GeoArabia, Vol. 6, No. 1, 2001 Gulf PetroLink, Bahrain Jurassic Gastropod Faunas, Central Saudi Arabia Jurassic Gastropod Faunas of Central Saudi Arabia Jean-Claude Fischer Laboratoire de Paléontologie du Muséum national d’Histoire naturelle, Paris, France Yves-Michel Le Nindre, Jacques Manivit and Denis Vaslet Bureau de Recherches Géologiques et Minières, Orléans, France ABSTRACT Mapping of Phanerozoic rocks at 1:250,000 scale by joint teams from the Saudi Arabian Deputy Ministry for Mineral Resources and the Bureau de Recherches Géologiques et Minières since 1980 has covered most of the Jurassic outcrops in central Saudi Arabia. Stratigraphic, sedimentologic and paleogeographic studies provided a precise framework for collected gastropod faunas that could be calibrated against ammonite zones and sequence-stratigraphic zones. Of more than 600 samples collected, about 440 gastropod specimens could be determined on at least a generic level. Their age range is from Bajocian to Oxfordian–Kimmeridgian. They correspond to 26 genera and 35 species from the Euomphalidae, Ataphridae, Pseudomelaniidae, Coelostylinidae, Procerithiidae, Nerineidae, Purpurinidae, Aporrhaidae, Naticidae, Acteonidae, Retusidae, and Akeridae families. Twelve species are new, and three (Kosmomphalus and Bifidobasis in the Euomphalidae, and Striatoonia in the Pseudomelaniidae) were proposed for new taxa of generic or subgeneric rank. Most of the identified species are of Middle Jurassic age, mainly Bathonian and Callovian and only six are Late Jurassic. All species are typical of an internal platform environment (upper infralittoral), in a lagoonal to back-reef setting, but some also colonized the external platform in the lower infralittoral fore-reef zone. Paleogeographically, most of the species are related to European and Sinai faunas; only seven are equivalent to North African or East African faunas, and one only was reported from Madagascar. INTRODUCTION The gastropod faunas studied were collected by joint field teams from the Saudi Arabian Deputy Ministry for Mineral Resources (DMMR) and the Bureau de Recherches Géologiques et Minières (BRGM). The collections were made during 1:250,000-scale field mapping of the Phanerozoic Cover Rock Program from 1980 to 1987. These are the first gastropod faunas from the Jurassic of central Saudi Arabia to be described and analyzed in detail, since the very limited data published by Newton and Crick (1908) and Newton (1921). The fossils are from the Shaqra Group of Toarcian to Kimmeridgian age. They were collected during mapping of the 1:250,00-scale quadrangles shown on Figure 1. From north to south they are: Qibah (Robelin et al., 1994), Buraydah (Manivit et al., 1986), Al Faydah (Vaslet et al., 1985a), Shaqra (Vaslet et al., 1988), Ar Riyad (Vaslet et al., 1991), Darma (Manivit et al., 1985b), Wadi ar Rayn (Vaslet et al., 1983), Wadi al Mulayh (Manivit et al., 1985a), Sulayyimah (Vaslet et al., 1985b). In addition, reconnaissance field trips were made in the As Sulayyil, Wadi al Faw and Al Mudafin quadrangles. GEOGRAPHIC AND GEOLOGIC DATA (Y.-M. Le Nindre, J. Manivit and D. Vaslet) The stratigraphic framework used for this study comes from papers by Le Nindre et al. (1990a, 1990b) and Manivit et al. (1990), supplemented with global bathymetric indicators (Le Nindre et al., 1988), and for eustasy (Le Nindre et al., 1990c). Many specialized papers have been published on the paleontology and biostratigraphy of these deposits: ammonites (Enay et al., 1986, 1987; Enay and Mangold, 1994); nautiloids (Tintant, 1987); brachiopods of Dogger age (Alméras, 1987); ostracods (Dépêche et al., 1987); calcareous nannofossils (Manivit, 1987); and dasycladacean algae (Bernier, 1987). 63 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/63/4559680/fischer.pdf by guest on 26 September 2021 Fischer et al. The sedimentology of the outcrops in the north of the area was studied by Le Strat et al. (1985), and later, for all of central Saudi Arabia, by Le Nindre et al. (1988, 1990b). Geographic Location The gastropod faunas were collected from the Jurassic deposits that crop out along 1,000 km of the Tuwayq escarpment, between latitudes 28°N. and 18°N. It is around Riyadh (lat. 24°00’N) that the outcrops of Jurassic deposits are the widest (85 km) and thickest (over 1,000 m). The Jurassic rocks are deposits of the Arabian platform on the southern margin of the Tethys Ocean bordering Gondwana. The open-sea domain during Jurassic times was over the area of the present Arabian Gulf (Dercourt et al., 1993; Al-Husseini, 1997). Lithostratigraphic and Biostratigraphic Setting The Shaqra Group is named after the town of Shaqra (Manivit et al., 1990), and comprises all Late Jurassic deposits in central Saudi Arabia. The group overlies the Late Permian to Late Triassic Buraydah Group (Le Nindre et al., 1990a) with a disconformable contact (erosion surface). The upper boundary of the Shaqra Group, corresponding to a pre-Early Cretaceous erosion