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Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh, Saudi Arabia
Accepted Manuscript Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh, Saudi Arabia Abdelbaset S. EL-Sorogy, Mahmoud A. Galmed, Khaled Al-Kahtany, Ali Al-Zahrani PII:
S1464-343X(17)30135-8
DOI:
10.1016/j.jafrearsci.2017.03.019
Reference:
AES 2855
To appear in:
Journal of African Earth Sciences
Received Date: 31 January 2017 Revised Date:
14 March 2017
Accepted Date: 20 March 2017
Please cite this article as: EL-Sorogy, A.S., Galmed, M.A., Al-Kahtany, K., Al-Zahrani, A., Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh, Saudi Arabia, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.03.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh,
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Saudi Arabia
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Abdelbaset S. EL-Sorogy1, 2, Mahmoud A. Galmed1, 3, Khaled Al-Kahtany1 and Ali Al-Zahrani1
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Geology and Geophysics Department, College of Science, King Saud University, Saudi Arabia.
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Geology Department, Faculty of Science, Zagazig University, Zagazig, Egypt.
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Geology Department, Faculty of Science, Cairo University, Egypt.
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Abstract
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In order to document the microfacies analysis and diagenetic alterations of the Middle
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Jurassic Dhruma Formation at southwest Riyadh City of central Saudi Arabia, a stratigraphic
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section was studied in detail at Khashm adh Dhi’bi area. Mudstones, wackstones, packstones,
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grainstones and boundstones are the main microfacies types in the studied area. These
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microfacies types with field investigations and fossil content indicated an environment ranging
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from deep shelf to organic buildup on platform margins for the studied carbonates. Cementation
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and recrystallization, dissolution, fragmentation and compaction, silicification, dolomitization,
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and bioerosion were the main diagenetic alterations affected the carbonate rocks of the Dhruma
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Formation. Cementation and recrystallization are represented by equant calcite crystals, Dog-
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tooth fringes of thin isopachous calcites and blocky low Mg-calcites. Gastrochaenolites,
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Trypanites and Meandropolydora spp. were the most bioeroders in coral heads and large bivalves
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and hardgrounds. These bioeroders indicated a long post-mortem period during the early
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diagenetic stage.
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Keywords: Microfacies, Diagenesis, Middle Jurassic, Dhruma Formation, Saudi Arabia, Saudi
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1. Introduction
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In Saudi Arabia, the Jurassic succession is divided into Marrat, Dhruma, Tuwaiq
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Mountain Limestone, Hanifa, Jubaila, Arab and Hith formations. It outcrops in central Saudi
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Arabia are with 1000 km length, exceeds 85 km width and 1100 m thickness (El-Asa’ad, 1989;
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El-Sorogy et al., 2014; El-Sorogy and Al-Kahtany, 2015; Al-Dabbagh and El-Sorogy, 2015). The
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Dhruma Formation conformably underlains by the Marrat Formation and overlain by the Tuwaiq
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Mountain Limestone. It has been suggested as one of the good oil reservoir rocks in some places
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and considered as source rocks in others in Saudi Arabia (Murris, 1980; Ayres et al., 1982).
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The Dhruma Formation exposed in belts parallel and adjacent to the Marrat Formation in
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west of Riyadh City, central Saudi Arabia. Both are exposed 10 km west of the city of Al Riyadh
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(Powers et al., 1966; Al-Saad, 2008). The Dhruma Formation has been assigned a Middle
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Jurassic age (Bajocian - Bathonian - Callovian) and shows a distinct lateral facies variation where
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the carbonate rocks in the north are replaced by siliciclastics to the south (Powers et al., 1966).
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Detailed studies have been carried out on the Dhruma Formation including geological,
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paleontological, paleoecological stratigraphical and chemostratigraphical points of view (e.g.
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Redmond, 1964 a, b, 1965; Powers et al., 1966; Powers, 1968; Murris, 1980; Ayres et al., 1982;
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Vaslet et al., 1983, 1985; Manivit et al., 1985; Beydon, 1988; Banner et al., 1991; Al-Husseini,
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1997; Sharland et al. 2001; Hughes, 2002, 2004; Al-Saad, 2008, Craigie, 2015). The main
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objectives of the present work are to document microfacies analyses, depositional environments
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and diagenetic alterations affected carbonates of Dhruma Formation in southwest Riyadh, Saudi
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Arabia.
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2. Materials and methods 2
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The study area is located southwest Riyadh City at Khashm adh Dhi’bi area, at Latitudes
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24° 12' 24˝ N and Longitudes 46° 07' 30˝ E (Fig. 1). A composite section was measured in detail
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and rock and fossil samples were collected. 50 thin sections were prepared for microfacies
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analysis, coral identification and diagenetic alterations. Some samples needed impregnation with
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resin under vacuum due to high porosity. Thin sections and selective hand specimens are
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investigated and photographed using Polarizing Microscope and SEM. The classification of
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carbonate rocks followed the nomenclature of Dunham (1962), Embry and Klovan (1971) and the
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energy index classification of Plumely et al. (1962).
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3. Geologic and stratigraphic setting
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Powers et al. (1966) divided Dhruma Formation at Khashm adh Dhibi (374.5 m) into
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lower, middle and upper Dhruma (Fig. 2). Further subdivision carried out to the formation into
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seven informal units (D1 to D7) based on lithostratigraphic and biostratigraphic evidences
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(Vaslet et al., 1983, 1985; Manivit et al., 1985).
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3.1. Lower Dhurma (D1 and D2)
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D1 conformably overlies the top of the Marrat Formation with 57 m thick. It begins with
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ammonitic claystone with laminated dolomite with stromatolitic laminite (Fig. 3A) and passes
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upward to ammonite-rich calcarenite. D2 consists of green claystone intercalated with
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bioturbated with bioclastic, nodular, fossiliferous limestone successions, 86 m thick. D2 contains
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the ammonites Dorsentensia, Normannites, Ermoceras, Stephanoceras spp., the brachiopods
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Burmirhynchia sp, as well as bivalves and gastropods.
