Zagros–Oman Mountains are now emplaced (Fig. 1). The Phanerozoic sediments that cover the platform reflect a history of deposition during periods of marine transgression and regression of gen-erally shal low shelf seas across the low–lying part of the Arabian Shield. The history of the carbonate platform during the Jurassic and Cretaceous of is overwhelming importance because of the extraordinary richness of hydrocarbons in these horizons (Shebl and Alsharhan, 1994; Alsharhan and Nairn, 1997). The oil and gas production from Mesozoic carbonates and siliciclastics in the central part of Yemen of the Wadi Al–Jawf–Marib–Shabwa’a Basin, regardless of age, is directly related to the occurrence of porous limestone, dolomite, and sandstone. In the Middle–Upper Jurassic sequence, the normally nonporous limestone became oil and gas reservoirs through fracturing or faulting and associated dolomitization (Al–Thour, in press).

The Middle to Late Jurassic in Yemen was a time of a gradual transgression of a shallow sea from the east and south (Al–Thour, 1997). At this time, the platform subsided steadily following continuous transgression. This subsidence occurred against a background of fluctuations in sea level. These could have been eustatic, but this is difficult to prove in the absence of refined biostratigraphic correlation. A quite stable pattern of sedimentation was established in the basin with abundant stromatoporoid and coral buildups. Mixed carbona tesiliciclastic sediments were deposited at a late stage, indicating tectonic differentiation of the platform. The first transgression phase led to deposition of the Al–Khothally Formation (Fig. 2). The second phase produced a deeper–water facies of the Raydah Formation. The relative sea-level changes together with increasing tectonic activity led to deposition of the mixed carbonate–siliciclastic sediments of the Wadi Al–Ahjur Formation, with the development of paleokarst surfaces at the top part of Al–Ayein traverse, indicating emergence.

Lithostratigraphic and paleogeographic studies by Beydoun (1988, 1991) and Alsharhan and Nairn (1997) show that the development of the carbonate platform, in the areas studied, began in Late Callovian time (Figs. 2, 3). The terrestrial clastic sandstone of the Kohlan Formation (Triassic–Middle Jurassic) is succeeded by shallow marine carbonate sediments and then mixed carbonate–siliciclastic deposits marking a general shallowing of the sea following an earlier phase of transgression. During Oxfordian–Kimmeridgian times some tectonic differentation was initiated. The carbonate platform was submerged in the Oxfordian–Kimmeridgian and the Raydah Formation was deposited, indicating a major rise of relative sea level. Gradual subsidence of the shelf could have been responsible for this submergence. However, the global picture suggests that this more likely reflects a eustatic rise of sea level (Hallam, 1978, 1988). The pattern of sedimentation changed sharply in the Tithonian, when a major sea-level fall occurred. Carbonate and siliciclastic deposits were established on the shelf, and the influx of terrigenous material increased sharply. As already indicated, however, a component of regional tectonism also increased (Al–Thour, 1997).

The overlying Upper Jurassic–Lower Cretaceous Sabatayn Group is composed of a very thick (6000 m) sedimentary succession of dominantly argillaceous and evaporitic facies overlain by detrital formations (Fig. 3). Synsedimentary tectonic activity from the Late Jurassic to the Early Cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsiding rift basin. The unstable margin of the Wadi Al–Jawf–Marib–Shabwa’a riftbasin was the site of rapid facies changes, local disconformities, and periods of emergence and dolomitization along the WNW– and NNW–striking boundary fault system (Al–Thour, 1988; El–Anuaawy and Al–Thour, 1989; Al–Ganad, 1991). The overlying Cretaceous Tawilah Group is composed of fluvial siliciclastic sediments (Al–Subbary et al., 1993; Alsharhan and Nairn, 1997).

Fig. 3.

—Generalized geologic column of various rock units in Yemen (modified from Beydoun, 1988, 1991; copyright material reporduced with permission of Scientific Press).

Fig. 3.

—Generalized geologic column of various rock units in Yemen (modified from Beydoun, 1988, 1991; copyright material reporduced with permission of Scientific Press).

Analytical Techniques

The chemical analyses, by inductively coupled plasma emission spectrometry (ICP), were made at the Earth Sciences laboratory of the University of Birmingham, UK. ICP analysis of carbonates was performed after petrographic study and was used to determine the chemical composition of fifty–three samples from seven traverses. About 0.02–0.04 g of the finely powdered sample drilled from the pure micrites was homogenized and dried at 75°C for 2–3 hours. About 10 mg of the powdered samples was digested in 10 ml of 5% Aristar–grade HC1 and the insoluble residue content determined. About 5 ml of the soluble part of the solution was separated and stored in polyethylene containers for ICP analysis.

The standards used were prepared from individual atomic absorption metal solution, and several standard reference sediment samples were analyzed to check the accuracy of the instrument. To run the analysis, the dissolved sample is nebulized into the ICP torch, where it is atomized in a high–temperature argon plasma (8000–10,000 K) and emits an atomic spectrum, which is resolved into the atomic lines of each element by an optical grating in an optical spectrometer. Each element–selective atomic spectral line is measured with a photomultiplier detector on the local plane of the spectrometer. The signal from each photomultiplier is amplified and printed through a microprocessor. All trace–element compositions are given in ppm of the carbonate fraction.

Facies Sequences In The Amran Group

The Amran Group carbonate platform is divisible into three formations (Al–Thour, 1997) (Figs. 2, 4): (1) The lower Al–Khothally Formation is upper Callovian–Oxfordian in age (Zones 1–5 of Simmons and Al–Thour, 1994). This unit consists of sandy, oolitic, oncolitic, peloidal, partly dolomitic, massive to thick–bedded limestone about 200 m thick. (2) The middle Raydah Formation is early Kimmeridgian in age (Zones 6–8 of Simmons and Al–Thour, 1994). This unit shows several lithological facies beginning with massive, thick, cherty, fossiliferous, bituminous grainstone, packstone, wackestone, mudstone, and silty limestones intercalated with marly fossiliferous limestones, capped by massive mudstones with echinoids, stromatoporoids, and corals. It is about 400 m thick. (3) The upper Wadi Al–Ahjur Formation is late Kimmeridgian–Tithonian in age (Zones 9 and 10 of Simmons and Al–Thour, 1994) and consists of several carbonate intervals of mudstone, wackestone, packstone, and grainstone intercalated with sandy dolomitic marly fossiliferous shale and partly silty limestone. It is about 300 m thick.

The carbonate sediments of the Amran Group are composed of outcrop–scale sedimentary cycles of various types and scales (Al–Thour, 1992b). They comprise three facies associations (Al– Thour, 1997). Their organization is characterized by interfingering relationships, indicating that these sediments were deposited in a variety of depositional settings. The facies are grouped on the basis of shared sedimentological and paleontological features and named after the dominant environment in which they were interpreted to have been deposited. The three facies associations (Fig. 5) are distinguished in the Amran Group: (Fl) carbonate platform facies, (F2) carbonate–marl alternations facies, and (F3) shallow–water coral and stromatoporoid buildup facies.

Fig. 4.

—Micropaleontological biozonation of the Amran Group (after simmons and Al-Thour, 1994).

Fig. 4.

—Micropaleontological biozonation of the Amran Group (after simmons and Al-Thour, 1994).

Fig. 5.

—Stratigraphic sections showing fades associations and abundance within the different rock units of the Amran Group (modified after Al–Thour, 1997).

Fig. 5.

—Stratigraphic sections showing fades associations and abundance within the different rock units of the Amran Group (modified after Al–Thour, 1997).

Fl dominates in all formations of the Amran Group. F2 is well developed in the Raydah and Wadi Al–Ahjur Formations, and F3 can be recognized in the upper part of the Raydah Formation but its distribution is limited.

The lateral and vertical changes of facies are interpreted to be due to relative sea–level changes responding to associated tectonism and eustasy. The repetition and interfingering of both fining–upward and shallowing–upward cycles in the study areas suggest that deposition occurred during a period fluctuating eustatic sea level.

