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Abstract :

The depositional environment and the diagenetic history of the Middle Jurassic carbonate (Bir Maghara Formation), north Sinai, Egypt, have been evaluated through comprehensive petrographic and geochemical studies of oncoid grains. Petrographically theoncoids are composed of micrite and vary from subspherical to spherical bodies. The abundance of the oncoids and their close association with ooids, pellets, and shelf fauna indicate that the Middle Jurassic carbonate sequence was deposited in a shal low marine environment within the photic zone.

The oncoids are characterized by relatively low Fe and Mn concentrations, normal Sr concentrations, light δ18O values (from –3.9 to –5.5%o PDB), and heavy δ13C values (from+2.2 to+3.1 %o PDB). Enhanced magnesium concentrations (3.6 mol % MgCO3) in these oncoids, relative to carbonate matrix (0.6 mol% MgCO3), suggest their original Mg–calcite mineralogy. The δ18O and δ13C values show a narrow range, and the oncoid grains from the same stratigraphic level do not yield any large difference in their isotopic composition. Isotopic comparison of these oncoids (mean δ18O = –5.4%o; mean δ13C =+2.1 %o PDB) with the sparry calcite cements (mean δ18O = –9.8%o; mean δ13C =+0.6%o PDB) suggests that meteoric modification was coincident with the development of subaerial exposure and meteoric influx during emergence of the carbonate platform. Such diagenetic modification is probably responsible for 18O depletion of the oncoids and the carbonate matrix.

Introduction

Oncoid grains are common in the Middle Jurassic shallow marine carbonate deposits of north Sinai, Egypt, and exhibit a wide range of morphologies. Similar oncoids are described worldwide in the Phanerozoic sedimentary column from marine, lacustrine, and fluvial environments (Abell et al., 1978; Nickel, 1983; Andrews,1986; Ratcliffe, 1988; Smith and Mason, 1991; Zamarreno et al., 1997). The examination of oncoids, which are constructed by algae, is of interest in both geological and biological studies, where the algae are important producers of both calcium carbonate and organic matter in many shelf areas from tropics to temperate regions (Milliman, 1974; Morse and Mackenzie, 1990). Therefore, the variation in oncoid morphologies and their excellent preservation in the study area provide the opportunity to examine the occurrence of oncoid grains in relation to their environmental setting.

Despite the wealth of literature relating to oncoids of Mesozoic carbonate shelves (e.g., Bishop, 1968; de Klasz, 1978; Purser, 1978; Becher and Moore, 1979; Peryt, 1981, 1983), few attempts have been made to understand the relationship between palaeoenvironmental and palaeotemperature conditions, oncoid morphology, and the resultant texture of carbonates. Meanwhile, research on the mineralogy and geochemistry (trace elements and stable isotopes) of oncoids is still scanty. The oxygen and carbon isotopic composition of oncoids record paleoenvironmental and paleotemperature conditions during the Jurassic if the effects of diagenesis have not modified the original signal and if the algae did not impart a fractionation of carbon and oxygen isotopes.

The present study reports detailed elemental and isotopic compositions of the Middle Jurassic oncoids from the north Sinai, Egypt. The major goal of this study is to unravel the environmental and diagenetic history of these oncolitic limestones. Diagenetic phases and their environments are identified by detailed petrography and geochemistry. Consequently, the stable–isotope compositions of the oncoids provide a reasonably reliable proxy for Middle Jurassic marine 5180 and 513C values, as well as of paleotemperature of sea water at that time.

Geological Setting

The Middle Jurassic Bir Maghara Formation is exposed in the Gebel Maghara area and in the subsurface in bore hole No. H–1 (Fig. 1). It overlies the Shusha Formation and is overlain by the clastic deposits of Safa Formation. The Bir Maghara Formation consists of two members, the Mahl Member and the Mowerib Member (Fig. 2). Stratigraphy and sedimentology of the Middle Jurassic rocks have been studied by a number of authors (e.g., Al Far, 1966; Goldberg et al., 1971; Jenkins, 1990, Said, 1990; Keeley and WalJis, 1991).

Fig.1

—Schematic facies distribution of Middle Jurassic in north Sinai (Modified from Schlumberger, 1984).

Fig.1

—Schematic facies distribution of Middle Jurassic in north Sinai (Modified from Schlumberger, 1984).

Fig.2

Lithostratigraphic sections of Middle Jurassic rocks. A) Bajocian–Bathonian surface section of Gebel Maghara. B) Subsurface section of bore hole No. H–1.

Fig.2

Lithostratigraphic sections of Middle Jurassic rocks. A) Bajocian–Bathonian surface section of Gebel Maghara. B) Subsurface section of bore hole No. H–1.