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3.2. Middle Dhurma (D3 to D6) 3
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D3 is 52.5 m thick of coarse grained calcarenite, bioclastic limestone with scleractinian
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corals, pelletoid, bioturbated, oncolitic calcarenite rich in echinoid fragments (Fig. 3B). D4 is 44
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m thick of bioclastic calcirudite, coarse grained calarenite, ooilitic mudstone, shelly, nodular
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limestone (Fig. 3C). D5 is 41 m thick of fossiliferous limestone, clayey limestone, marl, sparry
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pelletoid fossiliferous calarenite, bioclastic limestone with ammonites. D6 is 55.5 m thick of
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claystone, bioclastic limestone with bivalves, bioclastic calcarenite, bioturbated limestone,
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pelletoid calcarenite and claystone. Middle Dhruma contains ammonites of Ermoceras sp. and
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Thambites sp., as well as abundant bivalves, brachiopods, gastropods and echinoids.
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3.3. Upper Dhurma (D7)
The upper Dhruma is divided into Atash and Hisyan members (Powers, 1968). The Atash
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Member is 26 m thick of sparry calcarenite with coral, bioclastic limestone with foraminifers, and
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massive bioclastic limestone with large stromatoporoids. The overlying Hisyan Member is 57 m
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of clayey limestone, bioclastic limestone with corals, calarenite with oyster and brachiopod
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fauna, fossiliferous clay intercalated with fine grained limestone and thin coquinoid slabs and
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topped with white limestone (Fig. 3D). Upper Dhruma contains abundant bivalves, brachiopods,
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gastropods and echinoids, as well as the ammonites Grossouvria sp., Erymnoceras sp. in the
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uppermost part.
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4. Results and discussion
4.1. Microfacies analysis
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Throughout the Middle Jurassic Dhruma Formation, the carbonates are the most common
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lithology throughout the studied sections. The limestone is consisting of micritic and sparitic mud
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and/or matrix (with some microsparite) together with wide ranging amounts of allochems, 4
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organic matter, micrograined terrigenous detritus, and secondary minerals such as pyrite and
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hematite.
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4.1.1. Mudstone microfacies
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This microfacies consists of dark brown micritic matrix with some foramineferal tests,
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sponge spicules and other bioclastic fragments. Terrigenous quartz grains were recorded in many
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samples. In most samples, these grains consist of are of silt to very fine sand size. The quartz
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grains are mostly very angular to subangular (Fig. 4A and Fig. 6A). Mudstones are represented
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by sandy mudstone, sandy foraminiferal mudstone and bioclastic mudstone. This microfacies
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type is comparable with facies zone 2 of Flugel (2010), which deposited below fair-weather wave
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base but within the reach of extreme storm waves on deep shelf.
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4.1.2. Wackstone microfacies
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This microfacies type is represented by forminiferal wackestone, pelloidal wackestone,
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echinoidal wackestone, algal molluscan wackestone and bioclastic wackestone. It is very
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common facies (up more than 40%) of the limestone units. The matrix consists of micrite with
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some silt and sand sized bioclasts of echinoids, algae, molluscs and few miliolids (Fig. 4B). Non-
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skeletal grains include intraclasts and pelloids. The pelloids are rounded to elliptical in shape.
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This microfacies type is comparable with facies zone 3 of Flugel (2010). It deposited on slope
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below wave base.
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4.1.3. Packstone microfacies
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This microfacies is characterized by abundance of allochems of different types and shapes
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embedded within microcrystalline calcite matrix and in many samples in microspars cements 5
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and/or sparite (Fig. 4 C, D and Fig. 6B). The allochems consist of pelloids and shell fragments of
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bivalves and foramineferal tests. The pelloids are rounded to elliptical in shape and exhibit brown
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staining. Ooids are rarely observed with rounded to elliptical forms with or without nucleus and
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have several cortexes and some with surfacial layer. Packstones include bioclastic packstone,
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ooilitic bioclastic packstone and foraminiferal pelloidal packstone. These types could be
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comparable with facies zone 4 of Flugel (2010), which deposited in distinctly inclined sea floor
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seaward of platform margins.
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4.1.4. Grainstone microfacies
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This microfacies is characterized by large amounts of allochems of medium to coarse
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sand size. Allochems are represented by skeletal grains, peloids and ooids. The peloidal grains
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are oval, spherical to elliptical in shape and micritized (Fig. 4E). The skeletal are formed of
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molluscan fragments, foraminiferal tests and brachiopod fragments. All are embedded in sparitic
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cement (Fig. 6C). Fine to medium, subangular to angular sand size of detrital quartz grains are
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observed. Grainstones are represented by coquina grainstone, pelloidal grainstone, sandy
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molluscan grainstone and ooilitic bioclastic grainstone. These types are comparable with facies
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zone 6 of Flugel (2010), in elongate shoals, above fair-weather wave base and within the euphotic
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zone, strongly influenced by tidal currents.
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4.1.5. Boundstone microfacies This microfacies is represented by coralline framestone and coralline bafflestone from
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the reefal parts in the middle and the upper Dhruma Formation. The scleractinian corals
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Actinastrea, Coenastraea, Stylina, Cryptocoenia,
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Kobyastraea and Vallimeandropsis spp. act as framebuilder (Fig. 4F). Boundstones are compared 6
Isastrea, Collignonastraea, Ovalastrea,
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with the Facies zone 5 of Flugel (2010), as wave-resistant barrier reefs rimming the platform.
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Water depths generally some meters and very narrow facies belt characterized by in-situ growth
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of sessile organisms. In general, the presence of these fossil assemblages of bivalves, brachiopods, ammonites
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and corals suggests a deposition in the open sea back-reef and shallow lagoon shelf (Masse et al.,
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2004; Neagu and Cirnaru, 2004; Ivanova et al., 2008; El-Sorogy et al., 2014). The recorded
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microfacies types, field investigations and fossil content indicated an environment ranging from
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deeper shelf margin with slow sedimentation to winnowed platform edge sands and organic
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buildup of in situ sessile organisms on platform margins.
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4.2. Diagenesis
The petrographic observations of the studied Middle Jurassic Dhruma carbonate
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samples underwent several diagenetic processes. These diagenetic changes may take place in
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the submarine, subaerial fresh water and subsurface environments. The following is a detailed
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study on these diagenetic processes:
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4.2.1. Cementation and recrystallization Medium to coarse, subhedral equant calcite crystals are partially or completely filled the
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original intra-skeletal pores within the studied skeletons in the Dhruma rocks (Figs 5A, B and
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Figs 6B, C). These indicate an early marine cementation. Some remnant pores are filled with
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fine-grained sediment, indicating penecontemporaneous deposition. Dog-tooth spar fringes of
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thin isopachous calcite crystals are clearly observed around cavities of bioclasts, especially
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scleractinian corals (Fig. 7D). Changing of aragonitic skeletons, especially corals to more stable,
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blocky, low Mg-calcite during the meteoric diagenetic stages is a good example of 7
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recrystallization (Figs. 5B, C). Some foraminiferal tests and ostracods showed a wide variety of
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sparry calcite cement and other ones are partially to completely replace by pyrite or hematite.