Diagenetic Synthesis Of The Amran Group

Four major diagenetic sub–environments affected the Amran Group: (1) synsedimentary diagenesis, (2) shallow burial diagenesis, (3) deep burial diagenesis, and (4) uplift diagenesis. A generalized diagenetic scheme for the Amran Group (Fig. 6) showing the different types of cements and their relation to diagenetic sequences is based on study of over 450 thin sections using both standard petrography and cathodoluminescence. Diagenesis of the Amran Group encompasses many processes with conspicuous effects on carbonate sediments, such as cementation and dissolution to produce secondary porosity (Table 1), including the development of secondary microporosity (e.g., when unstable minerals such as aragonite were dissolved to leave molds) (Figs. 7, 8). Early cementation in the sediments is seen in the nuclei of ooids and oolitic coated grains and the internal framework of colonial organisms (Table 2). Cementation has replaced the nucleus of the ooids and/or skeletal grains prior to physical compaction. Compaction (both physical and chemical) becomes an increasingly significant process as a result of increasing overburden pressure. In the early stages of burial, physical compaction is more important and results in closer packing and fracturing of grains (Table 3). Eventually, chemical compaction leads to dissolution at grain contacts and the formation of planar and/or anastomosing stylolites by dissolution.

Fig. 6.

—Diagram showing the different types of cements and their relation to diagenetic sequences.

Fig. 6.

—Diagram showing the different types of cements and their relation to diagenetic sequences.

Fig. 7.

(opposite page).–A) Photomicrographofskeletal packstone microfacies showing interparticle porosity and the occurrence of micritization around bivalve and echinoid fragments, scale is 1 mm; B) Photomicrograph of skeletal wackestone microfacies showing the development of framework porosity within algae (Rivularia piae), scale is 0.5 mm; C) Photomicrograph of fenestral mudstone microfacies showing fenestrae which forms by dissolution; scale is 4 mm; D) Photomicrograph of boundstone microfacies showing shel-tering effects of compacted shallow marine gastropods; scale is 10 mm; E) Photomicrograph of dolomite microfacies showing the development of intercrystalline porosity; incipient dedolomitization has occurred; scale is 1 mm.; F) View of the Jurassic–Cretaceous paleokarst surface at the top of the AT Ayein traverse, Thula area, Wadi Al–Ahjur Formation

Fig. 7.

(opposite page).–A) Photomicrographofskeletal packstone microfacies showing interparticle porosity and the occurrence of micritization around bivalve and echinoid fragments, scale is 1 mm; B) Photomicrograph of skeletal wackestone microfacies showing the development of framework porosity within algae (Rivularia piae), scale is 0.5 mm; C) Photomicrograph of fenestral mudstone microfacies showing fenestrae which forms by dissolution; scale is 4 mm; D) Photomicrograph of boundstone microfacies showing shel-tering effects of compacted shallow marine gastropods; scale is 10 mm; E) Photomicrograph of dolomite microfacies showing the development of intercrystalline porosity; incipient dedolomitization has occurred; scale is 1 mm.; F) View of the Jurassic–Cretaceous paleokarst surface at the top of the AT Ayein traverse, Thula area, Wadi Al–Ahjur Formation

Fig. 8.

(opposite page).—A) Photomicrograph of skeletal grainstone-wackestone microfacies showing columnar stubby cement that developed within leached shells and grew out from grain to meet another similar stubby spar growing from opposite grain, scale is 1 mm; B) Photomicrograph of packstone–wackestone microfacies showing small stubby crystals that form crudely isopachous rinds of cement, particularly within leached shells; scale is 1 mm; C) Photomicrograph of oolitic grainstone microfacies showing the effect of physical compaction and dissolution of aragonite; the compaction of the outer ooid layer suggests dissolution of the aragonite layer. Note that most of the ooid has Mg–calcite; scale is 1 mm; D) Scanning electron micrograph showing microporosity within mudstones. Note that the micrite is inhomogeneous, which indicates several different sources of the micrite, scale is 0.1 mm; E) Photomicrograph of skeletal wackestone microfacies showing the development of dolomite within micrite. Note that dolomite rhombs are partly dedolomitized; scale is 1 m; F) Jurassic calcrete paleosols from Al–Geda’an section, Naihm area to the east of Sana’a, Yemen. Note the presence of black pebbles in both the calcrete and the overlying carbonate; object in center is about 10 cm.

Fig. 8.

(opposite page).—A) Photomicrograph of skeletal grainstone-wackestone microfacies showing columnar stubby cement that developed within leached shells and grew out from grain to meet another similar stubby spar growing from opposite grain, scale is 1 mm; B) Photomicrograph of packstone–wackestone microfacies showing small stubby crystals that form crudely isopachous rinds of cement, particularly within leached shells; scale is 1 mm; C) Photomicrograph of oolitic grainstone microfacies showing the effect of physical compaction and dissolution of aragonite; the compaction of the outer ooid layer suggests dissolution of the aragonite layer. Note that most of the ooid has Mg–calcite; scale is 1 mm; D) Scanning electron micrograph showing microporosity within mudstones. Note that the micrite is inhomogeneous, which indicates several different sources of the micrite, scale is 0.1 mm; E) Photomicrograph of skeletal wackestone microfacies showing the development of dolomite within micrite. Note that dolomite rhombs are partly dedolomitized; scale is 1 m; F) Jurassic calcrete paleosols from Al–Geda’an section, Naihm area to the east of Sana’a, Yemen. Note the presence of black pebbles in both the calcrete and the overlying carbonate; object in center is about 10 cm.

Table 1.

Porosuty in the Middle–upper Jurassic Carbonates of the Amran Group.

DiageneticFeaturesDescriptionInterpretationOccurrencesFigures
Interparticle Grains develop in mud-free carbonatesediments; their distribution is verylimited. Have spherical shape, theirsize ranges between 2 mm and 1. 5 cm. The shape variation of grains is afunction of their biological origin. Thetype of fossil grains indicate marine ornonmarine environment. Common in all traverses. 7A 
Intraparticle Developed within grains e. g. chambersof foraminifera, gastropods, bryozoa, corals and stromatoporoids. They showwell–developed pore spaces. Grains tend to become quickly filledduring early reef development by bothcoarse and fine–grained internalsediments, leading to a complexdepositional pore system. Thedevelopment of this porosity withingrains before, during, and shortly afterdeposition indicate a marineenvironment. Common in Al–Khothally, middle–upperpart of the Hamdah, middle part ofWadi Nager, and lower–middle part ofAl–Ayein, Jabal Salab, and rare in WadiAl–Ahjur and Al–Gedaan traverses. 7B 
Fenestral Pores are small and vary from onemicrofacies to another. Rock surfaceshave a distinctive mottled structure. They are a fabric–selective depositionalindicating a marine environment. Themottled structure indicates intensebioturbation. Common in Al–Khothally Hamdah, Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, andJabal Salab traverses, but rare in WadiNager traverse. 7C 
Shelter Occurs below shell fragments in aconvex–up position. Fossil fragmentshave been affected by dissolution, cementation, and compaction.Compaction may take place prior tocementation. Fossil grains provide an umbrella fromthe area beneath, protecting it fromfiner interstitial detritus. Sediments areunable to fill the cavities because ofthe “umbrella” effect of the shell. It is afabric–selective depositional type. Common in the middle and upper partof Al–Khothally, upper part of Hamdah, middle part of Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, and Jabal Salabtraverses, but is rare in Wadi Nagertraverse. 7D 
Vuggy Irregular secondary holes formed bydissolution that cut across grainsand/or cement boundaries. Associatedwith unconformities. Developed as a result of dissolutionunder meteoric conditions. Common in Al–Gedaan and rare in Al–Khothally Hamdah, middle part of WadiAger. 7E 
Moldic Formed by the dissolution of aragonitebioclasts. Have different shapes andsizes depending on their mode oforigin. Represents the first phase ofdissolution and may have been relatedto exposure and meteoric dissolution ofthe sediments at the end of the AmranGroup times. Well developed in all traverses. 7F 
Intercrystalline Associated with replacive dolomitesformed by recrystallization oflimestones and is the result ofdissolution and dolomitizationprocesses. A second phase ofdissolution occurred afterdolomitization, created minor amountsof intercrystalline porosity by thepartial dissolution of zoned dolomite.The crystal shape is variable and thesize ranges between 30 and 280 μm. Form an important reservoir type in anumber of settings ranging fromsupratidal/sabkha to normal marinesequences. Developed in Jabal Salab, Al–Gedaan, lower part of Al–Khothally, upper partof Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 7G 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Interparticle Grains develop in mud–free carbonatesediments; their distribution is verylimited. Have spherical shape, theirsize ranges between 2 mm and 1. 5 cm. The shape variation of grains is afunction of their biological origin. Thetype of fossil grains indicate marine ornonmarine environment. Common in all traverses. 7A 
Intraparticle Developed within grains e. g. chambersof foraminifera, gastropods, bryozoa, corals and stromatoporoids. They showwell–developed pore spaces. Grains tend to become quickly filledduring early reef development by bothcoarse and fine–grained internalsediments, leading to a complexdepositional pore system. Thedevelopment of this porosity withingrains before, during, and shortly afterdeposition indicate a marineenvironment. Common in Al–Khothally, middle–upperpart of the Hamdah, middle part ofWadi Nager, and lower–middle part ofAl–Ayein, Jabal Salab, and rare in WadiAl–Ahjur and Al–Gedaan traverses. 7B 
Fenestral Pores are small and vary from onemicrofacies to another. Rock surfaceshave a distinctive mottled structure. They are a fabric–selective depositionalindicating a marine environment. Themottled structure indicates intensebioturbation. Common in Al–Khothally Hamdah, Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, andJabal Salab traverses, but rare in WadiNager traverse. 7C 
Shelter Occurs below shell fragments in aconvex–up position. Fossil fragmentshave been affected by dissolution, cementation, and compaction.Compaction may take place prior tocementation. Fossil grains provide an umbrella fromthe area beneath, protecting it fromfiner interstitial detritus. Sediments areunable to fill the cavities because ofthe "umbrella’’ effect of the shell. It is afabric–selective depositional type. Common in the middle and upper partof Al–Khothally, upper part of Hamdah, middle part of Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, and Jabal Salabtraverses, but is rare in Wadi Nagertraverse. 7D 
Vuggy Irregular secondary holes formed bydissolution that cut across grainsand/or cement boundaries. Associatedwith unconformities. Developed as a result of dissolutionunder meteoric conditions. Common in Al–Gedaan and rare in Al–Khothally Hamdah, middle part of WadiAger. 7E 
Moldic Formed by the dissolution of aragonitebioclasts. Have different shapes andsizes depending on their mode oforigin. Represents the first phase ofdissolution and may have been relatedto exposure and meteoric dissolution ofthe sediments at the end of the AmranGroup times. Well developed in all traverses. 7F 
Intercrystalline Associated with replacive dolomitesformed by recrystallization oflimestones and is the result ofdissolution and dolomitizationprocesses. A second phase ofdissolution occurred afterdolomitization, created minor amountsof intercrystalline porosity by thepartial dissolution of zoned dolomite.The crystal shape is variable and thesize ranges between 30 and 280 μm. Form an important reservoir type in anumber of settings ranging fromsupratidal/sabkha to normal marinesequences. Developed in Jabal Salab, Al–Gedaan, lower part of Al–Khothally, upper partof Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 7G 
 