In general, the Middle Jurassic deposits of north Sinai comprise a section of predominantly marine shelf sediments dominated by carbonates. Sedimentation of these deposits was controlled by the three competing factors: (1) progressive eustatic sea level rise, (2) intermittent and relatively minor subsidence related to continuing deepening of the depositional basin, and (3) progradation of argillaceous and siliciclastic sediments from the south basement massif. These factors resulted in complex, cyclic deposition in the study area.

Each cycle culminated with either reduced deposition or erosion. Transgressions permitted carbonate deposition over north Sinai (including the study area), whereas shallowing of the depositional environment led to deposition of peri tidal carbonate and progradation of nonmarine sediments. As a result, widespread emergence occurred at the tops of individual cycles. In the study area, the carbonate facies comprise oolitic, oncolitic, and peloidal grainstones and packstones facies (Fig. 2). Meanwhile, the faunally restricted subtidal to peritidal wackestone facies are also present. Terrigenous shales and sandstones are interbedded with carbonates throughout the study sequence (Fig. 2).

Tectonically, during the Jurassic the continental margin of the Tethyan Sea was affected by block faulting and tensional movements (Bernoulli, 1972). This extensional regime persisted until the Late Cretaceous and was associated with high heat flow (Spohn and Schubert, 1982). The study area encompasses a transitional area from an extensional regime to a compressional regime (Freund et al., 1975). This compressional regime led to the development of the Syrian Arc system and was also manifested by a change from high to low heat flow. In the Cretaceous more gradual regional subsidence took place, which gave rise to post-uplift folding and compaction of the argillaceous Jurassic strata. Finally, the Jurassic sequences were uplifted and subsequently truncated by Early Cretaceous erosion. This erosion has variably reduced the thickness of the Jurassic section.

Sampling And Analytical Procedures

This study is based on extensive field work on surface and subsurface Middle Jurassic sections, examination of over 80 polished thin sections, 60 major element and trace–element analyses, and 60 oxygen and carbon isotope analyses from two sections of Bir Maghara Formation (Fig. 2). Approximately 10 samples of oncoid grains and matrix were studied using a JEOL scanning electron microscope (SEM) with an on–line Exl–EDS system. Gold-coated thin sections and broken surfaces were studied either unetched or after slight etching with dilute HC1 (IN) for up to 30 seconds. X–ray powder diffraction was used for routine determination of the mineralogic purity of all samples prior to elemental and isotopic analyses.

Documentation of primary textures, and of chemical changes that occurred in the studied marine oncoid grains during diagenesis, requires detailed geochemical analyses of various petrographic components (oncoid grains, matrix, and calcite cement). Accordingly, microsampling is essential. Isotopic analysis of oxygen and carbon was performed on a Finnigan MAT stable–isotope–ratio mass spectrometer at the Stable Isotope Laboratory, University of Michigan. The 0.5 mg powdered carbonate samples of oncoid grains, matrix, and calcite cement were obtained using a dental drill and dissolved in 100% anhydrous phosphoric acid at 70°C. All data have been corrected following procedure modified from Craig (1957). Precision was monitored through daily analyses of NBS–20, which is better than 0.08%o for oxygen and 0.05%o for carbon determinations.

Major–element and trace–element compositions for various petrographic components were measured by atomic absorption spectrophotometry (AAS). Powders of 0.1 g were dissolved in concentrated HC1. Despite the low insoluble residue contents in most of the analyzed samples, elements absorbed to clays and organic phases may have contributed to the measured trace-element contents. Analytical precision at the 95% confidence level determined on replicate analyses is better than 5%.

Results And Discussion

Petrography and Diagenesis

Oncoids are abundant in some layers of the studied Middle Jurassic carbonate sequences and are associated with ooids, pellets, and skeletal grains. Micrite is the predominant fabric of these oncoids (Fig. 3). In general, the Middle Jurassic carbonate include argillaceous mudstones/wackestones that contain oncoids, passing shoreward into oncoidal–pelodial and peloidal–bioclastic facies and then into oolite–flat grainstones. The studied oncolites formed by the physical movement of intraclasts or fossils (serving as the nuclei) that were coated by microbial mats (Fig. 3). Some oncoids are spherical and have continuous laminae that completely envelop the nucleus (Fig. 3B), which most probably indicates continuous agitation.

Fig.3

—Photomicrographs of the studied Middle Jurassic oncoids. A) Micritized subspherical dense oncoids with carbonate intraclasts as nuclei (surface sample). Note the grain–contact suture between oncoid grains. B) Laminated coating with late calcite filling the core of the oncoid (subsurface sample). C) Spherical oncoid displaying symmetrical growth (subsurface sample). Note the partial dissolution and the formation of vugs. D) Oncoid with micritic matrix (subsurface sample). Note the stylolite associated with authigenie calcite and dolomite crystals.

Fig.3

—Photomicrographs of the studied Middle Jurassic oncoids. A) Micritized subspherical dense oncoids with carbonate intraclasts as nuclei (surface sample). Note the grain–contact suture between oncoid grains. B) Laminated coating with late calcite filling the core of the oncoid (subsurface sample). C) Spherical oncoid displaying symmetrical growth (subsurface sample). Note the partial dissolution and the formation of vugs. D) Oncoid with micritic matrix (subsurface sample). Note the stylolite associated with authigenie calcite and dolomite crystals.