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4.2.2. Dissolution
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The originally aragonitic skeletons like scleractinian corals and some molluscs have been
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infilled by equant calcite cement or completely/partially dissolved or replaced by low-Mg calcite
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(Figs. 5A, C). It is apparent that many diagenetic alterations such cementations and silicification
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occurred during and/or after dissolution (Lawrence, 1994). Dissolution of aragonitic skeletons
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generates vuggy, moldic and enlarges intergranular porosities (Flügel, 2010; Ozer and Ahmed,
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2016). Dissolution of the smallest carbonate grains could also have furnished cementation, as it
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may saturate the waters with calcium carbonate.
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4.2.3. Fragmentation and compaction
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Shallow marine current and wave processes were the most important factors of the
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mechanical process (Gili et al., 1995). In our study area, mechanical fragmentation of the fossils
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was due to storms, currents, and/or winnowing. Some bivalve fragments showing transverse
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fractures (Figs. 5D and 7E). The presence of the mostly horizontal and vertical, but locally
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diagonal fractures indicate that the valves were exposed to intense physical compaction and
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fragmentation. Evidences of compaction include fusiform faecal pellets with attenuated lateral
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margins and completely flattened condition of some bivalve shell fragments (Fig. 4E).
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4.2.4. Silicification
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Silicification is less common diagenetic feature in the studied carbonate grains. This
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suggests that silica saturated waters and low pH conditions were occasionally developed in the 8
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environment resulting in authigenic silica pore-fill as explained in many studies (e.g. Schmitt and
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Boyd, 1981; Lawrence, 1994). The authigenic silica presents as pore-filling of the molluscan
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shell fragments (Fig. 5E). It is precipitated as spherulitic chalcedony. The contact of quartz
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crystals with the skeleton boundary is sharp, indicating that the quartz crystals were precipitated
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as cavity-filling cement after stabilization of the wall boundaries (El-Sorogy et al., 2016). It is
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apparent that the silicification occurred during and/or after dissolution, and has been explained in
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many studies (e.g. Jacka, 1974; Knauth, 1979; Holdaway and Clayton, 1982; Lawrence, 1994). In
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general chalcedony is usually destructive to the cellular structure of the bivalve skeletons, but the
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remains of primary calcites can be preserved within the chalcedony crystals. This indicates that
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silicification realized after calcite cementation of cells during late burial diagenesis.
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4.2.5. Dolomitization
Some studied carbonates are dolomitized. Dolomite rhombs are nonferroan, typical
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zoning and exhibited slightly to strongly undulose extinctions under crossed nicols. The original
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calcite matrix was replaced by fine crystalline, idiotopic to subidiotopic dolomite (Fig. 5F and
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Fig. 6D). The undulose extinctions associated with dolomites indicate a precipitated under deeper
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burial conditions or were altered by burial recrystallization (Pettijohn, 1975; Wendte et al., 1998;
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Tucker and Wright, 1990; Wierzbicki et al., 2006).
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4.2.6. Bioerosion
Bioerosion is one of the most characteristic and abundant feature in coral specimens, large
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bivalves and hard grounds in the upper part of the Dhruma Formation. Three species of traces
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were distinguished in the studied carbonates (Figs. 7. A-C): 1) Gastrochaenolites sp., which is
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the most common type of bioerosion structures, especially in coral heads. Kleeman (1980) stated 9
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that, the bioeroder Gastrochaenolites is mostly attributed to the bivalves such as Lithophaga and
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Gastrochaena spp. It has narrower aperture than the main chamber, which may be circular, oval,
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or dumb-bell shaped (Kelly and Bromley, 1984). 2) Trypanites sp., which is a narrow,
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cylindrical, unbranched boring with single entrance. Trypanites is the polychaetes and/or
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sipunculans which uses an acid or other chemical agent to dissolve the calcium carbonate
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(Bromley, 1972; Taylor and Wilson, 2003). 3) Worm borings of Meandropolydora sp. are very
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common as a long, cylindrical gallery with two apertures and run through the substrate in
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irregular contortions (Abdel-Fattah and Assal, 2015). The latter traces may indicate that the coral
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specimens were on the sea floor for a long post-mortem period during the pre-diagenetic phase
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but mostly the early diagenetic stage.
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5. Conclusions
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1. Based on lithostratigraphic and biostratigraphic criteria, the Dhruma Formation at Khashm adh
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Dhibi area in central Saudi Arabia is divided into lower (D1 and D2), middle (D3-D6) and upper
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Dhruma (D7). The carbonates of the Dhrurma Formation are distigushed into mudstones,
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wackestones, packstones, grainstones and boundstones. The most common allochemes are
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skeletal grains (molluscs, brachiopods, corals, echinoids, foraminifers and ostracods), pelloids
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and few ooids. Microfacies analysis and fossil content revealed a deposition in an environment
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ranged from deep shelf to organic buildup on platform margins.
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2. Cementation and recrystallization, dissolution, fragmentation and compaction, silicification,
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dolomitization and bioerosion are the main diagenetic alterations affected carbonate rocks of the
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Dhruma Formation. Equant, dog-tooth isopachous and blocky calcites are the recorded
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cementation and recrystallization in early marine and meteoric environment. Silica is precipitated
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as spherulitic chalcedony filling pores among shell fragments. Bioerosion by boring bivalves and
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worm tubes is one of the most characteristic coral heads and large oysters in the Dhruma
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carbonates.
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Acknowledgments
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group No. (RG-1438-060).
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Lawrence, M.J.F., 1994. Conceptual model for early diagenetic chert and dolomite, Amuri Limestone Group, north-eastern South Island, New Zealand. Sedimentol., 41, 479–498. Manivit, J., pelleton, C., Vaslet, D., le Nindre, Y-M., Brosse, J-M., Breton, J-P., Fourniquet, J.,
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1985. Geologic map of Dhruma Quadrangle, Sheet 24 H. kingdom of Saudi Arabia,
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Deputy Ministry for Mineral Resources, Geoscience Map, GM-101, Scale 1:250,000.