7H h– 
Common in all traverses. Well developed in Al–Ayein, Thula, Al–Gedaan, and Jabal Salabtraverses. 
Indicate tectonic deformation orsolution collapse associated withdissolution of limestone. Developed atany time during burial history of acarbonate sequence starting withshallow burial because of commonearly lithification. Indicates deposition under shallowmarine environment. Brecciation ofcarbonate rock sequences can occurin a number of situations, includinglimestone solution collapse, faulting, orsoil formation. These breccia zonesdeveloped during episodic subaerialexposure. 
Occurs in brittle, normallyhomogeneous carbonates. A tensionalphase causes the opening of fracturesor suture seams or if there aremineralogical changes in the rock suchas dolomitization. Formed as a result of tectonic activitythat led to the development of karstsurfaces. Associated with limestonesolution collapse, often resulting inenhanced porosity that may form eithera reservoir for hydrocarbons or a hostfor mineralization. 
 ?o ~ 
  
D ir 03 o) 
 WO _Q 
 o o _ 
CO ^ s 
LL 0– (0 5 
 
DiageneticFeaturesDescriptionInterpretationOccurrencesFigures
Interparticle Grains develop in mud-free carbonatesediments; their distribution is verylimited. Have spherical shape, theirsize ranges between 2 mm and 1. 5 cm. The shape variation of grains is afunction of their biological origin. Thetype of fossil grains indicate marine ornonmarine environment. Common in all traverses. 7A 
Intraparticle Developed within grains e. g. chambersof foraminifera, gastropods, bryozoa, corals and stromatoporoids. They showwell–developed pore spaces. Grains tend to become quickly filledduring early reef development by bothcoarse and fine–grained internalsediments, leading to a complexdepositional pore system. Thedevelopment of this porosity withingrains before, during, and shortly afterdeposition indicate a marineenvironment. Common in Al–Khothally, middle–upperpart of the Hamdah, middle part ofWadi Nager, and lower–middle part ofAl–Ayein, Jabal Salab, and rare in WadiAl–Ahjur and Al–Gedaan traverses. 7B 
Fenestral Pores are small and vary from onemicrofacies to another. Rock surfaceshave a distinctive mottled structure. They are a fabric–selective depositionalindicating a marine environment. Themottled structure indicates intensebioturbation. Common in Al–Khothally Hamdah, Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, andJabal Salab traverses, but rare in WadiNager traverse. 7C 
Shelter Occurs below shell fragments in aconvex–up position. Fossil fragmentshave been affected by dissolution, cementation, and compaction.Compaction may take place prior tocementation. Fossil grains provide an umbrella fromthe area beneath, protecting it fromfiner interstitial detritus. Sediments areunable to fill the cavities because ofthe “umbrella” effect of the shell. It is afabric–selective depositional type. Common in the middle and upper partof Al–Khothally, upper part of Hamdah, middle part of Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, and Jabal Salabtraverses, but is rare in Wadi Nagertraverse. 7D 
Vuggy Irregular secondary holes formed bydissolution that cut across grainsand/or cement boundaries. Associatedwith unconformities. Developed as a result of dissolutionunder meteoric conditions. Common in Al–Gedaan and rare in Al–Khothally Hamdah, middle part of WadiAger. 7E 
Moldic Formed by the dissolution of aragonitebioclasts. Have different shapes andsizes depending on their mode oforigin. Represents the first phase ofdissolution and may have been relatedto exposure and meteoric dissolution ofthe sediments at the end of the AmranGroup times. Well developed in all traverses. 7F 
Intercrystalline Associated with replacive dolomitesformed by recrystallization oflimestones and is the result ofdissolution and dolomitizationprocesses. A second phase ofdissolution occurred afterdolomitization, created minor amountsof intercrystalline porosity by thepartial dissolution of zoned dolomite.The crystal shape is variable and thesize ranges between 30 and 280 μm. Form an important reservoir type in anumber of settings ranging fromsupratidal/sabkha to normal marinesequences. Developed in Jabal Salab, Al–Gedaan, lower part of Al–Khothally, upper partof Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 7G 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Interparticle Grains develop in mud–free carbonatesediments; their distribution is verylimited. Have spherical shape, theirsize ranges between 2 mm and 1. 5 cm. The shape variation of grains is afunction of their biological origin. Thetype of fossil grains indicate marine ornonmarine environment. Common in all traverses. 7A 
Intraparticle Developed within grains e. g. chambersof foraminifera, gastropods, bryozoa, corals and stromatoporoids. They showwell–developed pore spaces. Grains tend to become quickly filledduring early reef development by bothcoarse and fine–grained internalsediments, leading to a complexdepositional pore system. Thedevelopment of this porosity withingrains before, during, and shortly afterdeposition indicate a marineenvironment. Common in Al–Khothally, middle–upperpart of the Hamdah, middle part ofWadi Nager, and lower–middle part ofAl–Ayein, Jabal Salab, and rare in WadiAl–Ahjur and Al–Gedaan traverses. 7B 
Fenestral Pores are small and vary from onemicrofacies to another. Rock surfaceshave a distinctive mottled structure. They are a fabric–selective depositionalindicating a marine environment. Themottled structure indicates intensebioturbation. Common in Al–Khothally Hamdah, Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, andJabal Salab traverses, but rare in WadiNager traverse. 7C 
Shelter Occurs below shell fragments in aconvex–up position. Fossil fragmentshave been affected by dissolution, cementation, and compaction.Compaction may take place prior tocementation. Fossil grains provide an umbrella fromthe area beneath, protecting it fromfiner interstitial detritus. Sediments areunable to fill the cavities because ofthe "umbrella’’ effect of the shell. It is afabric–selective depositional type. Common in the middle and upper partof Al–Khothally, upper part of Hamdah, middle part of Al–Ayein, Wadi Al–Ahjur, Al–Gedaan, and Jabal Salabtraverses, but is rare in Wadi Nagertraverse. 7D 
Vuggy Irregular secondary holes formed bydissolution that cut across grainsand/or cement boundaries. Associatedwith unconformities. Developed as a result of dissolutionunder meteoric conditions. Common in Al–Gedaan and rare in Al–Khothally Hamdah, middle part of WadiAger. 7E 
Moldic Formed by the dissolution of aragonitebioclasts. Have different shapes andsizes depending on their mode oforigin. Represents the first phase ofdissolution and may have been relatedto exposure and meteoric dissolution ofthe sediments at the end of the AmranGroup times. Well developed in all traverses. 7F 
Intercrystalline Associated with replacive dolomitesformed by recrystallization oflimestones and is the result ofdissolution and dolomitizationprocesses. A second phase ofdissolution occurred afterdolomitization, created minor amountsof intercrystalline porosity by thepartial dissolution of zoned dolomite.The crystal shape is variable and thesize ranges between 30 and 280 μm. Form an important reservoir type in anumber of settings ranging fromsupratidal/sabkha to normal marinesequences. Developed in Jabal Salab, Al–Gedaan, lower part of Al–Khothally, upper partof Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 7G 
 