The Middle Jurassic oncoids are composed of micrite and meteoric spar that form spongiostromate fabrics (Monty, 1981). Despite this, the obvious feature of these oncoids is their morphological variations, forming a continuous series from small (9 mm) subspherical–spherical forms to large (up to 1 cm) irregular forms (Fig. 3). The internal structure of the cortex of the subspherical–spherical oncoids is symmetrical, but this cortex is strongly asymmetrical in the irregular oncoids. For descriptive purposes the studied oncoids are divided into three morphological types, A, B, and C. Type A oncoids are small, smooth–surfaced, and spherical to subspherical, and composed dominantly of concentrically laminated fabric (Figs. 3A, 4A). This type dominates in peloidal mudstones to wackestones. The concentric laminae of this type suggest that oncoids were frequently, ifnotcontinuously, rolled during formation (Wright, 1983). The associated micritic matrix indicates that the environment was not sufficiently energetic to remove all the finegrained sediment. Type B oncoids have a greater size range than type A. Their cortex is continuous and composed of irregular laminae. Jones and Wilkinson (1978) suggested that the continuous cortex may grow around allochems with no rolling. Type C oncoids are irregular (Fig. 3D). Types B and C oncoids could not have formed in an environment where rolling occurred. Peryt (1981) concluded that all oncoids are associated with slow sedimentation rates, and that oncoids composed of spongiostromate fabrics typify high–energy conditions.

Fig.4

—SEM photomicrographs of oncoids, matrix, and calcite cement. A) Microstructure of oncoid types (b and c). Note the calcite cement (a) and the matrix (d). B) Enlargement of b. Note the calcite spars which forming the laminated coating. C) Enlargement of c. Note the relic of the original high-Mg calcite mineral. D) Matrix displaying calcite microspar and porosity.

Fig.4

—SEM photomicrographs of oncoids, matrix, and calcite cement. A) Microstructure of oncoid types (b and c). Note the calcite cement (a) and the matrix (d). B) Enlargement of b. Note the calcite spars which forming the laminated coating. C) Enlargement of c. Note the relic of the original high-Mg calcite mineral. D) Matrix displaying calcite microspar and porosity.

In general, early marine cementation, dissolution of metastable calcium carbonate components, and dolomitization were the dominant diagenetic processes that affected the studied carbonate sequences (Fig. 5). Marine calcite cementation in interparticle (especially ooids) pore spaces began penecontemporaneously with Middle Jurassic carbonate deposition. Meniscus and pendant meteoric cements were formed prior to marine cementation. In many samples, early marine cementation was followed by leaching of allochems and other mineralogically unstable particles. Oncoids tended to be partially dissolved by the formation of vugs within them (Fig. 3C). Meanwhile, oncoid grains resisted dolomitization. Fossils were commonly dissolved entirely. Early dolomitization and late replacement dolomites are a common but rarely abundant component of Middle Jurassic carbonate.

Fig.5

—Paragenetic sequence for the Middle Jurassic carbonate (Bir Maghara Formation), Sinai.

Fig.5

—Paragenetic sequence for the Middle Jurassic carbonate (Bir Maghara Formation), Sinai.

Other diagenetic processes that affected the studied carbonates include stylolites and other pressure–dissolution features, burial calcite and dolomite cements, and fracturing, both tectonic and caused by collapse of partially dissolved framework allochems. Stylolites crosscut oncoid and ooid grains. Authigenic calcite and dolomite crystals are occasionally associated with stylolites (Fig. 3D). The Middle Jurassic carbonates are characterized by stylolite zones, which developed mainly in the peloncolitic micrite facies (Fig. 3D). This observation is in agreement with the Jurassic Burgogne oncolitic mudstones (Bathurst, 1984) .

Grain–contact suturing is common between the oncoid grains (Fig. 3A). This may be because the oncoid grains are more susceptible to dissolution than the other allochems. Stylolites do not crosscut late void–filling calcite and dolomite cements. The coarse calcite crystals and the saddle dolomite cements are generally interpreted as having formed by burial diagenetic fluids at elevated temperature (Choquette and James, 1987). Finally, late sparry equant calcite cements were precipitated after uplift of the Jurassic rocks. The petrographic characteristics suggest that the studied Middle Jurassic carbonate underwent long–lasting, meteoric modification, and dependent on meteoric recharge. These modifications could have been coincident with the development of regional, post–Jurassic unconformity corresponding to emergence of the Jurassic carbonates.