316
Text, 33 p.
319 320
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Masse, J.P., Fenerci-Masse, M., Korbar T., Velic. I., 2004. Lower Aptian rudist faunas (Bivalvia, Hippuritoidea) from Croatia. Geol. Croat., 57 (2), 117–137.
Murris, R.J., 1980. Middle East: Stratigraphic evolution and oil habitat. Bulletin American
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Association Petroleum Geologists, 64, 597–618.
Neagu, T., Cirnaru, P., 2004. Lower Aptian agglutinated foraminifera from the Southern
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Dobrogea and SE part of the Moesian Platform. Acta Palaeontol. Romaniae, 4, 277–297.
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Ozer, S., Ahmad, F., 2016. Caprinula and Sauvagesia rudist faunas (Bivalvia) from the
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Cenomanian of NW Jordan. Stratigraphy and taxonomy. Cretaceous Research 58, 141–
325
159.
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Pettijohn, F.J., 1975. Sedimentary rocks, Third Edition. Harper and Row, New York.
327
Plumley, W.J., Risley, G.A., Graves, J.R.R.W., Kaley, M.E., 1962. Energy index for limestone
328
interpretation and classification. In: A.A.P.G., Classification of carbonate rocks - a
329
symposium (85-107), Tulsa.
331
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Powers, R.W., 1968. Lexique stratigraphie international 3, Asie, fasc. 10bl, Saudi Arabia. C.N.R.S. edit., 177p.
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Powers, R.W., Ramirez, L.F., Redmond, C.D., Elberg, E.L.J.R., 1966. Geology of the Arabian
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Peninsula, Sedimentary Geology of Saudi Arabia. U.S. Geological Survey Professional
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papers, 560, D, 147 p. 14
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336 337 338 339 340
Redmond, C.D., 1964a. Lituolid foraminifera from the Jurassic and Cretaceous of Saudi Arabia. Micropaleontology, 10, 405–414. Redmond, C.D., 1964b. The Foraminiferal family Pfenderinidae in the Jurassic of Saudi Arabia. Micropaleontology, 10, 251–263.
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Redmond, C.D., 1965. Three new genera of Foraminifera from the Jurassic of Saudi Arabia. Micropaleontology, 11, 133–140.
Schmitt, J.G., Boyd, D.W., 1981. Patterns of silicification in Permian pelecypods and
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brachiopods from Wyoming. Journal of Sedimetary Petrology, 51, 1297–1308.
343
Sharland, P.R., Archer, R., Casey, D.M., Davies, R.B., Hall, S.H., Heward, A.B., Horbury, A.D.,
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Simmons, M.D., 2001. Sequence Stratigraphy of the Arabian Plate. In: Husseini, M.
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(Ed.). GeoArabia, Special Publication, 2, 371 p.
348 349
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Taylor, P.D., Wilson, M.A. 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews, 62, 1–103.
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Tucker, M.E., Wright, V.P., 1990. Carbonate sedimentology. Blackwell Scientific Publications, Oxford-London-Edinburgh-Boston-Melbourne-Berlin-Paris-Vienna, 481pp. Vaslet, D., Delfour J., Manivit, J., Le Nindre Y.M., Brosse J.M., Fourniguc, J., 1983. Geologic
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Map of the Wadi a Rayn Quadrangle, sheet 23 H., Kingdom of Saudi Arabia, Saudi
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Arabian Deputy Ministry for Mineral Resources, Geoscience Map GM-63, Scale
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1:250,000 with text, 46p.
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Vaslet, D., Pelleton, C., Manivit, J., le Nindre, Y-M., Brosse, J-M., Fourniguet, J., 1985.
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Geologic map of The Sulayyimah Quadrangle, Sheet 21 H. Kingdom of Saudi Arabia
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(with text): Saudi Arabian Deputy Ministry of Mineral Resources, Geosciences Map
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GM-100 A, Scale 1:250,000.
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Wendte, J.C., Qinq, J.J., Dravis, S. O., Moore, L.L., Ward, S.G., 1998. High-temperature saline
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(thermoflux) dolomitization of Devonian Swan Hills platform and bank carbonates,
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Wild River area, west-central Alberta. Bull. Canad. Petrol. Geol., 46, 210–265.
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Wierzbicki, R., Dravis, J.J., Al-Aasm I., Harland, N., 2006. Burial dolomitization and dissolution
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of Upper Jurassic Abenaki platform carbonates, Deep Panuke reservoir, Nova Scotia,
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Canada. A.A.P.G.Bull., 90 (11), 1843–1861.
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Figure Captions
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Fig. 1. Location map of the study area (after El-Sorogy et al., 2017).
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Fig. 2. Composite stratigraphic section of the three members of the Dhruma Farmation at
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Khashm adh Dhibi.
386
Fig. 3. A, stromatolites at the base of the lower Dhruma Formation (D1); B, fossiliferous
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limestone with scleractinian corals and bivalves, middle Dhruma (D3); C, bedded, massive
388
fossiliferous limestone intercalated with thinly marls, middle Dhruma (D4); D, massive
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bioturbated limestone in the upper Dhruma (D7).
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Fig. 4. Examples of the microfacies associations of the Dhruma Formation (Crossed nicols). A,
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sandy mudstone, with very fine quartz grains, lower Dhruma; B, echinoidal wackestone with
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echinoid plates (arrows) and other bioclastics, embedded in micritic matrix, lower Dhruma; C,
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sandy foraminiferal peloidal packstone with biserial test (arrows), middle Dhruma; D, sandy
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molluscan peloidal packstone with molluscan fragments, middle Dhruma; E, molluscan
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grainstone with rounded molluscan fragments in sparitic cement, upper Dhruma; F, scleractinian
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framestone with recrystallized corals, upper Dhruma.