7H h– 
Common in all traverses. Well developed in Al–Ayein, Thula, Al–Gedaan, and Jabal Salabtraverses. 
Indicate tectonic deformation orsolution collapse associated withdissolution of limestone. Developed atany time during burial history of acarbonate sequence starting withshallow burial because of commonearly lithification. Indicates deposition under shallowmarine environment. Brecciation ofcarbonate rock sequences can occurin a number of situations, includinglimestone solution collapse, faulting, orsoil formation. These breccia zonesdeveloped during episodic subaerialexposure. 
Occurs in brittle, normallyhomogeneous carbonates. A tensionalphase causes the opening of fracturesor suture seams or if there aremineralogical changes in the rock suchas dolomitization. Formed as a result of tectonic activitythat led to the development of karstsurfaces. Associated with limestonesolution collapse, often resulting inenhanced porosity that may form eithera reservoir for hydrocarbons or a hostfor mineralization. 
 ?o ~ 
  
D ir 03 o) 
 WO _Q 
 o o _ 
CO ^ s 
LL 0– (0 5 
 

Table 2.

Cementation in the Middle–Upper Jurassic Carbonates of the Amran Group.

Diagenetic 
Features Description Interpretation Occurrences Figures 
Isopachous Consists of a uniform fringe of fibrouscalcite cement around micritized grains.The size of the crystals is 1 mm. Grewperpendicular to the edges of the grains. Represents the first generation ofcement which can be interpreted asmarine. Common in the upper part of the Al–Khothally, middle part of the Hamdah, Al–Ayein, Al–Gedaan, Jabal Salab, andlower part of Wadi Al–Ahjur traverses. 8A 
Columnar stubby Consists of small crystals (2–4 mm) thatform crudely isopachous rims ofcement, particularly in interparticlepores. The length–to–width ratio of thesecrystals is relatively small, ranging from1: 1 to 2: 1. Developed within leachedoriginally aragonitic shells and grew outfrom grain surfaces to meet similarstubby spar growing from the oppositegrain. Sometimes stubby cementfollowed by blocky cement. Represents the second phase ofcementation after the development ofisopachous cement, which can beinterpreted as marine. It is also verycommon in meteoric environment. Well developed in all studiedtraverses. 8B 
Blocky calcite Forms a drusy texture in which theprecipitation of calcite cement startswith the formation of small crystals onthe edges of the particles, then thesecrystals ultimately grow away from theparticle margin and their size increases, while they decrease in number. Fills thepore spaces inside and outsideparticles. Represents the second and/or thirdphase of cementation, after thedevelopment of either isopachouscement, columnar stubby, cement orboth, which can be interpreted as ashallow burial and/or deep burialenvironment. Well developed in all studiedtraverses. 8C 
Syntaxial 
overgrowth Found around the remains ofechinoderms in optical continuity withthe original single crystals. Have aporous structure progressively filled bythe growth of the cement. The growth ofrim cement is prevented by the micriteenvelope, and it is not extended outsidethe grain. Syntaxial calcite overgrowths onechinoderm debris associated withhardgrounds indicating seafloor cement.The overgrowth cement is regarded assynsedimentary in origin because it iscut by other grains or borings. Indicationof synsedimentary cement overgrowthsis shown by the growth of cementtowards the boring walls. Common in Al–Khothally, upper part ofHamdah, Wadi Nager, Al–Ayein, WadiAl–Ahjur, Al–Gedaan and Jabal Salabtraverses. 8D 
Micritization Skeletal fragments appear to besheathed in an envelope of micrite. Theenvelope is seen as patchy micrite andcomposed of micritic calcite. The outersurface coat has gentle smoothcontours, while the inner surface isirregular. Grows in two directions, bothinside and outside the grain. They arefine–grained carbonates, with crystalsize less than 4 |im. This carbonateappears as a dense or translucentgroundmass in thin sections. Micritization occurs mainly by repeatedboring of algae and fungi, infilling of themicroborings with microcrystallineprecipitates, or as a result ofrecrystallization. Micritization anddiminution of skeletal grains in shallowseas provide a major source of limemud. Recognized in all traverses. 8E 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Chert nodules Characterized, in the field, by theirnodular ellipsoidal shape; size isvariable, but can reach 60 cm long and 6cm wide. Have sharp boundaries whichtruncate the contained fossils and areenriched in iron and/or manganeseminerals. Their occurrence at the top ofwackestone beds indicates shallowburial depth. The absence of this kind ofcement in high–energy shallowing–upward sequences could indicate asubmarine process. Middle and top part of Al–Khothally, thelower and middle part of Hamdah, lowerpart of Wadi Nager, Jabal Salab, theupper part of Al–Ayein, and in themiddle part of Wadi Al–Ahjur traverses. 8F 
Silt deposition Represented by massive beds 20 cmthick. At certain horizons, the marlyshaly fossiliferous limestone areintercalated with silty mudstones. A storm–induced depositionalmechanism is the reasonable andaccepted interpretation for this sort ofalternation. Occurs at Wadi Al–Ahjur and in themiddle part of Al–Ayein traverses. 8G 
Jurassic calcrete The calcrete beds, 20–30 cm thick, areassociated with ferruginous hardgroundsurfaces, fenestrae, and geopetalstructures. Lamination around spar–filled alveoli and grains can berecognized and may reflect the firststage of calcrete formation. Theselaminae may surround all or only part ofthe void and/or grain. Their thickness isabout 2 mm. Irregularly shaped blackpebbles consist of angular fragments ofsoilstone crusts. Jurassic calcrete isdense, dull, and dirty. Common feature in semi–arid to aridclimate regimes. Black pebbles occurabove unconformities indicatingmeteoric conditions. Well developed in the middle and toppart of Wadi Al–Ahjur, middle part ofHamdah and middle–upper part of Al–Gedaan traverses. 8H 
Carbonate mud(micrite) A fine–grained, usually dark matrix iscommon and susceptible to diageneticalteration and may be replaced bycoarser mosaics of microspar throughaggrading neomorphism, which resultsin lower porosities. Micrite is nothomogeneous, which means that it didnot come from one source. Showsevidence of periods of subaerialexposure that led to a influx of meteoricwater during and at the end of theJurassic. Micrites (carbonate mud) occur in theshallow marine, less agitated centralparts of the platforms. The micropores inmicritic limestone could be primary orsecondary. Altered skeletal grains arethe major source of the mud. The ICPresults suggest that these micrites wereoriginally composed of carbonate mudfrom differing sources (e. g. aragoniteneedles from algal disintegration, primary HMC precipitates, and/orpossibly early microcrystalline cementsformed in sediments during submarinelithification). The micrite was derivedfrom many different sources for example(algae, bioerosion), and subsequentlywas recrystallized during shallow burial, or earlier during subaerial exposure andfreshwater influx. Occurs in most of the traverses. 81 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Isopachous Consists of a uniform fringe of fibrouscalcite cement around micritized grains.The size of the crystals is 1 mm. Grewperpendicular to the edges of the grains. Represents the first generation ofcement which can be interpreted asmarine. Common in the upper part of the Al–Khothally, middle part of the Hamdah, Al–Ayein, Al–Gedaan, Jabal Salab, andlower part of Wadi Al–Ahjur traverses. 8A 
Columnar stubby Consists of small crystals (2–4 mm) thatform crudely isopachous rims ofcement, particularly in interparticlepores. The length–to–width ratio of thesecrystals is relatively small, ranging from1: 1 to 2: 1. Developed within leachedoriginally aragonitic shells and grew outfrom grain surfaces to meet similarstubby spar growing from the oppositegrain. Sometimes stubby cementfollowed by blocky cement. Represents the second phase ofcementation after the development ofisopachous cement, which can beinterpreted as marine. It is also verycommon in meteoric environment. Well developed in all studiedtraverses. 8B 
Blocky calcite Forms a drusy texture in which theprecipitation of calcite cement startswith the formation of small crystals onthe edges of the particles, then thesecrystals ultimately grow away from theparticle margin and their size increases, while they decrease in number. Fills thepore spaces inside and outsideparticles. Represents the second and/or thirdphase of cementation, after thedevelopment of either isopachouscement, columnar stubby, cement orboth, which can be interpreted as ashallow burial and/or deep burialenvironment. Well developed in all studiedtraverses. 8C 
Syntaxial 
overgrowth Found around the remains ofechinoderms in optical continuity withthe original single crystals. Have aporous structure progressively filled bythe growth of the cement. The growth ofrim cement is prevented by the micriteenvelope, and it is not extended outsidethe grain. Syntaxial calcite overgrowths onechinoderm debris associated withhardgrounds indicating seafloor cement.The overgrowth cement is regarded assynsedimentary in origin because it iscut by other grains or borings. Indicationof synsedimentary cement overgrowthsis shown by the growth of cementtowards the boring walls. Common in Al–Khothally, upper part ofHamdah, Wadi Nager, Al–Ayein, WadiAl–Ahjur, Al–Gedaan and Jabal Salabtraverses. 8D 
Micritization Skeletal fragments appear to besheathed in an envelope of micrite. Theenvelope is seen as patchy micrite andcomposed of micritic calcite. The outersurface coat has gentle smoothcontours, while the inner surface isirregular. Grows in two directions, bothinside and outside the grain. They arefine–grained carbonates, with crystalsize less than 4 |im. This carbonateappears as a dense or translucentgroundmass in thin sections. Micritization occurs mainly by repeatedboring of algae and fungi, infilling of themicroborings with microcrystallineprecipitates, or as a result ofrecrystallization. Micritization anddiminution of skeletal grains in shallowseas provide a major source of limemud. Recognized in all traverses. 8E 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Chert nodules Characterized, in the field, by theirnodular ellipsoidal shape; size isvariable, but can reach 60 cm long and 6cm wide. Have sharp boundaries whichtruncate the contained fossils and areenriched in iron and/or manganeseminerals. Their occurrence at the top ofwackestone beds indicates shallowburial depth. The absence of this kind ofcement in high–energy shallowing–upward sequences could indicate asubmarine process. Middle and top part of Al–Khothally, thelower and middle part of Hamdah, lowerpart of Wadi Nager, Jabal Salab, theupper part of Al–Ayein, and in themiddle part of Wadi Al–Ahjur traverses. 8F 
Silt deposition Represented by massive beds 20 cmthick. At certain horizons, the marlyshaly fossiliferous limestone areintercalated with silty mudstones. A storm–induced depositionalmechanism is the reasonable andaccepted interpretation for this sort ofalternation. Occurs at Wadi Al–Ahjur and in themiddle part of Al–Ayein traverses. 8G 
Jurassic calcrete The calcrete beds, 20–30 cm thick, areassociated with ferruginous hardgroundsurfaces, fenestrae, and geopetalstructures. Lamination around spar–filled alveoli and grains can berecognized and may reflect the firststage of calcrete formation. Theselaminae may surround all or only part ofthe void and/or grain. Their thickness isabout 2 mm. Irregularly shaped blackpebbles consist of angular fragments ofsoilstone crusts. Jurassic calcrete isdense, dull, and dirty. Common feature in semi–arid to aridclimate regimes. Black pebbles occurabove unconformities indicatingmeteoric conditions. Well developed in the middle and toppart of Wadi Al–Ahjur, middle part ofHamdah and middle–upper part of Al–Gedaan traverses. 8H 
Carbonate mud(micrite) A fine–grained, usually dark matrix iscommon and susceptible to diageneticalteration and may be replaced bycoarser mosaics of microspar throughaggrading neomorphism, which resultsin lower porosities. Micrite is nothomogeneous, which means that it didnot come from one source. Showsevidence of periods of subaerialexposure that led to a influx of meteoricwater during and at the end of theJurassic. Micrites (carbonate mud) occur in theshallow marine, less agitated centralparts of the platforms. The micropores inmicritic limestone could be primary orsecondary. Altered skeletal grains arethe major source of the mud. The ICPresults suggest that these micrites wereoriginally composed of carbonate mudfrom differing sources (e. g. aragoniteneedles from algal disintegration, primary HMC precipitates, and/orpossibly early microcrystalline cementsformed in sediments during submarinelithification). The micrite was derivedfrom many different sources for example(algae, bioerosion), and subsequentlywas recrystallized during shallow burial, or earlier during subaerial exposure andfreshwater influx. Occurs in most of the traverses. 81 