Geochemistry

Elemental Compositions.—

Oncoid grains, matrix, and calcite cements from surface and subsurface sequences did not yield any large difference in their elemental compositions. The mean Mg and Sr concentrations of oncoid gra ins are more than the corresponding matrix values, but the mean Fe and Mn concentrations are consistently less than the corresponding matrix values (Table 1; Fig. 6). During meteoric stabilization and alteration in a system with relatively high water/rock ratio, a steady state is reached for the oncoid grains. Therefore, the elemental compositions (Sr, Fe, and Mn) of the oncoid grains are approximately consistent with compositions expected for abiotic marine precipitates formed in Jurassic sea water. The relatively high MgCO3 concentrations of the oncoid grains (up to 3.9 mol %) is probably due to their original high-Mg calcite mineral composition. There is no indication of dolomitization of oncoid grains. On the other hand, the relatively low Sr contents of the matrix are quite common in ancient carbonate sequences. Moreover, the petrographic characters suggest that the Fe and Mn entered the matrix during a recrystallization event.

Table. 1.

Chemical and isotopic compositions of Middle Jurassic carbonate.

(a) Surface Samples, Gebel EI Maghara

(b) Surface Samples, Borehole No. H-1

Sample No.MgCO3(mol%)MnCO3(mol%)FeCO3(mol%)SrCO3(mol%)δ18O%oδ13C%o
OMCOMCOMCOMCOMCOMC
2. 61 0. 67 0. 68 0. 26 0. 60 0. 71 0. 60 1. 40 0. 90 0. 14 0. 10 0. 03 –5. 4 –7. 5 –10. 5 +2. 3 –0. 02 –2. 9 
3. 42 0. 81 0. 92 0. 14 0. 80 0. 63 0. 20 096 0. 60 0. 10 0. 07 0. 02 –5. 1 –7. 0 –11. 5 +2. 2 +0. 1 –2. 0 
12 1. 56 0. 54 0. 41 0. 06 1. 20 0. 92 0. 10 0. 84 0. 80 0. 09 0. 08 0. 01 –4. 2 –6. 7 –10. 3 +2. 5 +1. 0 0. 0 
15 2. 80 1. 15 – 0. 10 0. 90 – 0. 15 0. 90 – 0. 16 0. 11 – –4. 6 –5. 6 – +2. 8 +1. 5 – 
17 3. 65 1. 20 – 0. 42 2. 10 – 0. 16 1. 50 – 0. 21 0. 13 – –5. 6 –6. 5 – +2. 1 +0. 3 – 
20 3. 81 1. 13 1. 10 0. 28 1. 60 1. 20 0. 08 1. 70 1. 20 0. 18 0. 10 0. 04 –3. 8 –5. 0 –10. 0 +3. 2 +2. 5 –1. 5 
23 1. 60 0. 94 0. 54 0. 18 1. 64 0. 81 0. 12 0. 74 1. 10 0. 22 0. 15 0. 03 –5. 3 –6. 6 –10. 8 +2. 4 +2. 7 +1. 5 
24 1. 92 1. 31 0. 35 0. 04 1. 40 0. 94 0. 14 1. 20 1. 30 0. 08 0. 08 0. 01 –5. 4 –7. 2 –9. 5 +2. 8 +1. 6 +0. 1 
26 0. 94 0. 42 0. 48 0. 08 0. 60 1. 20 0. 04 0. 60 1. 10 0. 15 0. 12 0. 02 –4. 6 –7. 3 –10. 6 +3. 1 +0. 5 –0. 4 
28 0. 82 0. 71 0. 62 0. 10 0. 84 1. 30 0. 06 1. 30 1. 40 0. 19 0. 16 0. 01 –4. 4 –5. 6 –10. 4 +2. 9 +2. 5 –0. 5 
Sample No.MgCO3(mol%)MnCO3(mol%)FeCO3(mol%)SrCO3(mol%)δ18O%oδ13C%o
OMCOMCOMCOMCOMCOMC
2. 61 0. 67 0. 68 0. 26 0. 60 0. 71 0. 60 1. 40 0. 90 0. 14 0. 10 0. 03 –5. 4 –7. 5 –10. 5 +2. 3 –0. 02 –2. 9 
3. 42 0. 81 0. 92 0. 14 0. 80 0. 63 0. 20 096 0. 60 0. 10 0. 07 0. 02 –5. 1 –7. 0 –11. 5 +2. 2 +0. 1 –2. 0 
12 1. 56 0. 54 0. 41 0. 06 1. 20 0. 92 0. 10 0. 84 0. 80 0. 09 0. 08 0. 01 –4. 2 –6. 7 –10. 3 +2. 5 +1. 0 0. 0 