397
Fig. 5. Examples of the diagenetic alterations affected carbonate rocks of the Dhruma Formation
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(crossed nicols). A, partial dissolution of bivalve fragments (arrows), middle Dhruma; B, C,
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cementation and recrystallization of scleractinian corals, bivalve fragments and ostracods, upper
400
Dhruma; D, fractures in the bivalve fragments due to compaction (arrows), middle Dhruma; E,
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silicification of bivalve fragment in the form of spherulitic chalcedony in pores after dissolution,
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upper Dhruma; F, dolomitization if the form of well crystalline, idiotopic to subidiotopic
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dolomite rhombs, lower Dhruma.
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Fig. 6. Representative SEM photos. A, detrital quartz grain embedded within microcrystalline
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calcite matrix, middle Dhruma; B, microsparite to sparite filling voids, middle Dhruma; C, well
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crystalline sparry calcite crystals, upper Dhruma; D, well crystalline dolomite rhomb in
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dolostone, lower Dhruma.
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Fig. 7. Examples of bioerosion in corals. A, Gastrochaenolites borings in scleractinian coral,
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middle Dhruma; B, Meandropolydora borings in the lower surface of coral head, middle
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Dhruma; C, meandering tunnels of Trypanites, upper Dhruma; D, dog-tooth spar fringes around
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cavities of scleractinian coral, upper Dhruma; E, different fractures as a result of intense physical
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compaction and fragmentation, middle Dhruma.
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Unit Lower T1
Member
Upper Dhurma
Tuwaiq
Lithology
Description Massive limestone
Calcarenite with corals, bioclastic limestone with foraminifers, massive bioclastic limestone with large stromatoporoids, clayey limestone, bioclastic limestone with corals, calarenite with oyster and brachiopod fauna.
D7
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Age
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D3
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Lower Dhurma
D2
Early Marrat Jurassic
Calcarenite, bioclastic limestone with scleractinian corals, pelletoid, bioturbated, oncolitic calcarenite, clayey limestone, pelletoid fossiliferous calarenite, bioclastic limestone with ammonites. bioclastic limestone with bivalves.
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Middle
Dhurma
D5
D4
Middle
Jurassic
D6
Ammonitic claystone with laminated dolomite and stromatolitic laminate, green claystone intercalated with bioturbated and bioclastic, nodular, fossiliferous limestone.
D1 10
m
5 0
Gypsum, bedded carbonates rocks.
Upper Marrat
Fig. 2
B
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F Fig. 4
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Highlights 1-
Mudstones, wackstones, packstones, grainstones and bountones are the main
microfacies investigated in Dhruma carbonates. Cementation,
dissolution,
compaction,
bioerosion are the diagenetic alterations. 3-
silicification,
dolomitization
and
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Dhruma carbonates are deposited in deeper shelf margin to organic buildup on
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platform margins.
S1464-343X(17)30135-8
DOI:
10.1016/j.jafrearsci.2017.03.019
Reference:
AES 2855
To appear in:
Journal of African Earth Sciences
Received Date: 31 January 2017 Revised Date:
14 March 2017
Accepted Date: 20 March 2017
Please cite this article as: EL-Sorogy, A.S., Galmed, M.A., Al-Kahtany, K., Al-Zahrani, A., Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh, Saudi Arabia, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.03.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Microfacies and diagenesis of the Middle Jurassic Dhruma carbonates, southwest Riyadh,
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Saudi Arabia
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Abdelbaset S. EL-Sorogy1, 2, Mahmoud A. Galmed1, 3, Khaled Al-Kahtany1 and Ali Al-Zahrani1
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Geology and Geophysics Department, College of Science, King Saud University, Saudi Arabia.
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2
Geology Department, Faculty of Science, Zagazig University, Zagazig, Egypt.
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3
Geology Department, Faculty of Science, Cairo University, Egypt.
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Abstract
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In order to document the microfacies analysis and diagenetic alterations of the Middle
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Jurassic Dhruma Formation at southwest Riyadh City of central Saudi Arabia, a stratigraphic
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section was studied in detail at Khashm adh Dhi’bi area. Mudstones, wackstones, packstones,
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grainstones and boundstones are the main microfacies types in the studied area. These
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microfacies types with field investigations and fossil content indicated an environment ranging
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from deep shelf to organic buildup on platform margins for the studied carbonates. Cementation
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and recrystallization, dissolution, fragmentation and compaction, silicification, dolomitization,
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and bioerosion were the main diagenetic alterations affected the carbonate rocks of the Dhruma
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Formation. Cementation and recrystallization are represented by equant calcite crystals, Dog-
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tooth fringes of thin isopachous calcites and blocky low Mg-calcites. Gastrochaenolites,
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Trypanites and Meandropolydora spp. were the most bioeroders in coral heads and large bivalves
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and hardgrounds. These bioeroders indicated a long post-mortem period during the early
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diagenetic stage.
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Keywords: Microfacies, Diagenesis, Middle Jurassic, Dhruma Formation, Saudi Arabia, Saudi
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Arabia.
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1. Introduction
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In Saudi Arabia, the Jurassic succession is divided into Marrat, Dhruma, Tuwaiq
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Mountain Limestone, Hanifa, Jubaila, Arab and Hith formations. It outcrops in central Saudi
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Arabia are with 1000 km length, exceeds 85 km width and 1100 m thickness (El-Asa’ad, 1989;
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El-Sorogy et al., 2014; El-Sorogy and Al-Kahtany, 2015; Al-Dabbagh and El-Sorogy, 2015). The
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Dhruma Formation conformably underlains by the Marrat Formation and overlain by the Tuwaiq
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Mountain Limestone. It has been suggested as one of the good oil reservoir rocks in some places
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and considered as source rocks in others in Saudi Arabia (Murris, 1980; Ayres et al., 1982).
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The Dhruma Formation exposed in belts parallel and adjacent to the Marrat Formation in
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west of Riyadh City, central Saudi Arabia. Both are exposed 10 km west of the city of Al Riyadh
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(Powers et al., 1966; Al-Saad, 2008). The Dhruma Formation has been assigned a Middle
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Jurassic age (Bajocian - Bathonian - Callovian) and shows a distinct lateral facies variation where
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the carbonate rocks in the north are replaced by siliciclastics to the south (Powers et al., 1966).
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Detailed studies have been carried out on the Dhruma Formation including geological,
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paleontological, paleoecological stratigraphical and chemostratigraphical points of view (e.g.
