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Knox.Pdf by Guest on 25 September 2021 Knox Et Al GeoArabia, Vol. 12, No. 4, 2007 Wajid Group (Cambrian to Permian), southwestern Saudi Arabia Gulf PetroLink, Bahrain Stratigraphic evolution of heavy-mineral provenance signatures in the sandstones of the Wajid Group (Cambrian to Permian), southwestern Saudi Arabia Robert W. O’B. Knox, Stephen G. Franks and Joshua D. Cocker ABSTRACT The Wajid Group of southwestern Saudi Arabia consists of a dominantly sandy succession of Cambrian to Permian age that spans several discrete phases in the tectonic evolution of the Arabian Peninsula. The principal aim of this study was to determine whether successive changes in the tectonic setting are reflected in changes in provenance- related mineralogy. Because of the relatively limited compositional range of the Wajid sandstones, heavy-mineral assemblages have been used as the primary tool for assessing changes in provenance signature. A comparison of heavy-mineral and petrological data has, however, also been carried out. Variation in the relative abundances of zircon, rutile, monazite, tourmaline and apatite has revealed significant changes in provenance signature between the Dibsiyah (Cambrian– Ordovician), Sanamah (Ordovician–Silurian), Khusayyayn (Devonian–Carboniferous) and Juwayl (Carboniferous–Permian) sandstones. Since previous studies have established that northward-flowing rivers deposited the fluvial sandstones of the Wajid Group, it appears that the source area lay to the south. In the absence of data from the region to the south, it is not possible to identify specific source areas. It is clear, however, that the successive changes in provenance signature must reflect exposure of new source rocks through progressive denudation, changes in the pattern of tectonic uplift or changes in the drainage system. It is also possible that some of the observed mineral variation is related to lateral influx of sands through long-shore drift during times of high sea level. Two distinct mineral compositions occur within the Dibsiyah sandstones, indicating that a major change in provenance took place during deposition of the Upper Dibsiyah sands. The boundary between the Dibsiyah and Sanamah formations is sharply defined, although the overall composition of the Sanamah sandstones is in many respects similar to that of the Dibsiyah sandstones. There is a relatively small difference in composition between the Sanamah sandstones and the associated diamictites. A major change in provenance is indicated at the base of the Khusayyayn Formation, with an increase in the proportion of monazite and staurolite. This change in composition persists into the Juwayl Formation although the greater variability displayed by the Juwayl heavy-mineral assemblages indicates contribution from several sources. Heavy-mineral assemblages in the Juwayl sandstones are comparable to those of the Unayzah C and B sandstones of central Saudi Arabia, but differences suggest mixing between a southern (Juwayl) and western (Shield) source for the Unayzah sandstones. Compositionally, Wajid sandstones range from quartz arenite to arkose. Comparison of the petrographic and heavy-mineral data is hampered by the different grain-size ranges studied. However, it would appear that samples with similar heavy-mineral provenance character do not necessarily possess similar feldspar percentages, even when the latter are corrected for in-situ kaolinization. The data set is too small to establish an explanation for this apparent discrepancy. INTRODUCTION The Wajid Group comprises a succession of Palaeozoic sandstones that crop out in southwestern Saudi Arabia, along the southeastern flank of the Arabian Shield (Figure 1). Its northern limit is defined by the southern flank of the Central Arabian Arch. The Wajid strata extend eastwards into the subsurface and are believed to be in lateral continuity with the Palaeozoic succession of the Rub’ Al-Khali Basin. 65 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/4/65/4570200/knox.pdf by guest on 25 September 2021 Knox et al. In April 2004, Saudi Aramco sponsored a reconnaissance field trip to the Wajid area, led by Saudi Aramco geologists Mike Hulver, Kent Norton, and Abdelmuttaleb Al-Qahtani. The opportunity was taken during this field trip to conduct preliminary sampling for petrography and heavy-mineral analysis. The Wajid Group provides the principal surface exposure of Palaeozoic rocks in southwestern Arabia and is thus key to understanding the early Phanerozoic tectonic and palaeogeographic evolution of the region. Four of the tectonostratigraphic megasequences (AP2–AP5) of Sharland et al. (2001) are represented, as well as two periods of Gondwana glaciation (latest Ordovician and Permian– 44° 52° 60° 44° 44°30' 45° 45°30' IRAQ 31° 31° IRAN N 300 KUWAIT 0 km 20°30' BAHRAIN 20°30' QATAR Arabian Shield UAE 23° Riyadh 23° Jabal al W37–41 SAUDI ARABIA Juwayl OMAN R Location ed S ea Map YEMEN Arabian Sea 15° 15° Aden ETHIOPIA Gulf of 20° W48 44° 52° W15–16 46° thlith Ta W42,43 Juwayl Formation Khusayyayn Formation W08,19 W34–36 Qalibah Formation W10–14 W33 W02–03 W20–23 Sanamah Formation 19°30' Dibsiyah Formation 19°30' t n e Proterozoic basement m p r a c s y E a q w i h a g i w u H N T h 0 50 d 19° 19° a y i km R - n a j r a N 18°30' 18°30' W54A Riyadh-Shara 18° 18° wra Road W61 W68 Sharawra 17°30' 44° 44°30' 45° 45°30' 46° 17°30' Figure 1: Location map (modified from Al-Husseini, 2004, figure 22) showing the location of samples analysed for heavy minerals. 