15 2. 80 1. 15 – 0. 10 0. 90 – 0. 15 0. 90 – 0. 16 0. 11 – –4. 6 –5. 6 – +2. 8 +1. 5 – 
17 3. 65 1. 20 – 0. 42 2. 10 – 0. 16 1. 50 – 0. 21 0. 13 – –5. 6 –6. 5 – +2. 1 +0. 3 – 
20 3. 81 1. 13 1. 10 0. 28 1. 60 1. 20 0. 08 1. 70 1. 20 0. 18 0. 10 0. 04 –3. 8 –5. 0 –10. 0 +3. 2 +2. 5 –1. 5 
23 1. 60 0. 94 0. 54 0. 18 1. 64 0. 81 0. 12 0. 74 1. 10 0. 22 0. 15 0. 03 –5. 3 –6. 6 –10. 8 +2. 4 +2. 7 +1. 5 
24 1. 92 1. 31 0. 35 0. 04 1. 40 0. 94 0. 14 1. 20 1. 30 0. 08 0. 08 0. 01 –5. 4 –7. 2 –9. 5 +2. 8 +1. 6 +0. 1 
26 0. 94 0. 42 0. 48 0. 08 0. 60 1. 20 0. 04 0. 60 1. 10 0. 15 0. 12 0. 02 –4. 6 –7. 3 –10. 6 +3. 1 +0. 5 –0. 4 
28 0. 82 0. 71 0. 62 0. 10 0. 84 1. 30 0. 06 1. 30 1. 40 0. 19 0. 16 0. 01 –4. 4 –5. 6 –10. 4 +2. 9 +2. 5 –0. 5 
Sample No.MgCO3(mol%)MnCO3(mol%)FeCO3(mol%)SrCO3(mol%)δ18O%oδ13C%o
OMCOMCOMCOMCOMCOMC
41 1. 86 0. 80 – 0. 34 0. 56 – 0. 12 1. 71 – 0. 11 0. 05 – –4. 0 –6. 1 – +2. 2 +1. 8 – 
44 2. 9 1. 20 – 0. 08 0. 40 – 0. 20 0. 92 – 0. 08 0. 07 – –4. 0 –4. 2 – +2. 7 +2. 5 – 
47 3. 6 1. 90 0. 85 0. 49 1. 06 0. 7 0. 80 1. 20 0. 81 0. 21 0. 10 0. 04 –4. 1 –4. 9 –9. 9 +3. 0 +2. 6 –1. 8 
50 3. 80 0. 45 0. 38 0. 13 0. 49 0. 65 0. 14 0. 64 0. 74 0. 19 0. 11 0. 05 –5. 1 –6. 4 –9. 6 +2. 3 +1. 6 +0. 8 
54 2. 75 0. 60 0. 6 0. 28 1. 12 0. 87 0. 19 1. 22 0. 68 0. 24 0. 18 0. 08 –5. 5 –5. 5 –9. 4 +2. 5 +2. 2 +0. 4 
58 1. 96 0. 84 0. 26 0. 33 0. 98 0. 49 0. 35 1. 43 1. 20 0. 30 0. 13 0. 05 –5. 0 –3. 8 –9. 7 +3. 0 +3. 0 +0. 7 
61 3. 90 1. 44 – 0. 64 1. 27 – 0. 04 0. 82 – 0. 18 0. 06 – –4. 5 –4. 8 – +3. 1 +2. 8 – 
66 2. 6 0. 64 1. 2 0. 51 0. 96 0. 74 0. 40 1. 66 0. 94 0. 28 0. 17 0. 06 –4. 5 –5. 4 –9. 0 +2. 5 +1. 6 –1. 2 
69 1. 75 0. 38 – 0. 22 1. 06 – 0. 31 0. 88 – 0. 19 0.06 – –4. 3 –6. 4 – +2. 8 +1. 4 – 
72 3. 4 0. 86 0. 4 0. 57 2. 18 1. 40 0. 41 2. 53 1. 12 0. 32 0. 12 0. 03 –4. 4 –6. 2 –8. 2 +2. 2 +1. 5 +1. 2 
Sample No.MgCO3(mol%)MnCO3(mol%)FeCO3(mol%)SrCO3(mol%)δ18O%oδ13C%o
OMCOMCOMCOMCOMCOMC
41 1. 86 0. 80 – 0. 34 0. 56 – 0. 12 1. 71 – 0. 11 0. 05 – –4. 0 –6. 1 – +2. 2 +1. 8 – 
44 2. 9 1. 20 – 0. 08 0. 40 – 0. 20 0. 92 – 0. 08 0. 07 – –4. 0 –4. 2 – +2. 7 +2. 5 – 
47 3. 6 1. 90 0. 85 0. 49 1. 06 0. 7 0. 80 1. 20 0. 81 0. 21 0. 10 0. 04 –4. 1 –4. 9 –9. 9 +3. 0 +2. 6 –1. 8 
50 3. 80 0. 45 0. 38 0. 13 0. 49 0. 65 0. 14 0. 64 0. 74 0. 19 0. 11 0. 05 –5. 1 –6. 4 –9. 6 +2. 3 +1. 6 +0. 8 
54 2. 75 0. 60 0. 6 0. 28 1. 12 0. 87 0. 19 1. 22 0. 68 0. 24 0. 18 0. 08 –5. 5 –5. 5 –9. 4 +2. 5 +2. 2 +0. 4 
58 1. 96 0. 84 0. 26 0. 33 0. 98 0. 49 0. 35 1. 43 1. 20 0. 30 0. 13 0. 05 –5. 0 –3. 8 –9. 7 +3. 0 +3. 0 +0. 7 
61 3. 90 1. 44 – 0. 64 1. 27 – 0. 04 0. 82 – 0. 18 0. 06 – –4. 5 –4. 8 – +3. 1 +2. 8 – 
66 2. 6 0. 64 1. 2 0. 51 0. 96 0. 74 0. 40 1. 66 0. 94 0. 28 0. 17 0. 06 –4. 5 –5. 4 –9. 0 +2. 5 +1. 6 –1. 2 
69 1. 75 0. 38 – 0. 22 1. 06 – 0. 31 0. 88 – 0. 19 0.06 – –4. 3 –6. 4 – +2. 8 +1. 4 – 
72 3. 4 0. 86 0. 4 0. 57 2. 18 1. 40 0. 41 2. 53 1. 12 0. 32 0. 12 0. 03 –4. 4 –6. 2 –8. 2 +2. 2 +1. 5 +1. 2 
O = oncoid grains, M = Matrix, C = meteoric calcite cement