Table 3.

Compaction in the Middle–Upper Jurassic Carbonates of the Amran Group.

Diagenetic 
Features Description Interpretation Occurrences Figures 
Physical Represented by fractured andflattened grains. The timing ofcompaction events relative tocementation is both important andrelatively easy to observe. Thelack of porosity in mudstone andwackestone micro–faciesindicates that the carbonatesediments of the Amran Groupunderwent substantial physicalcompaction. Reflects the increase inpressure brought uponsediments as a result ofdeep burial. Recognized in alltraverses. 9A 
Chemical Stylolites well developed and varymuch in amplitude and in formfrom one occurrence to another.Marked by concentrations oforganic matter, clays, pyrite, anddetrital silicates and commonlydolomite residue of syn-styloliteorigin. Formed along contactsbetween rigid grains or nodules orin clay–rich zones and claypartings. Both wispy seamstylolites and anastomosingwispy seams can be recognized. Burial depth withattendant lithostatic loadis one of the most criticalfactors determining theonset and ultimateefficiency of chemicalcompaction. The originalmineralogy of thesediments undergoingcompaction is also avery important controlover the nature of itsearly chemicalcompactional history. Recognized in alltraverses. 9B 
 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Physical Represented by fractured andflattened grains. The timing ofcompaction events relative tocementation is both important andrelatively easy to observe. Thelack of porosity in mudstone andwackestone micro–faciesindicates that the carbonatesediments of the Amran Groupunderwent substantial physicalcompaction. Reflects the increase inpressure brought uponsediments as a result ofdeep burial. Recognized in alltraverses. 9A 
Chemical Stylolites well developed and varymuch in amplitude and in formfrom one occurrence to another.Marked by concentrations oforganic matter, clays, pyrite, anddetrital silicates and commonlydolomite residue of syn-styloliteorigin. Formed along contactsbetween rigid grains or nodules orin clay–rich zones and claypartings. Both wispy seamstylolites and anastomosingwispy seams can be recognized. Burial depth withattendant lithostatic loadis one of the most criticalfactors determining theonset and ultimateefficiency of chemicalcompaction. The originalmineralogy of thesediments undergoingcompaction is also avery important controlover the nature of itsearly chemicalcompactional history. Recognized in alltraverses. 9B 
 

Late cements are coarser than early cements and are up to several millimeters in size. They can be seen in the field usually occluding secondary porosity developed by dissolution within the skeletal grains. The same cement occludes primary porosity (e.g., fenestrae) and secondary pore spaces developed by dissolution of colonial organisms. Petrographically, these cements are nonferroan and have a drusy fabric. They infill both interparticle and intraparticle porosity. Dolomite cements are common in these limestones, and most were precipitated during later diagenesis in cavities and fractures (Table 4). Replacive dolomitization (Sellwood et al., 1987) occurred during shallow burial (small rhombic crystals 36–280 pm in size) and during burial diagenesis with the formation of saddle dolomite. Saddle dolomite crystals are perpendicular to the walls of allochems, e.g., foraminifera and bivalves, and/or in fractures as a result of dissolution and/or tectonism. In Jabal Salab two major periods of dolomitization are recognized. The first period was widespread and affected large volumes of the carbonates. It occurred before emergence and deep karstic erosion. The second period was associated with fractures and microcavities filled during sulfide mineralization (Al-Ganad, 1991).

