Toarcian and Bajocian Ammonites from the Haushi
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GeoArabia, 2011, v. 16, no. 4, p. 87-122 Gulf PetroLink, Bahrain Toarcian and Bajocian ammonites from the Haushi-Huqf Massif of southwestern Oman and the Hawasina Nappes of the Oman Mountains: Implications for paleoecology and paleobiogeography Raymond Énay ABSTRACT New and rare Jurassic ammonites have been found in Oman. A latest Bajocian Arabian Platform-type species was discovered in the Haushi-Huqf Massif autochthon of southwestern Oman, and Bajocian species typical of the Mediterranean Tethys and northwestern Europe were found in the Kawr-Misfah exotic unit of the Hawasina Nappes in the Oman Mountains. The dates provided by the new fauna have resulted in a reinterpretation of the geologic history of the containing rocks, and of their paleoecology and paleobiogeography. It is significant that ammonites from shallow-marine environments of the Arabian Platform are in close proximity to species from open-sea environments of the Mediterranean Tethys and northwestern Europe. This shows that endemism of the Arabian Province resulted from ecological isolation, whereas open-marine environments on the Oman margin, especially the pelagic seamounts off the margin, form part of a migration route between western and eastern Tethys (or Indo-Southwest Pacific), and perhaps far beyond. The occurrences among the Tethyan and pandemic components of ammonite faunas in the Canadian Pacific Cordillera of most of the taxa of the open-marine environments on the Oman margin reopens the question of Pacific biogeography during the Early Jurassic before the Hispanic oceanic corridor was completely open. Among the proposed models, the Pantropic Distribution Model of Newton is examined in the light of the Cretaceous paleobiogeography, with particular reference to rudists. INTRODUCTION During the last twenty years, important progress has been made in the understanding of the geology of Oman. Previously, work was directly connected to petroleum exploration and had, as its major interest, the study of lithostratigraphic facies and structures, whereas chronostratigraphy was poorly constrained. Paleontologic and biostratigraphic data were scarce, especially in relation to the macrofaunas. This situation changed as a result of regional geologic mapping by the French Bureau de Recherches Géologiques et Minières (BRGM) for the Ministry of Petroleum and Minerals of the Sultanate of Oman. From 1982 to 1984, BRGM geologists mapped the mountains of northern Oman at 1:100,000-scale and, from 1990 onwards, they mapped the Haushi-Huqf region, east of Dhofar (Figure 1). Concurrently, data from the field work were used in doctoral theses and papers, of which those by Béchennec (1988), Béchennec et al. (1988, 1989, 1990) and Roger et al. (1992) relate to the areas that are the subject of this paper. Also taken into account are the works of geologists from the universities of Paris 6 and Lausanne concerning the exotic blocks of the Hawasina Nappes (Baud et al., 1990a, b; Pillevuit, 1993; Pillevuit et al., 1997). During the field work, rare Jurassic ammonites were discovered that have allowed the dating of units with few other biostratigraphic markers. The new dates were used in the 1:250,000-scale geologic maps compiled by BRGM. The maps covered the Haushi-Huqf autochthon of the Mafraq quadrangle (Roger et al., 1992) and the Khaluf quadrangle (Dubreuilh et al., 1992) and the nappes and the autochthons of the mountains of northern Oman that crop out in a tectonic window in the Seeb quadrangle (Béchennec, Roger, Le Métour, and Wyns, 1992) and the Ibri quadrangle (Béchennec, Roger, Le Métour, Wyns and Chevrel, 1992) (Figure 1). 87 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/16/4/87/4568765/enay.pdf by guest on 30 September 2021 Énay 54°E 55° 56° 57° 58° TURKEY59° Caspian60° Sea 27°N N27° SYRIA 0 300 Med Sea km IRAN IRAQ IRAN JORDAN KUWAIT Figure 1 Musandam BAHRAIN Peninsula QATAR 26° EGYPT Arabian 26° Shield UAE OMAN SAUDI ARABIA Red SUDAN Sea YEMEN Arabian Sea ERITREA 25° 25° Gulf of Oman N 0 200 km 24° Jabal Hamrat al Hasan 24° Muscat Ibri Seeb UNITED ARAB Jabal Saih EMIRATES Akhdar Hatat Jabal Misfah 23° 23° Jabal Kawr Hamrat Ba'id Duru Wadi Alwa SAUDI ARABIA Jabal Safra 22° 22° OMAN Maastrichtian and Tertiary Mafraq Semail Nappes 21° Hawasina Nappes, Allochthonous units 21° Para-Autochthonous; Sumeini Group Arabian Sea Khaluf Autochthonous B, Permian – Cretaceous Autochthonous A, Pre-Late Permian Haushi 54° 55° 56° 57° Huqf 58° 59° 60° 20° Figure 1: Simplified geologic map of part of Oman (from Glennie et al., 1974; modified in Pillevuit, 1993) showing the locations of the Haushi-Huqf Massif and the Hawasina Nappes. Also shown are the outlines of the four quadrangles mapped at 1:250,000 scale by BRGM and mentioned in the text. The Saiwan and Haushi-Huqf regions are located within the Khaluf and Mafraq quadrangles. The new dates have contributed to an improved knowledge of the Jurassic evolution of the exotic units of the Hawasina Nappes. However, the ammonite finds