Fig.6

—Element variations for the studied Middle Jurassic carbonate. A) For oncoids. B) For the matrix. • surface samples; ? subsurface samples.

Fig.6

—Element variations for the studied Middle Jurassic carbonate. A) For oncoids. B) For the matrix. • surface samples; ? subsurface samples.

The Fe and Mn contents covary in the matrix and the meteoric sparry calcite cements and show a negative correlation with Sr content (Fig. 6). In general, the geochemical data indicate that the matrix and the calcite cement are richer in Fe and Mn and poorer in Sr and Mg than the oncoid grains. Therefore, the primary geochemical signature of the matrix was reset by later fluids enriched inFe and Mn. Concentrations of MgC03 (0.35 to 1.2 mol %) and SrC03 (0.01 to 0.08 mol %) contents of the meteoric sparry calcite cements are lower than those of the Jurassic marine calcite cement and the studied oncoid grains (Table 1). In general, marine calcite cements formed in shallow depositional settings range from 10 to 21 mol % MgCO3 with a modal composition of 12 mol % (Carpenter and Lohmann, 1992; Burton, 1993). In contrast, calcite cements with less than 2 mol % MgCO3 are typical of cementation from water with low Mg2t/Ca2+ ratio (Allan and Matthews, 1982). Sr concentrations are highly variable in modern abiotic calcite (200–2000 ppm; average, 1200 ppm; Burton 1993). In comparison, the studied calcite cement has substantially lower MgCO3 and SrCO3 contents (Table 1). Such low values can be attributed to their formation from meteoric water. Meanwhile, the concentrations of Mn and Fe in the studied calcite cements are higher than those of the oncoid grains (Table 1). Modern calcite cement precipitated from oxic marine water has Mn = 1 ppm and Fe = 39 ppm (Burton, 1993). Consequently, there is compelling evidence that trace quantities of Fe and Mn present in the studied calcite cements were added during the cement formation from meteoric water and the primary mineral composition of these cements was low-Mg calcite.

Carbon Isotopes.—

The mean δ13C value of +2.6%o for the oncoid grains in the study Middle Jurassic carbonate sections (Fig. 7) is typical of inorganic precipitates from marine pore fluids. Unfortunately, there is no record of stable–isotope compositions for Middle Jurassic marine carbonate in Egypt. Because marine carbonates normally have δ13C values of about 0 ± 3%o (Lundegard and Land, 1986), the δ13C values of oncoids (between +2.1 and +3.2%o PDB) suggest that marine carbonate was dominant and isotopically light carbon was not involved in formation of the oncoids. Positive carbon-isotope values also suggest that meteoric water was not involved in the diagenesis of these oncoids, at least not in large proportion, because extensive meteoric water influx would have led to a relative depletion in 13C (James and Choquette, 1984; Holail, 1992). This might indicate that the stabilization of the original high-Mg calcite to low-Mg calcite occurred in a marine environment. This would have led to loss of Mg, but the δ13C values of the low-Mg calcite are identical to the marine signal. Moreover, the enriched 13C of oncoids reflect that the vital effects have not modified the original signal.

Fig.7

—Cross–plot showing the carbon and oxygen isotopic compositions of oncoids, matrix, and meteoric calcite cement from Middle Jurassic carbonate, Sinai, Egypt. A) Gebel Maghara section. B) Bore hole No. H–1 section.

Fig.7

—Cross–plot showing the carbon and oxygen isotopic compositions of oncoids, matrix, and meteoric calcite cement from Middle Jurassic carbonate, Sinai, Egypt. A) Gebel Maghara section. B) Bore hole No. H–1 section.