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Redmond, 1964 a, b, 1965; Powers et al., 1966; Powers, 1968; Murris, 1980; Ayres et al., 1982;
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Vaslet et al., 1983, 1985; Manivit et al., 1985; Beydon, 1988; Banner et al., 1991; Al-Husseini,
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1997; Sharland et al. 2001; Hughes, 2002, 2004; Al-Saad, 2008, Craigie, 2015). The main
44
objectives of the present work are to document microfacies analyses, depositional environments
45
and diagenetic alterations affected carbonates of Dhruma Formation in southwest Riyadh, Saudi
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Arabia.
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2. Materials and methods 2
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The study area is located southwest Riyadh City at Khashm adh Dhi’bi area, at Latitudes
50
24° 12' 24˝ N and Longitudes 46° 07' 30˝ E (Fig. 1). A composite section was measured in detail
51
and rock and fossil samples were collected. 50 thin sections were prepared for microfacies
52
analysis, coral identification and diagenetic alterations. Some samples needed impregnation with
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resin under vacuum due to high porosity. Thin sections and selective hand specimens are
54
investigated and photographed using Polarizing Microscope and SEM. The classification of
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carbonate rocks followed the nomenclature of Dunham (1962), Embry and Klovan (1971) and the
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energy index classification of Plumely et al. (1962).
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3. Geologic and stratigraphic setting
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Powers et al. (1966) divided Dhruma Formation at Khashm adh Dhibi (374.5 m) into
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lower, middle and upper Dhruma (Fig. 2). Further subdivision carried out to the formation into
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seven informal units (D1 to D7) based on lithostratigraphic and biostratigraphic evidences
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(Vaslet et al., 1983, 1985; Manivit et al., 1985).
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3.1. Lower Dhurma (D1 and D2)
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D1 conformably overlies the top of the Marrat Formation with 57 m thick. It begins with
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ammonitic claystone with laminated dolomite with stromatolitic laminite (Fig. 3A) and passes
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upward to ammonite-rich calcarenite. D2 consists of green claystone intercalated with
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bioturbated with bioclastic, nodular, fossiliferous limestone successions, 86 m thick. D2 contains
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the ammonites Dorsentensia, Normannites, Ermoceras, Stephanoceras spp., the brachiopods
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Burmirhynchia sp, as well as bivalves and gastropods.
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3.2. Middle Dhurma (D3 to D6) 3
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D3 is 52.5 m thick of coarse grained calcarenite, bioclastic limestone with scleractinian
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corals, pelletoid, bioturbated, oncolitic calcarenite rich in echinoid fragments (Fig. 3B). D4 is 44
75
m thick of bioclastic calcirudite, coarse grained calarenite, ooilitic mudstone, shelly, nodular
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limestone (Fig. 3C). D5 is 41 m thick of fossiliferous limestone, clayey limestone, marl, sparry
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pelletoid fossiliferous calarenite, bioclastic limestone with ammonites. D6 is 55.5 m thick of
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claystone, bioclastic limestone with bivalves, bioclastic calcarenite, bioturbated limestone,
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pelletoid calcarenite and claystone. Middle Dhruma contains ammonites of Ermoceras sp. and
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Thambites sp., as well as abundant bivalves, brachiopods, gastropods and echinoids.
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3.3. Upper Dhurma (D7)
The upper Dhruma is divided into Atash and Hisyan members (Powers, 1968). The Atash
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Member is 26 m thick of sparry calcarenite with coral, bioclastic limestone with foraminifers, and
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massive bioclastic limestone with large stromatoporoids. The overlying Hisyan Member is 57 m
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of clayey limestone, bioclastic limestone with corals, calarenite with oyster and brachiopod
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fauna, fossiliferous clay intercalated with fine grained limestone and thin coquinoid slabs and
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topped with white limestone (Fig. 3D). Upper Dhruma contains abundant bivalves, brachiopods,
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gastropods and echinoids, as well as the ammonites Grossouvria sp., Erymnoceras sp. in the
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uppermost part.
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4. Results and discussion
4.1. Microfacies analysis
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Throughout the Middle Jurassic Dhruma Formation, the carbonates are the most common
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lithology throughout the studied sections. The limestone is consisting of micritic and sparitic mud
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and/or matrix (with some microsparite) together with wide ranging amounts of allochems, 4
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organic matter, micrograined terrigenous detritus, and secondary minerals such as pyrite and
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hematite.
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4.1.1. Mudstone microfacies
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This microfacies consists of dark brown micritic matrix with some foramineferal tests,
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sponge spicules and other bioclastic fragments. Terrigenous quartz grains were recorded in many
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samples. In most samples, these grains consist of are of silt to very fine sand size. The quartz
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grains are mostly very angular to subangular (Fig. 4A and Fig. 6A). Mudstones are represented
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by sandy mudstone, sandy foraminiferal mudstone and bioclastic mudstone. This microfacies
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type is comparable with facies zone 2 of Flugel (2010), which deposited below fair-weather wave
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base but within the reach of extreme storm waves on deep shelf.
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4.1.2. Wackstone microfacies
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This microfacies type is represented by forminiferal wackestone, pelloidal wackestone,
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echinoidal wackestone, algal molluscan wackestone and bioclastic wackestone. It is very
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common facies (up more than 40%) of the limestone units. The matrix consists of micrite with
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some silt and sand sized bioclasts of echinoids, algae, molluscs and few miliolids (Fig. 4B). Non-
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skeletal grains include intraclasts and pelloids. The pelloids are rounded to elliptical in shape.
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This microfacies type is comparable with facies zone 3 of Flugel (2010). It deposited on slope
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below wave base.
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4.1.3. Packstone microfacies
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This microfacies is characterized by abundance of allochems of different types and shapes
120
embedded within microcrystalline calcite matrix and in many samples in microspars cements 5
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and/or sparite (Fig. 4 C, D and Fig. 6B). The allochems consist of pelloids and shell fragments of
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bivalves and foramineferal tests. The pelloids are rounded to elliptical in shape and exhibit brown
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staining. Ooids are rarely observed with rounded to elliptical forms with or without nucleus and
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have several cortexes and some with surfacial layer. Packstones include bioclastic packstone,
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ooilitic bioclastic packstone and foraminiferal pelloidal packstone. These types could be
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comparable with facies zone 4 of Flugel (2010), which deposited in distinctly inclined sea floor
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seaward of platform margins.