have an importance that goes beyond simple improved dating. Although they are not numerous, the Oman ammonite fauna shows adaptations to the conjunction of two quite different environmental conditions. On the one hand, shallow- marine environments on the Arabian Platform were inhabited by ammonite species that were often endemic, whereas the more open-water environment in which the rocks of the Hawasina Nappes were deposited favored ammonites that are identical to those known from the same environments in the Submediterranean Tethyan margins and northwestern Europe. Recently discovered Jurassic ammonites of Oman and their paleoecology and paleobiogeography are the subject of this paper. These ammonites, discovered by geologists from BRGM and Lausanne University, Switzerland, are used in the development of a new paleobiogeography that is based on paleogeographic and evolutionary frameworks established by the authors listed above. 88 88 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/16/4/87/4568765/enay.pdf by guest on 30 September 2021 Toarcian and Bajocian ammonites of Oman GENERAL STRUCTURAL AND PALEOGEOGRAPHIC FRAMEWORK For an historical account of the geology of the area under consideration, reference should be made to the doctoral theses of Béchennec (1988) and Pillevuit (1993) and to the relevant geologic maps of Oman at 1:100,000 and 1:250,000 scales and their explanatory notes. The mountains of Oman are composed of nappes overlying relative autochthonous rocks and parautochthonous units (the Sumeini Group). According to Pillevuit (1993), thrusting of between 5 and 40 km has taken place. The whole area is overlain discordantly by neoautochthonous units of Middle-Late Maastrichtian to Early Miocene age. Autochthonous Rocks. The autochthonous rocks crop out within two tectonic windows in the core of the Oman Mountains at Saih Hatat and Jabal Akhdar, and in the Musandam Peninsula to the north (Figure 1). Autochthonous rocks also form small isolated outcrops in the foreland of the nappe structures, and more widely in the Haushi-Huqf Massif of southwestern Oman (Figure 1). Nappes. The two nappes are the Semail ophiolitic nappe (not dealt with in this paper) and the Hawasina Nappes formed of rock units defined on lithologic, sedimentologic and biostratigraphic criteria by various authors; for example, Béchennec, 1988; Béchennec, Roger et al., 1992; Béchennec, Roger, Le Metour and Wyns, 1992; Pillevuit, 1993; and Pillevuit et al., 1997. The Hawasina Nappes originated in the Hawasina Basin (Béchennec, 1988) and were thrust onto the margin of the Arabian Platform at the time of the general ophiolite obduction in the Campanian – Maastrichtian. Figure 2 is taken from Pillevuit et al. (1997) and shows diagrammatically the nappes structure and their palinspastic reconstruction, as modified from the work of BRGM. Paleogeographic interpretation. Glennie et al. (1974) proposed two models for the evolution of the Oman Arabian margin: the Hawasina Nappes would be a structural pile of sedimentary units that accumulated either on oceanic crust (model 1) or on continental crust (model 2). • Model 1 was adopted by Pillevuit (1993) and Pillevuit et al. (1997), with slight changes. In this interpretation, the exotic blocks (or units) were islands built on oceanic crust. Differences between the sedimentary successions in the various units were assumed to be facies changes within a single basin and the proponents did not accept that the so-called Hamrat Duru, Al Aridh and Umar basins were true “basins bounded by topographic swells” (Figure 3a). • BRGM favored model 2. The sedimentary units of the exotic units were interpreted as horsts (Ba’id, Misfah-Kawr) and basins (Hamrat Duru, Al Aridh and Umar) on continental crust, resulting from Triassic extension (Figure 3b). (a) Figure 2: (a) Schematic representation of 4 Semail Nappe Mesoautochthonous the units of the Oman Mountains; and 5 (b) their palinspastic reconfiguration by Umar Group Kawr Group Pillevuit et al. (1997). Key: AA, Al Aridh Al Aridh Group Group; AB, Al Buda’ah Group; HD, 3 Hamrat Ramaq Group Al Buda’ah Group Hamrat Duru Group; Kw, Kawr Group; Nappes Duru Hawasina Group 2 Sumeini Group Ra, Ramaq Group; Um, Umar Group. Autochthonous B 1 Autochthonous A (b) Hawasina Nappes Autochthonous B Ra Sumeini Group AB Kw Autochthonous A HD AA Um Semail Nappe 89 89 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/16/4/87/4568765/enay.pdf by guest on 30 September 2021 Énay South North a BA’ID SEAMOUNT MISFAH SEAMOUNT Al Buda’ah Group Kwar Group Limestone T1-T3 Oceanic sediments P2 T2-T3 Oceanic lithosphere TETHYS Basement Flexural Margin 1 km 100 Southwest Northeast b ARABIAN HAWASINA TETHYS PLATFORM BASIN Hamrat Duru Basin Ba’id Al Aridh Misfah Umar Basin Horst Trough Horst 450 km Basement Shallow Carbonates Bathyal Deposits Pre-Permian Triassic Radiolarites Volcanics Late Permian Breccias Oceanic lithosphere Turbidites Figure 3: Palinspastic reconstructions of the Oman margin during the Late Triassic including synthetic stratigraphic sections of the Ba’id and Misfah horsts / seamounts.