The mean 813C value of+1.7%o for the studied Middle Jurassic carbonate matrix (Fig. 7) is a characteristic of matrix alteration and recrystallization in systems with high water–rock ratios. These values are quite similar to that reported as typical for Jurassic carbonate matrix (Marshall and Ashton, 1980). The relative depletion in 13C for the matrix suggests that the carbon-isotope compositions were probably influenced by the 13C-depleted C02 inputs from meteoric fluids. Therefore, the relative depletion in 13C is related to diagenesis.

The δ13C values range widely, from +1.5 to –2.9%o for the meteoric sparry calcite cements (Figs. 7). Samples from the two studied sections have similar δ13C values. These values are lighter than those for other modern marine cements, which might reflect incorporation of isotopically light carbon from meteoric or subsurface water. This means that these values were strongly influenced by relatively 13C-enriched C02 coming from dissolution of the Middle Jurassic carbonates and by 13C-depleted CO2 coming from recharge of meteoric water through a soil zone. The wide range of δ13C compositions generally follow Lohmann’s (1988) meteoric–water line.

Oxygen Isotopes.—

The δ18O value is a sensitive parameter for evaluating water-rock ratio during Middle Jurassic carbonate diagenesis. This is because of the comparatively large temperature-related isotopic fractionation of oxygen isotopes, the large size of the pore–water oxygen reservoir, and the potentially large difference in δ18O values of marine vs. meteoric waters.

The mean δ18O value of -4.6%o for the studied Middle Jurassic oncoids is significantly shifted from values consistent with precipitation from Jurassic sea water (Fig. 7). The surface and subsurface oncoid samples do not yield any large difference in their oxygen-isotope compositions (from –3.8 to –5.6 %o and from –4.0 to –5.5 %o PDB, respectively). These values show a narrow range, however, and are much heavier than those of the Middle Jurassic meteoric cements and are close to those reported for presumed marine cements (–2.0 to –3.0%o PDB; Marshall and Ashton, 1980). Assuming the precipitation of oncoids from sea water with δ18O = –1 %o SMOW (1%o lighter than modern marine sea water), the paleothermometry equation of Friedman and O’Neil (1977) predicts formation of the Middle Jurassic oncoids at temperatures ranging from 29°C (δ18O = –3.8%o PDB) to 33°C (δ18O = –5.6%o PDB). These temperatures are quite high and are not in agreement with those predicted for normal sea water and are most probably related to the reseting of the oncoid δ18O values by diagenesis. The paleothermometry equation of Friedman and O’Neil (1977) for low-Mg calcite is used because the mean mineral composition of oncoid is approximately 3.6 mol % MgC03 (Table 1).

Stable oxygen–isotope compositions also suggest that if the studied oncoids lost their Mg during alteration. Land and Epstein (1970) showed that when high-Mg calcite fossils were converted to low-Mg calcite in meteoric waters, the oxygen–isotope composition of the resulting low-Mg calcite became lighter than the precursor carbonate, approaching isotopic equilibrium with meteoric waters. Oxygen–isotope data (Fig. 7) from the Middle Jurassic oncoids, which were originally of high-Mg calcite composition, show influence of meteoric waters. Therefore, the oncoids have stable–isotope composition consistent with alteration (stabilization) in meteoric waters (δ18O = –3.8 to –5.6%o PDB; δ13C =+2.2 to+3.2%o PDB).

Regarding the studied carbonate matrix, its δ18O and δ13C values define a trend distinct from the oncoids, with the subsurface matrix being isotopically heavier (δ18O = –3.8 to –6.4%o; δ13C = +1.4 to +3.6%o) than the surface matrix (δ18O = –5.0 to –7.5%o; δ13C = –0.1 to –2.7%o). There is great danger in just comparing numbers, and the matrix petrography and elemental composition should be considered to place constraints on the isotope interpretations. One particular problem is that there may have been secular fluctuations in sea–water δ18O and δ13C (Lohmann and Walker, 1989; Veizer and Hoefs, 1976), so that there could be variations through time of the isotopic signal of marine carbonate matrix. Therefore, the interpretation of these isotopic data assumes that the matrix had an initial Jurassic marine composition of about+3.0%o δ13C and –2.0 to –3.0%o δ18O PDB. Judging from these data the isotopically heaviest matrix samples are interpreted to have been least altered. The isotopically lighter matrix samples are interpreted to have been subjected to different recrystallization episodes.

