GeoArabia, Vol. 9, No. 2, 2004 Gulf Petrolink, Bahrain Stratigraphic architecture of the northern Oman continental margin - Mesozoic Hamrat Duru Group, Hawasina complex, Oman Ingo Blechschmidt, Paulian Dumitrica, Albert Ma�er, Leopold Krystyn and Tjerk Peters ABSTRACT The Triassic to Late Cretaceous deep-marine sediments of the Hamrat Duru Group, Oman Mountains, represent a subunit of the Hawasina nappe-complex which was deposited in a deep marine basin. During the Late Cretaceous SSW-directed obduction of the Semail Ophiolite, the Hawasina complex was emplaced onto the autochthonous cover of the Arabian basement, while the original configuration of the basin was destroyed. New lithostratigraphic results and high-resolution radiolarian and conodont biostratigraphy lead to a revised stratigraphic scheme of the Hamrat Duru Group which conforms with the standard stratigraphical nomenclature. The Hamrat Duru Group is divided into six formations: (1) The Early Triassic (Olenekian) to Late Triassic (Upper Norian) Zulla Formation (Limestone and Shale Member, Sandstone and Shale Member, Radiolarian Chert Member and Halobia Limestone Member); (2) The Late Triassic (late Norian to Rhaetian) Al Ayn Formation; (3) The Early Jurassic (late Pliensbachian) to Middle Jurassic (early Callovian) Guwayza Formation (Tawi Sadh Member and Oolitic Limestone Member); (4) Middle Jurassic (Callovian) to Late Cretaceous (Cenomanian?) Sid’r Formation (Lower Member, Upper Member); (5) Late Cretaceous (Cenomanian? to Santonian?) Nayid Formation; and (6) Late Jurassic (early Callovian) to Early (Late?) Cretaceous Wahrah Formation. Most of the lithostratigraphic units (formations and members) show isochronous boundaries between the different outcrop areas. The stratigraphic architecture of the Hamrat Duru Group megasequence is controlled by alternating siliciclastic and carbonate sedimentation possibly related to the second-order sea-level variations. The sediments accumulated on the continental rise of the Arabian margin mostly by submarine sediment-gravity flows and hemipelagic to pelagic rainout. A close relationship of the evolution of the Arabian Platform and the adjoining slope and basinal environments is evident. Changes in carbonate supply, oceanographic circulation and/or variations in silica productivity resulted in two distinct phases of radiolarian sedimentation. The first phase corresponds to the Triassic late Anisian-early Norian time interval; the second started in the Early Jurassic late Pliensbachian and lasted, with some interruptions, up to the Late Cretaceous Coniacian. The litho- and biostratigraphic similarities between the Mesozoic Hamrat Duru Basin of the northern/ central Oman Mountains and the Mesozoic Batain Basin of northeastern Oman are seen as related to Neo-Tethys-wide palaeoceanographic changes and suggest a strong interdependence of the two basins with the evolution of the Arabian Platform. INTRODUCTION AND PREVIOUS WORK Regional Geotectonic Framework During the last decades of geological research it has been generally accepted that the allochthonous cover of the northeastern Arabian Plate originated from the southern margin of the Neo-Tethys Ocean (Figure 1a). The Late Permian and Mesozoic Hawasina sediments were deposited in the Hawasina Basin which consisted of at least two sub-basins (Figure 1b), before being emplaced in a SSW direction onto the autochthonous cover of the Arabian Platform, coeval with the late Coniacian to Campanian (84-80 Ma) obduction of the Semail Ophiolite (Allemann and Peters, 1972; Glennie 81 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/81/4564638/blechschmidt.pdf by guest on 30 September 2021 Blechschmidt et al. Stratigraphic architecture of Hamrat Duru Group, Oman 20 IRANIAN TERRANES Palaeo-Tethys Ocean Neo-Tethys Ocean 20 New Passive Margin AFGHAN TURKEY B TERRANES Misfar Platform TIBET 0 Broad carbonate Early Jurassic evaporite shelf rifting in eastern Hamrat Duru Basin Mediterranean A Batain Basin ARABIAN PLATE 0 AFRICAN INDIA-PAKISTAN PLATE PLATE A Arabian Platform Continental Slope Hawasina Basin Hamrat Duru Basin Al Aridh Misfah Trough? Platform Umar Basin Spreading Ridge B Figure 1: (a) Opening of the Neo-Tethys, northern extension and passive margin post-ri� thermal subsidence (Middle Permian to Early Jurassic, 255-182 Ma; modified a�er Sharland et al., 2001). Note the positions of the Hamrat Duru and the Batain Basin. (b) Mesozoic pre-emplacement Hawasina Basin configuration (modified a�er Cooper, 1990; Le Métour et al., 1995; and Pillevuit et al., 1997). Figure 2 (facing page): Geological map of the central and eastern Oman Mountains (simplified a�er Glennie et al., 1974; Pillevuit et al., 1997) showing the main outcrop areas and studied sections. Key to sections: 1 - Wadi Zulla (=Wadi Dil), 2 - Qabil, 3 - Qusayd-Shulayshil, 4 - Al Jil, 5 - Al Ayn, 6 - Tawi Shu’ah, 7 - Tawi Shannah, 8 - Wadi Musallah, 9 - Wadi Yail, 10 - Wadi Saal, 11 - Wadi Sid’r, 12 - Wadi Sid’r North, 13 - Wadi Nayid, 14 - Al Dhaby, 15 - Mudaybi, 16 - Kathmah, 17 - Firq, 18 - Wadi Mu’aydin, 19 - Zukayt, 20 - Al Sawad, 21 - Jabal Safra, 22 - Kadrah Bani Dafa’a, 23 - Al Khashbah, 24 - Al Hammah Range, 25 - Dawwah, 26 - Wadi Bani Khalid, and 27 - Khabbah. Structural section across the central Oman Mountains and tectono-stratigraphic organisation of the present-day Oman Mountains are also shown (modified a�er Glennie et al., 1973, 1974). 