Marine Flooding Events of the Early and Middle Ordovician of Oman and the United Arab Emirates and Their Graptolite, Acritarch and Chitinozoan Associations
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GeoArabia, v. 15, no. 4, 2010, p. 81-120 Gulf PetroLink, Bahrain Marine flooding events of the Early and Middle Ordovician of Oman and the United Arab Emirates and their graptolite, acritarch and chitinozoan associations Barrie R. Rickards, Graham A. Booth, Florentin Paris and Alan P. Heward ABSTRACT Graptolites and their associated acritarch floras and chitinozoan faunas are described from three localities in Oman and the United Arab Emirates (UAE). They are characteristic of intervals of deeper water sediments in the Floian – Dapingian (Arenig, identified here as maximum flooding surface MFS O25) and mid Darriwilian (Llanvirn, MFS O30). These marine flooding events have distinctive and unique palynological signatures. Most of the recorded Darriwilian fauna and flora have northern Gondwana affinities. Oman and the UAE seem to have occupied relatively elevated positions during the Ordovician and only to have been inundated at times of highest sea level. Exotic rafts of Ordovician Rann and Ayim formations that crop out in the Kub Melange of the Dibba Zone range in age from Floian – Dapingian, through Darriwilian, to Katian (Arenig – Llanvirn- Caradoc) and may provide a glimpse of a relatively condensed Ordovician stratigraphy that occurs at depth beneath Musandam Peninsula and the Arabian Gulf. INTRODUCTION The origins of this paper lay in the chance discovery of a graptolite in the Kub Melange (Figure 1) of the United Arab Emirates (UAE) by Dan Schelling, working on behalf of Indago Petroleum Ltd. The green shale matrix of the melange was thought to be Late Triassic or Late Cretaceous, yet it contained a graptolite. Some of the shales clearly consist of Ordovician Rann Formation. R.B. Rickards identified the graptolite and thought it was the first graptolite found in the UAE or Oman. He was unaware of unpublished records from the Ghaba-1 (GB-1) and Baqlah-1 (BQ-1) boreholes in Oman (Figure 1) and the substantial work on the associated acritarch floras and chitinozoan faunas in relation to oil and gas exploration (e.g. Molyneux et al., 2006). This paper integrates graptolite, acritarch and chitinozoan interpretations to provide biostratigraphic ties between their respective zonations. It presents, as far as possible, data for the three fossil groups over the same stratigraphic intervals. It also presents new fossil evidence for the range of ages represented by the Rann Formation of the UAE. Graptolites have not been described from outcropping Ordovician rocks in Oman, possibly because the outcrops are of shallow-water facies or due to unfavourable deformation and cleavage development in Saih Hatat. They have only occasionally been encountered in subsurface cores as these are not normally cut in shale intervals. The Ghaba-1 graptolites were originally identified by Sudbury in the late 1950s, for the Iraq Petroleum Company. Sudbury has kindly provided reports (IR/RGSH/364/365), research notes and drawings and made helpful comments on the present interpretations. Acritarchs have proved important in furthering understanding of the Lower Palaeozoic stratigraphy of Oman. Lovelock et al. (1981) described limited acritarch assemblages from the Amdeh Formation outcrops in Saih Hatat, but much of the Lower Palaeozoic section in Oman is not exposed at the surface. The data that has permitted biozonation of the interval has all been derived from boreholes. Droste (1997) first reported a biozonation employed in Petroleum Development Oman (PDO) founded on studies of well material using acritarchs, chitinozoans and cryptospores. An update of the zonation was discussed in detail by Molyneux et al. (2006), where the sequence stratigraphic implications and the usefulness of the scheme in resolving stratigraphic issues were clearly demonstrated. Chitinozoans are well-represented in the Ordovician and Lower Silurian subsurface strata of Oman. However, with the exception of a few references to the group (e.g. Molyneux et al., 2004, 2006), a brief report of Lower – Middle Ordovician chitinozoans from outcrops in Saih Hatat (Sansom et al., 2009), 81 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/4/81/4565801/rickards.pdf by guest on 25 September 2021 Rickards et al. 54°E 55° 56° 57° 58° 59° IRAN 26°N Musandam 26° Hormuz Salt Basin KM Dubai Dibba Zone 25° 25° N 0 100 Gulf of Oman Abu Dhabi km F r o n t Oman Mountains o f 24° H 24° a w a s in Muscat a T UNITED ARAB h r u Outcropping EMIRATES s ts Ordovician S h e Saih Hatat e 23° ts 23° Fahud Salt Basin BQ-1 22° 22° Ghaba Salt Basin SAUDI ARABIA GB-1 21° 21° Huqf Area OMAN 20° 20° Arabian Sea 34°E 38° 42° 46° 50° 54° 60° 38°N TURKEY Caspian 38° 19° South Oman Sea 19° Salt Basin CYPRUS SYRIA N 34° LEBANON 0 300 34° Med Sea IRAQ km JORDAN 30° 30° Gulf KUWAIT IRAN of Suez BAHRAIN 26° SAUDI ARABIA QATAR EGYPT Gulf of