The isotopic data also constrain the origin of the sparry calcite cements, which are the main occluders of porosity in the studied Middle Jurassic carbonates. It is clear that in the interpretation of the origins of these cements, the isotopic values are ambiguous. Light δ18O values indicate either light (meteoric) water at low temperatures or heavy (marine or subsurface) water at high temperatures. The formation of these cements from marine water (δ18O = –1.0%o SMOW) would necessitate unrealistically high temperatures (i.e., 80°C ± 10°C; calcite–water geothermometer of Friedman and O’Neil, 1977). Because these calcite cements are isotopically distinct from the oncoids and the carbonate matrix (Fig. 7), it is improbable that isotopic reequilibration at higher temperature occurred. It appears likely that these calcite cements formed from low lsO pore water (meteoric). Late diagenetic fluids with a δ18O value of –6.0 to – 7.0%o (SMOW) result in a more realistic formation temperature (21°–24°C) for the late calcite. Carbonate–water interactions and mixing with meteoric water can yield low 5lsO late diagenetic fluids. Moreover, the relatively low S13C values of these cements (from –2.9 to+1.5%o PDB) indicate that l3C-depleted C02 coming from soil gas and the meteoric influx was a major source of carbon, in addition to normal marine bicarbonate (0.0%o PDB).

Conclusions

Petrographic and geochemical data complement the study of Middle Jurassic oncoids in north Sinai, Egypt. The following major conclusions can be drawn from this study.

  1. Apart from the textural evidence, the geochemical signatures support a shallow marine origin for Middle Jurassic carbonate. Meanwhile, the oncoid distribution and fluctuation are superimposed on the overall regressive pattern, and the influx of meteoric water associated with this pattern had a major influence on diagenesis.

  2. δ18O resetting is least obvious in the oncoid grains. Correlation of various geochemical parameters demonstrates that δ18O values of these oncoids decrease with decreasing Sr and with increasing Fe and Mn contents. The preserved oncoids are closest to the initial isotopic composition and least altered trace–element composition.

  3. The wide range of δ13C values for sparry calcite cement is related to variable carbon sources. The different values correspond to differences in mixing of 13C–depleted CO2 from soil gas (meteoric end member) and 13C–enriched CO2 coming from the dissolution of Middle Jurassic carbonate (marine end member). These changes and the highly depleted 18O values are linked to meteoric diagenesis.

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Acknowledgments

I express my appreciation to Dr. R. Rifai and the Geological Survey of Egypt for providing access to core samples. I would like to thank Profs. K. C. Lohmann and B. Wilkinson for using the Stable Isotope Laboratory at the University of Michigan, USA.

The manuscript benefitted from the comments and suggestions of L. S. Land and S. J. Mazzullo. I commend the editors A.S. Alsharhan and R.W. Scott for their efforts on our behalf.

Figures & Tables

Contents

GeoRef

References

References

Abell
,
P.I.
Awramik
,
S.M.
and
Osborne
,
R.H.
1978
,
Oxygen, and carbon isotopic variation in Pleistocene lacustrine stromatolites from Lake Turkana, Kenya
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Far
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Allan
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817
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48
55
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James
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Geoscience Canada
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11
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161
194
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Jenkins
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R.
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The Geology of Egypt
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Rotterdam
,
A. A. Balkema
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361
380
.
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,
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and
Wilkinson
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B.H.
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,
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48
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1103
1110
.
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,
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:
Journal of Petroleum Geology
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14
, p.
49
64
.
Land
,
L.S.
and
Epstein
,
S.
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,
Late Pleistocene d iagenesis and dolomitization, North Jamaica
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Sedimentology
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14
, p.
187
200
.
Lohmann
,
K.C.
1988
,
(Geohemica) patterns of meteoric diagenetic systems and their application to studies of paleokarst
, in
James
,
N.P.
and
Choquette
,
P.W.
eds.,
Paleokarst: Berlin, Springer–Verlag
 , p.
58
80
.
Lohmann
,
K.C.
and
Walker
,
J.C.
1989
,
The 5lsO record of Phanerozoic abiotic marine calcite cements
:
Geophysical Research Letters
 , v.
16
, p.
316
322
.
Li \degard
,
P.D.
and
Land
,
L.S.
1986
,
Carbon dioxide and organic acids: their role in porosity enhancement and cementation: Paleogene of the Texas Gulf Coast
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Gautier
,
D.L.
ed.,
The Role of Organic Matter in Sediment Diagenesis
 :
SEPM, Special Publication
38
, p.
129
146
.
Marshall
,
J.F.
and
Ashton
,
M.
1980
,
Isotopic and trace element evidence for submarine lithification hard–grounds in Jurassic of Eastern England
:
Sedimentology
 , v.
27
, p.
271
289
.
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,
J.D.
1974
,
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:
Berlin, SprLnger–Verlag
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375
p.
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,
C.L.
1981
,
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Monty
,
C.L.
ed.,
Phanerozoic Stromatolites: Berlin, Springer–Verlag
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273
275
.
Morse
,
J.W.
and
Mackenzie
,
F.T.
1990
,
Geochemistry of Sedimentary Carbonates
:
Amsterdam, Elsevier
 ,
707
p.
Nickel
,
E.
1983
,
Environmental significance of freshwater oncoids, Eocene Guarga Formation, Southern Pyrenees, Spain
, in
Peryt
,
T.
ed.,
Coated Grains
 :
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