The sediments of the Amran Group also show some evidence of dedolomitization (Table 4). A most distinctive petrographic texture present within the Amran Group is the rhombohedral zones of calcite and iron oxide. The rhombohedral shape suggests that selected dolomite zones have been either replaced or that calcite and iron oxide grew on dolomite substrates during interruptions in dolomite development. Dedolomitization occurred when meteoric waters invaded rocks and washed the dolomite crystals, removing the magnesium while helping the precipitation of calcite. Dedolomitization occurred by oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering.

Table 4.

Dolomitization and Dedolomization in the Middle-Upper Jurassic Carbonates of the Amran Group.

Diagenetic 
Features Description Interpretation Occurrences Figures 
Dolomitization Several types have beenrecognized. The first typecomprises a replacive dolomite;crystals range in size from 36 to280 μm. The second type showsthe development of saddledolomite in voids and/or fractures, crystals occurr perpendicular tothe walls of allochems, e. g., foraminifera, bivalves, and/or infractures. Occurred beforeemergence and deep karsticerosion. Associated withfractures and microcavitieswithsulfide mineralization. Dolomite distribution ofvarious shapes indicatesthat the precipitation didnot take place in oneevent, but over a muchlonger period of time, ranging from shallowburial to deeper burial.The coarse dolomitecrystal size suggests aslow crystallization rate, and the weak sweepingextension of crystalsseen in saddle dolomiteindicates a formationtemperature between60°C and 150°C. Thesefacts suggest ahydrothermal or deepburial environment ofdolomitization. Well developed in WadiAl–Khothally, Wadi Al–Ahjur, and Al–Ayeintraverses. 10A 
Dedolomitization Rhombohedral zones of calciteand iron oxide. The rhombohedralshape suggests either thatselected dolomite zones havebeen replaced or that calcite andiron oxide grew on dolomitesubstrates during interruption indolomite development. Dedolomitizationprocess can berecognized whenmeteoric waters invadedolomite rocks andremove magnesium tohelp in the precipitationof calcite. Occurrence ofnonferroan calcite andferric oxides inrhombohedral zones indolomite indicate thatdedolomitizationoccurred by oxidationand alteration of ferroandolomite zones andprobably reflectsalteration related torecent weathering. Occurs in the upper partof Al–Khothally, Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 10B 
Diagenetic 
Features Description Interpretation Occurrences Figures 
Dolomitization Several types have beenrecognized. The first typecomprises a replacive dolomite;crystals range in size from 36 to280 μm. The second type showsthe development of saddledolomite in voids and/or fractures, crystals occurr perpendicular tothe walls of allochems, e. g., foraminifera, bivalves, and/or infractures. Occurred beforeemergence and deep karsticerosion. Associated withfractures and microcavitieswithsulfide mineralization. Dolomite distribution ofvarious shapes indicatesthat the precipitation didnot take place in oneevent, but over a muchlonger period of time, ranging from shallowburial to deeper burial.The coarse dolomitecrystal size suggests aslow crystallization rate, and the weak sweepingextension of crystalsseen in saddle dolomiteindicates a formationtemperature between60°C and 150°C. Thesefacts suggest ahydrothermal or deepburial environment ofdolomitization. Well developed in WadiAl–Khothally, Wadi Al–Ahjur, and Al–Ayeintraverses. 10A 
Dedolomitization Rhombohedral zones of calciteand iron oxide. The rhombohedralshape suggests either thatselected dolomite zones havebeen replaced or that calcite andiron oxide grew on dolomitesubstrates during interruption indolomite development. Dedolomitizationprocess can berecognized whenmeteoric waters invadedolomite rocks andremove magnesium tohelp in the precipitationof calcite. Occurrence ofnonferroan calcite andferric oxides inrhombohedral zones indolomite indicate thatdedolomitizationoccurred by oxidationand alteration of ferroandolomite zones andprobably reflectsalteration related torecent weathering. Occurs in the upper partof Al–Khothally, Al–Ayein, Wadi Al–Ahjur, and JabalSalab traverses. 10B 

Synsedimentary Diagenesis

Synsedimentary diagenesis is characterized by the formation of syntaxial and isopachous cements in developing hardgrounds (with associated borings and burrows, and primary shelter, fenestral, framework, interparticle, and intraparticle porosity), geopetal structure, intraclasts, and cementation. Columnar stubby cement is associated with high–energy facies.

Both isopachous and columnar stubby cements are nonluminescent under CL, and their better preservation suggests an original calcite mineralogy. Specific features can be related to the basic regional development. Hardgrounds were formed repeatedly at several horizons in all formations of the Amran Group, indicating that these carbonate deposits received early cement during deposition. Truncation of shells indicates that the sediment must have been cemented at the time of boring. Burrows are commonly filled with calcite. Leached burrows may show fenestral structure. The shelter porosity was developed under marine conditions very close to shoals. Physical compaction affected the sediment and was followed by dissolution and finally cementation. Micritiza tion of grains was particularly common in the low–energy facies. Micritiza tion and marine cementation were controlled by the facies and the depositional setting of the sediments on the platform. For example the packstone microfacies shows more micritization and less marine cementation than the skeletal oolitic grainstone microfacies. The relationship of fractured and collapsed micrite envelopes, fractured remnant grain material, infilling of dissolved grains, fractured marine cements, and subsequent spar generations and stylolitization provide petrographic evidence to constrain the diagenetic setting of the dissolution. Spar infilling voids produced by dissolution of grain material is postdated by stylolitization, showing that these cements predate significant burial. Thus, the processes responsible for creating the pore spaces must have occurred at shallow burial depths, suggesting that meteoric fluids caused dissolution (Moss and Tucker, 1995).

Shallow Burial Diagenesis

Shallow burial diagenesis shows other specific features in all formations. Leaching, recrystallization, and early dolomitization (both replacive and void–filling) are the main features developed during shallow burial diagenesis of the Amran Group. The mold– filling cements are likely to have been precipitated during very shallow burial and before any compaction. The moldic and vuggy porosity distribution of early compaction, collapse breccia, and silt deposition indicate that the Amran Group continued to receive meteoric water after sediment stabilization, enlarging some molds and vugs.

The earliest of these late diagenetic phenomena is leaching (Bathurst, 1975; Choquette and Pray, 1970; Druckman and Moore, 1985; Moore, 1989). Evidence of dissolution is found in approximately sixty percent of the samples examined. It is important to mention that porosity, developed by leaching, has been obscured by the development of both columnar stubby and blocky calcite cement. Much of this cement was created in the shallow burial environment with the influence of meteoric water, but undoubtedly some solution of shallow burial cement has taken place. In some cases the secondary porosity thus created was partly destroyed by dolomite cements, but when dolomite content reached 50%, porosity is expected to increase with increasing dolomite percentage. The dolomite cement that fills leached porosity is the same as that filling nearby primary pores. The highest proportion of leaching effects is seen in coated grains and algal debris.

Deep Burial Diagenesis

The deep burial diagenesis is characterized by some specific features such as dissolution, blocky calcite cement, compaction (fractures, sutured grains, and stylolites) and the formation of saddle dolomite (Scholle and Halley, 1985; Choquette and James, 1987). Chemical compaction was followed by further cementation of blocky calcite in void spaces. Burial pressure solution is the most likely explanation both for stylolites with organic material along grain boundaries and locally highly compacted nodular lithologies. Blocky calcite cements exhibit a consistent luminescent zonation, with an initial nonluminescent zone followed by a bright one. This pattern may reflect a meteoric system initially open to oxygenated waters. Fe and Mn cannot be incorporated into the calcite lattice under oxygenated conditions, so the initial calcite zones exhibit no luminescence. The bright luminescence of the second calcite zone indicates that precipitation fluids later became reducing, allowing incorporation of Fe and Mn into the crystal lattice (Moore, 1989). Finally, the development of dolomite either in veins, fractures, and/or molds or as a replacement of micrite have been recognized under burial conditions. Saddle dolomite and fracturing were developed indicating post–burial conditions.