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4.1.4. Grainstone microfacies
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This microfacies is characterized by large amounts of allochems of medium to coarse
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sand size. Allochems are represented by skeletal grains, peloids and ooids. The peloidal grains
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are oval, spherical to elliptical in shape and micritized (Fig. 4E). The skeletal are formed of
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molluscan fragments, foraminiferal tests and brachiopod fragments. All are embedded in sparitic
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cement (Fig. 6C). Fine to medium, subangular to angular sand size of detrital quartz grains are
135
observed. Grainstones are represented by coquina grainstone, pelloidal grainstone, sandy
136
molluscan grainstone and ooilitic bioclastic grainstone. These types are comparable with facies
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zone 6 of Flugel (2010), in elongate shoals, above fair-weather wave base and within the euphotic
138
zone, strongly influenced by tidal currents.
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4.1.5. Boundstone microfacies This microfacies is represented by coralline framestone and coralline bafflestone from
142
the reefal parts in the middle and the upper Dhruma Formation. The scleractinian corals
143
Actinastrea, Coenastraea, Stylina, Cryptocoenia,
144
Kobyastraea and Vallimeandropsis spp. act as framebuilder (Fig. 4F). Boundstones are compared 6
Isastrea, Collignonastraea, Ovalastrea,
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with the Facies zone 5 of Flugel (2010), as wave-resistant barrier reefs rimming the platform.
146
Water depths generally some meters and very narrow facies belt characterized by in-situ growth
147
of sessile organisms. In general, the presence of these fossil assemblages of bivalves, brachiopods, ammonites
149
and corals suggests a deposition in the open sea back-reef and shallow lagoon shelf (Masse et al.,
150
2004; Neagu and Cirnaru, 2004; Ivanova et al., 2008; El-Sorogy et al., 2014). The recorded
151
microfacies types, field investigations and fossil content indicated an environment ranging from
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deeper shelf margin with slow sedimentation to winnowed platform edge sands and organic
153
buildup of in situ sessile organisms on platform margins.
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4.2. Diagenesis
The petrographic observations of the studied Middle Jurassic Dhruma carbonate
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samples underwent several diagenetic processes. These diagenetic changes may take place in
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the submarine, subaerial fresh water and subsurface environments. The following is a detailed
159
study on these diagenetic processes:
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4.2.1. Cementation and recrystallization Medium to coarse, subhedral equant calcite crystals are partially or completely filled the
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original intra-skeletal pores within the studied skeletons in the Dhruma rocks (Figs 5A, B and
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Figs 6B, C). These indicate an early marine cementation. Some remnant pores are filled with
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fine-grained sediment, indicating penecontemporaneous deposition. Dog-tooth spar fringes of
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thin isopachous calcite crystals are clearly observed around cavities of bioclasts, especially
167
scleractinian corals (Fig. 7D). Changing of aragonitic skeletons, especially corals to more stable,
168
blocky, low Mg-calcite during the meteoric diagenetic stages is a good example of 7
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recrystallization (Figs. 5B, C). Some foraminiferal tests and ostracods showed a wide variety of
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sparry calcite cement and other ones are partially to completely replace by pyrite or hematite.
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4.2.2. Dissolution
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The originally aragonitic skeletons like scleractinian corals and some molluscs have been
174
infilled by equant calcite cement or completely/partially dissolved or replaced by low-Mg calcite
175
(Figs. 5A, C). It is apparent that many diagenetic alterations such cementations and silicification
176
occurred during and/or after dissolution (Lawrence, 1994). Dissolution of aragonitic skeletons
177
generates vuggy, moldic and enlarges intergranular porosities (Flügel, 2010; Ozer and Ahmed,
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2016). Dissolution of the smallest carbonate grains could also have furnished cementation, as it
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may saturate the waters with calcium carbonate.
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4.2.3. Fragmentation and compaction
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Shallow marine current and wave processes were the most important factors of the
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mechanical process (Gili et al., 1995). In our study area, mechanical fragmentation of the fossils
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was due to storms, currents, and/or winnowing. Some bivalve fragments showing transverse
185
fractures (Figs. 5D and 7E). The presence of the mostly horizontal and vertical, but locally
186
diagonal fractures indicate that the valves were exposed to intense physical compaction and
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fragmentation. Evidences of compaction include fusiform faecal pellets with attenuated lateral
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margins and completely flattened condition of some bivalve shell fragments (Fig. 4E).
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4.2.4. Silicification
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Silicification is less common diagenetic feature in the studied carbonate grains. This
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suggests that silica saturated waters and low pH conditions were occasionally developed in the 8
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environment resulting in authigenic silica pore-fill as explained in many studies (e.g. Schmitt and
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Boyd, 1981; Lawrence, 1994). The authigenic silica presents as pore-filling of the molluscan
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shell fragments (Fig. 5E). It is precipitated as spherulitic chalcedony. The contact of quartz
196
crystals with the skeleton boundary is sharp, indicating that the quartz crystals were precipitated
197
as cavity-filling cement after stabilization of the wall boundaries (El-Sorogy et al., 2016). It is
198
apparent that the silicification occurred during and/or after dissolution, and has been explained in
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many studies (e.g. Jacka, 1974; Knauth, 1979; Holdaway and Clayton, 1982; Lawrence, 1994). In
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general chalcedony is usually destructive to the cellular structure of the bivalve skeletons, but the
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remains of primary calcites can be preserved within the chalcedony crystals. This indicates that
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silicification realized after calcite cementation of cells during late burial diagenesis.
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4.2.5. Dolomitization
Some studied carbonates are dolomitized. Dolomite rhombs are nonferroan, typical
206
zoning and exhibited slightly to strongly undulose extinctions under crossed nicols. The original
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calcite matrix was replaced by fine crystalline, idiotopic to subidiotopic dolomite (Fig. 5F and
208
Fig. 6D). The undulose extinctions associated with dolomites indicate a precipitated under deeper
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burial conditions or were altered by burial recrystallization (Pettijohn, 1975; Wendte et al., 1998;
210
Tucker and Wright, 1990; Wierzbicki et al., 2006).