Oman Arabian UAE 18° 22° Shield 18° OMAN SUDAN Red Sea 18° ERITREA YEMEN Arabian Sea 14° 14° SOCOTRA ETHIOPIA Gulf of Aden 54° 55° 56° 57° 58° 34° 38° 42° 46°59° 50° 54° 60° Figure 1: Location map Mirbatshowing fossiliferous Kub Melange (KM) outcrops in Ras Al Salalah Khaimah, UAE, and the Ghaba-1 (GB-1) and Baqlah-1 (BQ-1) oil exploration boreholes in Oman. Ordovician outcrops are limited in NE Arabia to the Amdeh Formation in Saih Hatat (Oman) and the Rann Formation of the UAE. Ordovician strata, however, occur extensively in the subsurface of Oman, in the Oman Salt Basins and to the west. 82 82 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/4/81/4565801/rickards.pdf by guest on 25 September 2021 Early and Middle Ordovician flooding events, Oman and UAE and the description of new species (Al-Ghammari et al., 2010), no detailed data have been published. Most of the information is contained in unpublished Petroleum Development Oman internal reports (authored by F. Paris). LOCATION OF SAMPLES AND BIOSTRATIGRAPHY Kub Melange, Southeast of Idhn Village, Ras Al Khaimah, UAE (25°25.555’ N, 56°03.920’ E; UTM WGS84 2,812,439 N, 406,008 E) The Kub Melange crops out in a few square kilometres of the Dibba Zone in the northern Oman Mountains (Figure 1). It has been interpreted as the sheared equivalent of the Haybi Complex, an assemblage of exotic rocks that lie between the deep-sea sediments of the Hawasina Nappes and the Semail Ophiolite. In contrast to the normal exotics of the Haybi Complex, the Kub contains a variety of rafts of Palaeozoic age in an organic-rich, shaly-sandstone matrix that has not yet been dated directly (Searle and Graham, 1982; Robertson et al., 1990). The mélange may have originated as a fault-scarp deposit associated with a transform margin of the Tethys Ocean. Subsequently it was thrust and deformed during the emplacement of the Semail Ophiolite. Rafts of Ordovician Rann Formation have been dated as Mid to Late Ordovician and considered correlative with the upper part of the Amdeh Formation of Oman (Am5; Figure 2; Hudson et al., 1954; Glennie et al., 1974; Omatsola et al., 1981; Robertson et al., 1990; Goodenough et al., 2006). Few details though have been published of the Rann trilobite faunas, trace fossils and facies. The rafts are a few hundred metres across and less than 100 m thick, or smaller. The shales and thinly bedded sandstones of the Rann are intensely folded and sheared, whereas the quartzite-rich intervals tend to be intact and form small hills. The relatively limited outcrops of the Kub Melange in the Jabal Qamar South – Jabal Ar Raan area are being extensively quarried. The graptolite sample was collected from from the corner of a quarry in shales, which was inactive (Figure 3), but in 2009 began being intensively worked again. Graptolite fragments are rare, but with searching more were found in June 2008. The green shale of the initial sample was sub-sampled for palynology. Despite its unpromising green colour, a rich assemblage of acritarchs was obtained on processing. In-situ samples were collected from the same locality in June 2008 and yielded similar assemblages (e.g. Rann 4), but proved less productive. Rann shales and sandstones, 250–400 m further to the east, are fossilifererous containing trilobites (Neseuretus and Taihungshania) and trilobite traces (Cruziana furcifera, C. goldfussi and C. rugosa), brachiopods and hyolithids. Another outcrop nearby has yielded different trilobites (Vietnamia, Deanaspis and Dreyfussina), brachiopods and orthoconic nautiloids. The trilobite faunas indicate sediments of Floian and Katian age in close juxtaposition. The details of sedimentary facies, trilobite and conodont faunas and trace fossils will be described in a subsequent paper (Fortey et al., in preparation). Goodenough et al. (2006) reported an Upper Ordovician age for orthoconic nautiloid-rich, nodular limestone rafts within the Kub Melange. These are probably attributable to the Ayim Formation of Robertson et al. (1990), rather than strictly to the Rann. Rafts of Ayim occur commonly in proximity to Rann quartzites. More recent work on conodont faunas from the Ayim indicates they are of latest Middle rather than Late Ordovician age (Fortey et al., in preparation). Graptolites The quarry southeast of Idhn village yielded a single well-preserved specimen of Baltograptus deflexus (Elles and Wood, 1901; Figure 4), which indicates the varicosus - victoriae Biozones, mid Floian – early Dapingian (Arenig), Early Ordovician. The specimen (Dan 303) is described in the Systematic Palaeontology section below. Given its occurrence in a thrust mélange it shows surprisingly little evidence of shearing or tectonic deformation. The further fragmentary material collected in 2008 are of indeterminate dichograptids. 83 83 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/4/81/4565801/rickards.pdf by guest on 25 September 2021 on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/15/4/81/4565801/rickards.pdf Rickards etal.