Uplift Diagenesis

Uplift diagenesis is characterized by reopening of stylolites through fractures and development of dolomitization under meteoric conditions Frank 1981 suggested that the occurrence of nonferroan calcite and ferric oxides in rhombohedral zones in dolomite indicates that dedolomitization occurred by oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering Subaerial exposurepunctuated the development of the platform and led to the formation of a paleokarst surfaceat the toppart of AAyeiln TraverseThula area WadiAAhjulr Formation.

Conclusions

Diagenesis of the Middle–Upper Jurassic carbonates of the Amran Group involved many processes and produced conspicuous effects, such as cementation and dissolution, resulting in secondary porosity, including the development of secondary microporosity. Compaction (both physica I and chemical) became an increasingly significant process as a result of increasing overburden pressure. In the early stages of burial, physical compaction was more important and resulted in closer packing and fracturing of grains. Eventually, chemical compaction led to dissolution at grain contacts and the formation of planar and/or anastomosing stylolites by dissolution. Dolomite cements are common in these limestones and were mostly precipitated 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. Dedolomitization occurred in response to oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering.

However, the diagenetic events in the Amran Group carbonate platform reveals the importance of several controls on carbonate diagenesis as a site for cementation and dolomitization of platform carbonates. Diagenesis in the Amran Group sediments was controlled by several factors. Original mineralogy and position of relative sea level controlled both early meteoric and later shallow–burial diagenesis by preferential dissolution of the less stable aragonltic components. The secondary porosity then provided sites for further calcite cementation. Climate was important in controlling the extent of meteoric diagenesis that the platform experienced during periods of exposure, so that karstification and meteoric diagenesis were limited. Sea–level changes were important for determining the timing, extent, and duration of exposure of the platform. Late burial was important for determining the possible sources of fluids for shallow–burial and deep–burial cementation as well as dolomitization. The major phase of calcite cementation, in most samples, postdates early physical compaction features, and this calcite occludes the porosity produced during meteoric exposures and synsedimentary deposition. Dolomitization can also be seen to postdate early compactional features.

Four major subenvironments where diagenesis of the Amran Group were operative are: (1) synsedimentary diagenesis, (2) shallow burial diagenesis, (3) deep burial diagenesis, and (4) uplift diagenesis.

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Philobbos
,
A.
Purser
,
B.
, eds.,
Geodynamic and Sedimentation of the Red SeaGulf of Aden Rift System
 :
Geological Society of Egypt, Special Publication
, p.
383
408
.
Al–Thour
,
K.A.
,
1988
,
Geological and sedimentological studies on Al–Balaqarea, Marib, Yemen Arab Republic
:
M.Sc. thesis, Sana’a University
 ,
214
p.
Al–Thour
,
K.A.
,
1992a
,
Sabatain Group: The product of main and post rift phase during Jurassic-Cretaceous time, Yemen Republic (abstract)
, in
Marshall
,
J.
,
convenor, 9th meeting of carbonate sedimentologists
:
The Bathurst Meeting
 ,
The University of Liverpool
,
U.K.
, Book of abstracts,
2
p.
Al–Thour
,
K.A.
,
1992b
,
Stratigraphy, sedimentology and diagenesis of the Amran Group (Jurassic) of the region to the west and northwest of Sana’a, Yemen Republic
:
Ph.D. thesis, The University of Birmingham
 ,
293
p.
Al–Thour
,
K.A.
,
1997
,
Facies sequences of the Middle-Upper Jurassic carbonate platform (Amran Group) in the Sana’a region, Republic of Yemen
:
Marine and Petroleum Geology
 ,
v. 14
, p.
643
660
.
Al–Thou’R
,
K.A.
, in press,
Sedimentary facies and hydrocarbon potential of the Middle-Upper Jurassic carbonates (Amran Group) in the Wadi Al–Jawf-Mareb Shabwa’a Basin, Yemen
:
The Science Conference
 ,
Yemeni Scientific Research Foundation Special Publication series, Sana’a
.
Bathurst
,
R.G.
,
1975
,
Carbonate Sediments and Their Diagenesis
, 2nd Edition:
Amsterdam, Elsevier
 ,
658
p.
Beydoun
,
Z.R.
,
1988
,
Hie Middle East
:
Regional Geology and Petroleum Resources
 :
London
,
Scientific Press Ltd
.,
292
p.
Beydoun
,
Z.R.
,
1991
,
Arabian plate hydrocarbon geology and potential—A plate tectonic approach
:
American Association of Petroleum Geologists, Studies in Geology
 , no.
33
,
77
p.
Choquette
,
P.W.
James
,
N.P.
,
1987
, Diagenesis # 12:
Diagenesis in limestones—3, The deep burial environment
:
Geoscience Canada
 ,
v. 14
, p.
335
.
Choquetie
,
P.W.
Pray
,
L.C.
,
1970
,
Geological nomenclature and classification of porosity in sedimentary carbonates
:
American Association of Petroleum Geologists, Bulletin
 ,
v. 54
, p.
207
250
.
Druckman
,
Y.
Moore
,
C.H.
,
1985
,
Late subsurface secondary porosity in a Jurassic grainstone reservoir, Smackover Formation, Mt. Vernon Field, Southern Arkansas
, in
Roehl
,
P.O.
Choquette
,
P.W.
, eds.,
Carbonate Petroleum Reservoirs
 :
New York, Springer-Verlag
, p.
369
383
.
El–Anuaawy
,
M.I.
Al–Thour
,
K. A.
,
1989
,
Sedimentological evolution, diagenesis and hydrocarbon potentiality of Late Jurassic carbonates, Eastern region, Yemen Arab Republic
:
The Journal of the University of Kuwait (Science)
 ,
v. 16
, p.
401
123
.
Frank
,
J.R.
,
1981
,
Dedolomitization in the Taum Sauk limestone (Upper Cambrian), Southeast Missouri
:
Journal of Sedimentary Petrology
 ,
v. 51
, p.
7
18
.
Haliam
,
A.
,
1978
,
Eustatic cycles in the Jurassic
:
Palaeogeography, Palaeodimatology and Palaeoecology
 ,
v. 23
, p.
1
32
.
Haliam
,
A.
,
1988
,
A reevaluation of Jurassic eustasy in the light of new data and the revised Exxon curve
, in
Wilgus
,
C.K.
Hastings
,
B.S.
Kendall
,
C.G.St.C.
Posamentier
,
H.W.
Ross
,
C.A.
Wagoner
,
J.C. Van
, eds.,
Sea-Level Changes
 :
An Integrated Approach: SEPM, Special Publication
42
, p.
261
273
.
Hird
,
K.
Tucker
,
M.
,
1988
,
Contrasting diagenesis of two Carboniferous oolites from South Wales: a tale of climatic influence
:
Sedimentology
 ,
v. 35
, p.
587
602
.
Moore
,
C.H.
,
1989
,
Carbonate Diagenesis and Porosity
:
Amsterdam, Elsevier, Developments in Sedimentology
 
46
,
388
p.
Moss
,
S.
Tucker
,
M.
,
1995
,
Diagenesis of Barremian-Aptian platform carbonates (the Urgonian Limestone Formation of SE France): near-surface and shallow-burial diagenesis
:
Sedimentology
 ,
v. 42
, p.
853
874
.
Powers
,
R.W.
Ramirez
,
L.F.
Redmond
,
C.D.
Elberg
,
J.E.
,
1966
,
Geology of the Arabian Peninsula (Sedimentary Geology of Saudi Arabia)
:
U.S. Geological Survey, Professional Paper
 
560
D,
150
p.
Scholle
,
P.
Halley
,
R.
,
1985
,
Burial diagenesis: out of sight, out of mind
, in
Schneidermann
,
N.
Harris
,
P.M.
, eds.,
Carbonate Cements
 :
SEPM, Special Publication
36
, p.
309
334
.
Sellwood
,
B.W.
Scott
,
J.
James
,
B.
Evans
,
R.
Marshall
,
J.
,
1987
,
Regional significance of dedolomitization in Great Oolite reservoir facies of Southern England
, in
Brooks
,
J.
Glennie
,
K.W.
, eds.,
Petroleum Geology of Northwest Europe
 :
London
,
Graham & Trotman
, p.
129
137
.
Shebl
,
H.T.
Alsharhan
,
A.S.
,
1994
,
Sedimentary facies and hydrocarbon potential of Berriasian-Hauterivian carbonates in Central Arabia
, in
Simmons
,
M.D.
, ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
159
174
.
Simmons
,
M.D.
Al–Thour
,
K.
,
1994
,
MicropaJeontological biozonation of the Amran Series (Jurassic) in the Sana’a Region, Yemen Republic
, in
Simmons
,
M.D.
, ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
43
60
.
Wright
,
V.P.
,
1988
,
Paleokarsts and paleosols as indicators of paleoclimate and porosity evolution: a case study from the Carboniferous of South Wales
, in
James
,
N.P.
Choquette
,
P.W.
, eds.,
Paleokarst
 :
New York
,
Springer-Verlag
, p.
329
341
.