surface, is overlain by the Early Cretaceous Thamama Group (Powers et al., 1966). These two erosion surfaces correspond to regional disconformities that are locally conformable at outcrop scale. The Shaqra Group mainly consists of carbonates and claystone, with locally terrigenous deposits in the south and the northwest that correspond to margins of the platform. The Shaqra Group is divided into seven lithostratigraphic units mappable at 1:250,000 scale as defined by Powers et al. (1966) and Powers (1968), and revised by Manivit et al. (1990) (Figure2). The abundant fossils sampled in the Shaqra Group permit a precise definition of the stratigraphy of Jurassic deposits in central Arabia. The biostratigraphic framework was based on ammonites (Enay et al., 1987). From the late Toarcian to the early Kimmeridgian, this Jurassic succession is divided into 16 ammonite biozones and 2 subzones. This biozonation is complemented by zonation based on other fauna (nautiloids, echinoderms, brachiopods, bivalves, gastropods, and hexacorals) and on microfauna (foraminifers, ostracods) and nannoflora. An Arabian ammonite zonation was later proposed by Enay and Mangold (1994). Reference is made to the West European and Mediterranean ammonite zonation (Groupe Français d’Etude du Jurassique, 1997) for the correlation of Arabian and European gastropod faunas, and a correlation between European and Arabian ammonite zonations is proposed in Figure 2. From bottom to top, the Shaqra Group is subdivided into the following formations: 1. The Marrat Formation (126 m thick in the type section at Khashm adh Dhibi) is informally subdivided into lower (47 m), middle (40 m) and upper Marrat (39 m). It consists of interbedded marine sandstone, carbonate and claystone deposits that are Toarcian to older in age. According to the faunal contents, the upper part of the lower Marrat and the middle Marrat belong to the early Toarcian (Serpentinum zone). The basal part of the upper Marrat yielded a Nejdia fauna assigned to the middle Toarcian (Bifrons zone). The middle and upper parts of the upper Marrat deposits did not provide any paleontologic markers. A possible Aalenian hiatus exists between the Marrat and Dhruma formations. 2. The Dhruma Formation (450 m thick in the type section at Khashm adh Dhibi) comprises units D1 to D7 (Manivit et al., 1990). These are lower Dhruma (D1–D2: 143 m), middle Dhruma (D3–D6: 193 m) and upper Dhruma (D7: 111 m, type section at Khashm al Mashriq). The Dhruma Formation is mainly composed of limestone and claystone in the central part of the outcrop area. In the north and the south of this area, the formation is composed of detrital rocks. The age of the Dhruma Formation is early Bajocian to middle Callovian. 64 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/63/4559680/fischer.pdf by guest on 26 September 2021 Jurassic Gastropod Faunas, Central Saudi Arabia Tertiary—Quaternary QIBAH Cretaceous N Jurassic Buraydah Permian—Triassic BURAYDAH Cambrian— Carboniferous Precambrian Shield Shaqra 1:250,000 scale DARMA quadrangle AL FAYDAH SHAQRA J City A B Riyadh Town Darma A L DARMA T U W A Y Q Al Hawtah WADI AR RAYN WADI AL MULAYH SULAYYIMAH As Sulayyil AS SULAYYIL 0 100 km WADI AL FAW Figure 1: Jurassic outcrops in central Saudi Arabia and location of 1:250,000-scale AL MUDAFIN quadrangles in which sampling was undertaken. 65 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/1/63/4559680/fischer.pdf by guest on 26 September 2021 Fischer et al. D1 The basal part of the lower Dhruma yielded an Euhoploceras and Hyperlioceras fauna of early Bajocian age (upper part of the Discites zone). The Shirbuirnia fauna found in the upper part of D1 is also dated as early Bajocian (lower part of the Laeviuscula zone). D2 The basal part of D2 yielded a Dorsetensia, Normannites and Stephanoceras fauna together with the first primitive forms of Ermoceras dated as late early Bajocian (lower part of the Humphriesianum zone). The upper part of D2, the Dhibi Limestone Member, contains the main Ermoceras fauna of the late Bajocian Niortense zone. D3 The basal and middle parts of D3 yielded late Bajocian ammonite faunas of the Garantiana zone (Ermoceras mogharense fauna) and the Parkinsoni zone (Thambites fauna). The Zigzagiceras fauna discovered at the top of D3 is early Bathonian in age (Zigzag zone). D4 Yielded a Tulites fauna assigned to the early Bathonian Zigzag zone. D5 Characterized by a Micromphalites fauna of early Bathonian age (Aurigerus zone). D6 Yielded an endemic fauna with Dhrumaites and two species of nautiloids (Tinant, 1987) that suggest an early Bathonian age. The early Bathonian age is retained here and accords with a major break in sedimentation observed at the top of the middle Dhruma. However, this age is brought into question by the brachiopod faunas that could suggest a late Bathonian age for D6 (Almeras, 1987). D7 The upper Dhruma is divided into two members, from bottom to top, the Atash Member and the Hisyan Member.