82 83 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/81/4564638/blechschmidt.pdf by guest on 30 September 2021 Blechschmidt et al. Stratigraphic architecture of Hamrat Duru Group, Oman et al., 1973; Béchennec, 1987; Bernoulli et al., 1990; Cooper, 1988). The Hamrat Duru sedimentary rocks represent a base of slope and abyssal plain sedimentary facies of a distinct sub-basin of the Hawasina Basin along the Arabian Platform. The sedimentation in the Hamrat Duru Basin was mostly characterised by submarine sediment-gravity flows such as turbidity currents and debris flows, as well as hemipelagic rain out and radiolarian productivity (Glennie et al., 1974; Murris, 1981; Cooper, 1986). The first comprehensive study of the Oman Mountains was presented by Glennie et al. (1973, 1974) who recognised five main structural units (Figure 2). From bo�om to top, these are: 1. Autochthonous A (folded Proterozoic to Palaeozoic rocks) and B (Permian to Cretaceous carbonates and deeper marine sediments), representing the lowermost outcropping tectonic units, 2. Sumeini Nappes, characterized by Neo-Tethyan slope deposits of Permian to Cretaceous age, 3. Hawasina Nappes, subdivided into several groups comprising Permian to Cretaceous basinal sediments, 4. Semail Ophiolite, sequence of Cretaceous oceanic crust, and 5. Neoautochthonous, sedimentary platform sequence ranging in age from the Late Cretaceous to Miocene, which unconformably overlies all older units. Gulf of Oman 0 N 25 Hawasina Window 1 Muscat km B Jabal Wahrah Saih Hatat 4 Wadi Al Ayn 2 5 6 7 Ibri 3 Jabal Akhdar 17 20 8 18 Nizwa 27 19 Hamrat Ad Duru R. 22 9 A 13 14 21 15 10 Ibra 12 Jabal 23 11 Hammah Jabal 25 16 Safra 24 26 Al Hammah R. 1 - 27 Studied sections Gulf of Oman Southwest Northeast Muscat UAE J. AKHDAR A HAMRAT AD DURU B 0 25 SAUDI ARABIA km Neoautochthonous OMAN Semail Ophiolite Arabian Sea Hawasina Nappes Sumeini Nappes N 0 150 B km Autoch- thonous A 82 83 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/81/4564638/blechschmidt.pdf by guest on 30 September 2021 Blechschmidt et al. Stratigraphic architecture of Hamrat Duru Group, Oman Previous Stratigraphic Work and Palinspastic Reconstructions Glennie et al. (1973, 1974) interpreted the Hawasina Complex as a stack of nappes with sediments deposited in an oceanic basin along the northern Arabian passive continental margin. Mainly based on lithostratigraphic criteria, they subdivided the Hawasina sediments into the Hamrat Duru Group and into several formations such as Halfa Formation, Haliw Formation, Wahrah Formation, Al Ayn Formation, Ibra Formation, Al Aridh Formation, and the Oman Exotics. Glennie’s et al. (1974) first palinspastic reconstruction of the Hawasina Basin assumed Permian to Late Triassic-Early Jurassic ri�ing and simple in-sequence thrusting during emplacement. Later workers (see below) correlated the facies between the tectonic units to set up regional stratigraphic schemes, grouping similar sediment types of similar age throughout the area as a whole. Béchennec (1987) synthesised the stratigraphic work of the BRGM (Bureau de Recherches Géologiques et Minières) which was carried out during regional mapping of the Central Oman Mountains. The geological studies carried out between 1984 and 1993 by the BRGM resulted in the publication of geological maps at different scales of the entire Oman territory (e.g. Béchennec et al., 1993; Le Métour et al., 1995; Michel, 1993). New palaeontological data allowed more accurate dating of many stratigraphic sequences (Béchennec et al., 1993; Béchennec et al., 1990; De Wever et al., 1988). Most of the formations established by Glennie et al. (1974) were abandoned or were redefined by the BRGM geologists (Figure 3). This has led to considerable confusion with respect to the Hamrat Duru lithostratigraphy, especially as Béchennec and co-workers modified their stratigraphic scheme repeatedly as their mapping project progressed (Figure 3). The problems related to this approach are discussed in Robertson et al. (1990). According to the BRGM, the evolution