Acknowledgments

The author thanks his numerous colleagues for a stimulating discussion. Drs. C.G.St. Kendall, Ihsan Al–Aasm, A.S. Alsharhan, and R. W. Scott provided excellent reviews and suggestions which improved the paper.

Figures & Tables

Contents

GeoRef

References

References

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,
I.
,
1991
,
Etude geologique et metallogenique du Gesment ZnPbAg de Jabali (Bordure sue du basin de Wadi Al–Jawf, Republic de Yemen)
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,
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,
Cretaceous-Tertiary Pre-rift fluvial shallow marine sediments in Yemen
, in
Philobbos
,
A.
Purser
,
B.
, eds.,
Geodynamic and Sedimentation of the Red SeaGulf of Aden Rift System
 :
Geological Society of Egypt, Special Publication
, p.
383
408
.
Al–Thour
,
K.A.
,
1988
,
Geological and sedimentological studies on Al–Balaqarea, Marib, Yemen Arab Republic
:
M.Sc. thesis, Sana’a University
 ,
214
p.
Al–Thour
,
K.A.
,
1992a
,
Sabatain Group: The product of main and post rift phase during Jurassic-Cretaceous time, Yemen Republic (abstract)
, in
Marshall
,
J.
,
convenor, 9th meeting of carbonate sedimentologists
:
The Bathurst Meeting
 ,
The University of Liverpool
,
U.K.
, Book of abstracts,
2
p.
Al–Thour
,
K.A.
,
1992b
,
Stratigraphy, sedimentology and diagenesis of the Amran Group (Jurassic) of the region to the west and northwest of Sana’a, Yemen Republic
:
Ph.D. thesis, The University of Birmingham
 ,
293
p.
Al–Thour
,
K.A.
,
1997
,
Facies sequences of the Middle-Upper Jurassic carbonate platform (Amran Group) in the Sana’a region, Republic of Yemen
:
Marine and Petroleum Geology
 ,
v. 14
, p.
643
660
.
Al–Thou’R
,
K.A.
, in press,
Sedimentary facies and hydrocarbon potential of the Middle-Upper Jurassic carbonates (Amran Group) in the Wadi Al–Jawf-Mareb Shabwa’a Basin, Yemen
:
The Science Conference
 ,
Yemeni Scientific Research Foundation Special Publication series, Sana’a
.
Bathurst
,
R.G.
,
1975
,
Carbonate Sediments and Their Diagenesis
, 2nd Edition:
Amsterdam, Elsevier
 ,
658
p.
Beydoun
,
Z.R.
,
1988
,
Hie Middle East
:
Regional Geology and Petroleum Resources
 :
London
,
Scientific Press Ltd
.,
292
p.
Beydoun
,
Z.R.
,
1991
,
Arabian plate hydrocarbon geology and potential—A plate tectonic approach
:
American Association of Petroleum Geologists, Studies in Geology
 , no.
33
,
77
p.
Choquette
,
P.W.
James
,
N.P.
,
1987
, Diagenesis # 12:
Diagenesis in limestones—3, The deep burial environment
:
Geoscience Canada
 ,
v. 14
, p.
335
.
Choquetie
,
P.W.
Pray
,
L.C.
,
1970
,
Geological nomenclature and classification of porosity in sedimentary carbonates
:
American Association of Petroleum Geologists, Bulletin
 ,
v. 54
, p.
207
250
.
Druckman
,
Y.
Moore
,
C.H.
,
1985
,
Late subsurface secondary porosity in a Jurassic grainstone reservoir, Smackover Formation, Mt. Vernon Field, Southern Arkansas
, in
Roehl
,
P.O.
Choquette
,
P.W.
, eds.,
Carbonate Petroleum Reservoirs
 :
New York, Springer-Verlag
, p.
369
383
.
El–Anuaawy
,
M.I.
Al–Thour
,
K. A.
,
1989
,
Sedimentological evolution, diagenesis and hydrocarbon potentiality of Late Jurassic carbonates, Eastern region, Yemen Arab Republic
:
The Journal of the University of Kuwait (Science)
 ,
v. 16
, p.
401
123
.
Frank
,
J.R.
,
1981
,
Dedolomitization in the Taum Sauk limestone (Upper Cambrian), Southeast Missouri
:
Journal of Sedimentary Petrology
 ,
v. 51
, p.
7
18
.
Haliam
,
A.
,
1978
,
Eustatic cycles in the Jurassic
:
Palaeogeography, Palaeodimatology and Palaeoecology
 ,
v. 23
, p.
1
32
.
Haliam
,
A.
,
1988
,
A reevaluation of Jurassic eustasy in the light of new data and the revised Exxon curve
, in
Wilgus
,
C.K.
Hastings
,
B.S.
Kendall
,
C.G.St.C.
Posamentier
,
H.W.
Ross
,
C.A.
Wagoner
,
J.C. Van
, eds.,
Sea-Level Changes
 :
An Integrated Approach: SEPM, Special Publication
42
, p.
261
273
.
Hird
,
K.
Tucker
,
M.
,
1988
,
Contrasting diagenesis of two Carboniferous oolites from South Wales: a tale of climatic influence
:
Sedimentology
 ,
v. 35
, p.
587
602
.
Moore
,
C.H.
,
1989
,
Carbonate Diagenesis and Porosity
:
Amsterdam, Elsevier, Developments in Sedimentology
 
46
,
388
p.
Moss
,
S.
Tucker
,
M.
,
1995
,
Diagenesis of Barremian-Aptian platform carbonates (the Urgonian Limestone Formation of SE France): near-surface and shallow-burial diagenesis
:
Sedimentology
 ,
v. 42
, p.
853
874
.
Powers
,
R.W.
Ramirez
,
L.F.
Redmond
,
C.D.
Elberg
,
J.E.
,
1966
,
Geology of the Arabian Peninsula (Sedimentary Geology of Saudi Arabia)
:
U.S. Geological Survey, Professional Paper
 
560
D,
150
p.
Scholle
,
P.
Halley
,
R.
,
1985
,
Burial diagenesis: out of sight, out of mind
, in
Schneidermann
,
N.
Harris
,
P.M.
, eds.,
Carbonate Cements
 :
SEPM, Special Publication
36
, p.
309
334
.
Sellwood
,
B.W.
Scott
,
J.
James
,
B.
Evans
,
R.
Marshall
,
J.
,
1987
,
Regional significance of dedolomitization in Great Oolite reservoir facies of Southern England
, in
Brooks
,
J.
Glennie
,
K.W.
, eds.,
Petroleum Geology of Northwest Europe
 :
London
,
Graham & Trotman
, p.
129
137
.
Shebl
,
H.T.
Alsharhan
,
A.S.
,
1994
,
Sedimentary facies and hydrocarbon potential of Berriasian-Hauterivian carbonates in Central Arabia
, in
Simmons
,
M.D.
, ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
159
174
.
Simmons
,
M.D.
Al–Thour
,
K.
,
1994
,
MicropaJeontological biozonation of the Amran Series (Jurassic) in the Sana’a Region, Yemen Republic
, in
Simmons
,
M.D.
, ed.,
Micropalaeontology and Hydrocarbon Exploration in the Middle East
 :
London
,
Chapman & Hall
, p.
43
60
.
Wright
,
V.P.
,
1988
,
Paleokarsts and paleosols as indicators of paleoclimate and porosity evolution: a case study from the Carboniferous of South Wales
, in
James
,
N.P.
Choquette
,
P.W.
, eds.,
Paleokarst
 :
New York
,
Springer-Verlag
, p.
329
341
.

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