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4.2.6. Bioerosion
Bioerosion is one of the most characteristic and abundant feature in coral specimens, large
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bivalves and hard grounds in the upper part of the Dhruma Formation. Three species of traces
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were distinguished in the studied carbonates (Figs. 7. A-C): 1) Gastrochaenolites sp., which is
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the most common type of bioerosion structures, especially in coral heads. Kleeman (1980) stated 9
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that, the bioeroder Gastrochaenolites is mostly attributed to the bivalves such as Lithophaga and
218
Gastrochaena spp. It has narrower aperture than the main chamber, which may be circular, oval,
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or dumb-bell shaped (Kelly and Bromley, 1984). 2) Trypanites sp., which is a narrow,
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cylindrical, unbranched boring with single entrance. Trypanites is the polychaetes and/or
221
sipunculans which uses an acid or other chemical agent to dissolve the calcium carbonate
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(Bromley, 1972; Taylor and Wilson, 2003). 3) Worm borings of Meandropolydora sp. are very
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common as a long, cylindrical gallery with two apertures and run through the substrate in
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irregular contortions (Abdel-Fattah and Assal, 2015). The latter traces may indicate that the coral
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specimens were on the sea floor for a long post-mortem period during the pre-diagenetic phase
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but mostly the early diagenetic stage.
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5. Conclusions
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1. Based on lithostratigraphic and biostratigraphic criteria, the Dhruma Formation at Khashm adh
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Dhibi area in central Saudi Arabia is divided into lower (D1 and D2), middle (D3-D6) and upper
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Dhruma (D7). The carbonates of the Dhrurma Formation are distigushed into mudstones,
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wackestones, packstones, grainstones and boundstones. The most common allochemes are
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skeletal grains (molluscs, brachiopods, corals, echinoids, foraminifers and ostracods), pelloids
234
and few ooids. Microfacies analysis and fossil content revealed a deposition in an environment
235
ranged from deep shelf to organic buildup on platform margins.
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2. Cementation and recrystallization, dissolution, fragmentation and compaction, silicification,
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dolomitization and bioerosion are the main diagenetic alterations affected carbonate rocks of the
238
Dhruma Formation. Equant, dog-tooth isopachous and blocky calcites are the recorded
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cementation and recrystallization in early marine and meteoric environment. Silica is precipitated
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as spherulitic chalcedony filling pores among shell fragments. Bioerosion by boring bivalves and
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worm tubes is one of the most characteristic coral heads and large oysters in the Dhruma
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carbonates.
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Acknowledgments
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group No. (RG-1438-060).
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Figure Captions
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Fig. 1. Location map of the study area (after El-Sorogy et al., 2017).
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Fig. 2. Composite stratigraphic section of the three members of the Dhruma Farmation at
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Khashm adh Dhibi.
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Fig. 3. A, stromatolites at the base of the lower Dhruma Formation (D1); B, fossiliferous
387
limestone with scleractinian corals and bivalves, middle Dhruma (D3); C, bedded, massive
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fossiliferous limestone intercalated with thinly marls, middle Dhruma (D4); D, massive
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bioturbated limestone in the upper Dhruma (D7).
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Fig. 4. Examples of the microfacies associations of the Dhruma Formation (Crossed nicols). A,
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sandy mudstone, with very fine quartz grains, lower Dhruma; B, echinoidal wackestone with
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echinoid plates (arrows) and other bioclastics, embedded in micritic matrix, lower Dhruma; C,
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sandy foraminiferal peloidal packstone with biserial test (arrows), middle Dhruma; D, sandy
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molluscan peloidal packstone with molluscan fragments, middle Dhruma; E, molluscan
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grainstone with rounded molluscan fragments in sparitic cement, upper Dhruma; F, scleractinian
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framestone with recrystallized corals, upper Dhruma.
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Fig. 5. Examples of the diagenetic alterations affected carbonate rocks of the Dhruma Formation
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(crossed nicols). A, partial dissolution of bivalve fragments (arrows), middle Dhruma; B, C,
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cementation and recrystallization of scleractinian corals, bivalve fragments and ostracods, upper
400
Dhruma; D, fractures in the bivalve fragments due to compaction (arrows), middle Dhruma; E,
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silicification of bivalve fragment in the form of spherulitic chalcedony in pores after dissolution,
402
upper Dhruma; F, dolomitization if the form of well crystalline, idiotopic to subidiotopic
403
dolomite rhombs, lower Dhruma.
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Fig. 6. Representative SEM photos. A, detrital quartz grain embedded within microcrystalline
405
calcite matrix, middle Dhruma; B, microsparite to sparite filling voids, middle Dhruma; C, well
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crystalline sparry calcite crystals, upper Dhruma; D, well crystalline dolomite rhomb in
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dolostone, lower Dhruma.
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Fig. 7. Examples of bioerosion in corals. A, Gastrochaenolites borings in scleractinian coral,
409
middle Dhruma; B, Meandropolydora borings in the lower surface of coral head, middle
410
Dhruma; C, meandering tunnels of Trypanites, upper Dhruma; D, dog-tooth spar fringes around
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cavities of scleractinian coral, upper Dhruma; E, different fractures as a result of intense physical
412
compaction and fragmentation, middle Dhruma.
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Fig. 1
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Unit Lower T1
Member
Upper Dhurma
Tuwaiq
Lithology
Description Massive limestone
Calcarenite with corals, bioclastic limestone with foraminifers, massive bioclastic limestone with large stromatoporoids, clayey limestone, bioclastic limestone with corals, calarenite with oyster and brachiopod fauna.
D7
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Fm
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Age
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D3
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Lower Dhurma
D2
Early Marrat Jurassic
Calcarenite, bioclastic limestone with scleractinian corals, pelletoid, bioturbated, oncolitic calcarenite, clayey limestone, pelletoid fossiliferous calarenite, bioclastic limestone with ammonites. bioclastic limestone with bivalves.
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Middle
Dhurma
D5
D4
Middle
Jurassic
D6
Ammonitic claystone with laminated dolomite and stromatolitic laminate, green claystone intercalated with bioturbated and bioclastic, nodular, fossiliferous limestone.
D1 10
m
5 0
Gypsum, bedded carbonates rocks.
Upper Marrat
Fig. 2
B
D
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A A
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F Fig. 4
B
D
E
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F Fig. 5
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Highlights 1-
Mudstones, wackstones, packstones, grainstones and bountones are the main
microfacies investigated in Dhruma carbonates. Cementation,
dissolution,
compaction,
bioerosion are the diagenetic alterations. 3-
silicification,
dolomitization
and
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Dhruma carbonates are deposited in deeper shelf margin to organic buildup on
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platform margins.