66 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/4/65/4570200/knox.pdf by guest on 25 September 2021 Wajid Group (Cambrian to Permian), southwestern Saudi Arabia Carboniferous). The aim of this paper is to document the evolution of the Wajid heavy-mineral assemblages with time, and in particular to assess the relative roles of tectonism, provenance and climate in determining the composition of the assemblages. STRATIGRAPHY The term Wajid Sandstone was introduced by Powers et al. (1966) to comprise the clastic succession that lies between the limestones of the Khuff Formation and the Proterozoic basement between latitudes 17°30'N and 20°30'N and longitudes 43°30'E and 46°30'E, in southwestern Saudi Arabia (Figure 1). Powers (1968) formally defined the Wajid Sandstone Formation and proposed a composite type section comprising outcrops between Jabal Wajid (19°25'N) and Bani Ruhaiya (19°50'N). Subsequently, Kellogg et al. (1986) defined four members within the Wajid Sandstone Formation of the Wadi Tathlith Quadrangle. In ascending sequence, these are the Dibsiyah Member, the Sanamah Member, the Khusayyayn Member and the Juwayl Member. Although Evans et al. (1991) recommended abandonment of these terms in favour of those established for central and northern Saudi Arabia, Stump and van der Eem (1995) retained them (in modified form) on the grounds that they better reflect the distinctive character of the Wajid succession. The revisions that Stump and van der Eem (1995) proposed were to elevate the Wajid Sandstone to group status and the component members to formation status. They also recognised the Qalibah Formation (Qusaiba Member) as an additional component of the Wajid Group. The scheme of Kellogg et al. (1986), as modified by Stump and van der Eem (1995), is followed in this paper. Scarcity of fossils has hampered precise dating of the Wajid Group succession, but the group is now considered to range from Mid-Cambrian to Permian in age (Stump and van der Eem, 1995). The stratigraphic framework is summarised in Figure 2. The present study is restricted to sandstones exposed in the central and northern part of the Wajid Group outcrop (Figure 1). A brief description of the four sandstone formations is given below, in ascending order. The Dibsiyah Formation (c. 170 m) rests unconformably on the Proterozoic basement. Stump and van der Eem (1995) recognised two informal divisions (‘lower’ and ‘upper’) within the Dibsiyah Formation. The basal part of the ‘lower’ Dibsiyah Formation consists of a palaeosol overlain by quartzite pebble- bearing litharenitic and sublitharentic sandstones of highly variable thickness. The remainder of the ‘lower’ Dibsiyah Formation consists of mature, cross-bedded quartz arenitic sandstones. The ‘lower’ Dibsiyah sandstones were interpreted as having been deposited by a northward-prograding braided stream system. The ‘upper’ Dibsiyah consists of cross-bedded quartz arenites. The trace- fossil Tigillites (= Skolithos) is locally abundant, indicating deposition in intertidal environments. The Dibsiyah Formation was given a tentative Mid-Cambrian to ?Early Ordovician age by Stump and van der Eem (1995). It is equivalent, at least in part, to the Saq Sandstone of central and northern Saudi Arabia and is part of a vast sequence of quartz-rich sandstones deposited across northern Gondwana, from North Africa to Arabia. Considerable effort has gone into trying to understand the source of this “continent-wide braided stream system” with a consistently south to north transport direction (Avigard et al., 2003, 2005; Kolodner et al., 2006). The Sanamah Formation (0 to c. 140 m) occupies channels incised into the Dibsiyah Formation. It consists of a basal conglomerate of mixed pebble type, overlain by structureless, variably pebbly arkosic and subarkosic sandstones with rare diamictites. The bulk of the formation was interpreted as glacio-fluvial in origin by Stump and van der Eem (1995) who argued for a fluvial origin for the ‘tillites’ (diamictites) and ‘striated pavements’ described by some previous authors (McClure et al., 1988; Vaslet, 1990; Evans et al., 1991). The presence of bioturbated sandstones at the top of the formation in the type section (Stump and van der Eem, 1995) indicates an upward transition from a fluvial to marine environment. No biostratigraphically significant taxa have been recovered from the Sanamah Formation, but a Late Ordovician to early Silurian age has been inferred by Stump and van der Eem (1995).
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