Extensive palynological work in sequences in Oman and Saudi Arabia, has allowed a framework of eight biozones for the uppermost Carboniferous to Permian rocks in that region. Five of these are established in the palyniferous uppermost Carboniferous to Lower Permian sequence in Oman, and to some extent these are recognisable in sequences of central and southern Saudi Arabia. OSPZ1 (Oman and Saudi Arabia Palynological Zone 1), associated with the lower parts of the Al Khlata Formation and the Unayzah C member, is probably Stephanian in age. OSPZ2 is Asselian-Sakmarian in age, and is associated with the upper part of the Al Khlata Formation and the Unayzah B member. OSPZ3, which is subdivided into three sub-biozones, is associated with the Lower Gharif member; its age may lie in the range late Sakmarian to Artinskian, based on palynology and brachiopod-based ages for the Haushi Limestone, locally present toward the top of the Lower Gharif member. The composition of assemblages from OSPZ1 to OSPZ3 is similar to that of coeval sequences in other former Gondwana countries and the chronostratigraphical ages assigned to them are partly gained from correlation with faunally-calibrated Western Australian palynological biozones. At the level of OSPZ4, such correlation becomes difficult because significant differences are evident between Western Australian and Arabian assemblages, and because other Gondwana palynological biozonations, with which Arabia has greater affinity, are poorly constrained chronostratigraphically. OSPZ4 and the succeeding two biozones are established in the sporadically palyniferous Middle and Upper Permian sequences of Oman and Saudi Arabia. OSPZ4, though poorly characterised due to low palynological recovery, is probably ‘mid-Artinskian’ to Kungurian in age, and is associated with the Middle Gharif member. OSPZ5, present in southeast Saudi Arabia and Oman appears to be associated with the lower to middle parts of the Upper Gharif member, and is constrained in age, partly by the faunally-dated overlying Oman Khuff carbonates, as being Roadian or earliest Wordian in age. So far the assemblages of the highest biozone, OSPZ6, have been recovered only from the ‘basal Khuff clastics’ of central Saudi Arabia, and are believed to be younger than those of OSPZ5; dating by palynological means is difficult but recent microfaunal studies of the ‘basal Khuff clastics’ suggest a Capitanian age. Further study in appropriate palyniferous sections in other Middle Eastern countries may allow a more complete palynological succession to be established.
The Permian rocks of the southern Arabian Plate (Figures 1, 2) consist, at the base, of glacigenic diamictites and glaciolacustrine and glaciofluvial clastic sedimentary rocks, overlain by fluvial and other non-marine clastics, which are in turn overlain by carbonates, with evaporites and minor clastics. Though the lithostratigraphy of the Permian section is relatively well known (Hughes Clarke, 1988; Sharland et al., 2001), the biostratigraphy is not. Much of the Permian clastic sequence yields only palynomorphs, and even in these sections, thick sequences are palynologically barren. The carbonate units are abundantly fossiliferous, but it is only recently that progress has been made in relating Arabian carbonate faunas to those of the international Permian stages (Angiolini in Broutin et al., 1995; Angiolini et al., 1997, 1998, 2001; Dickens, 1999).
Several accounts of the palynology of the Arabian Permian have been published, most notably by Hemer (1965), McClure (1980), Kruck and Thiele (1983), Besems and Schuurman (1987), Love (1994), Stephenson (1998), Stephenson and Filatoff (2000a, b) and Stephenson and Osterloff (2002), but most information on palynostratigraphy is in the databases and reports of the principal oil companies operating in the region, i.e. Petroleum Development Oman (PDO) and the Saudi Arabian Oil Company (Saudi Aramco). Both these companies have in-house palynostratigraphical schemes, based on many years work in the region. Recent integration of these has allowed a provisional palynostratigraphical framework for Oman and Saudi Arabia. In this paper, some of the characteristics of this framework are described and illustrated. It is hoped that this may form a basis for a fuller regional palynozonation in the future. This paper also attempts to establish the dates of palynozones here defined against the new International Commission for Stratigraphy (ICS) scale for the Permian (Jin et al., 1997). For reasons of palaeophytogeographic variation and differing approaches to taxonomy adopted by workers in different areas, this is possible only to a variable degree, though new age data from work on the macropalaeontology of the region has helped to better constrain the age of some palynozones.
A schematic diagram illustrating the main lithostratigraphical units in Saudi Arabia and Oman, and the chronostratigraphical nomenclature used, is shown in Figure 2. Many of the lithostratigraphical units at member level are informally named, hence these are referred to with a lower case first letter in the word ‘member’. The main features of lithological units and their ages are discussed in the following section.
The Al Khlata Formation, Oman
The glacigene, non-marine Al Khlata Formation (the lower formation of the Haushi Group; Hughes Clarke, 1988) rests on the ‘Hercynian’ unconformity and as such overlies Precambrian to lower Carboniferous rocks (Osterloff et al., in press, a). The formation was deposited in a large intra-cratonic basin comprising much of the southern part of Oman south of the Oman Mountains. The northwestern extent of the basin, beneath the Rub’ Al-Khali, is not well known (Hughes Clarke, 1988), and its eastern and southern limits are obscure due to erosion related to the Haushi-Huqf uplift in eastern Oman. Widespread glaciolacustrine shales in the upper part of the Al Khlata Formation are informally termed the Rahab member by PDO geologists. This unit occurs throughout south and south central Oman and is considered a reliable correlative datum (Hughes Clarke, 1988).
For the Al Khlata Formation itself, only dating using palynostratigraphical schemes originating from the Gondwana region has been employed (Love, 1994; Besems and Schuurman, 1987; Stephenson, 1998; Stephenson and Filatoff 2000a), since no macrofossils are present. Besems and Schuurman (1987) and Love (1994) considered the Al Khlata Formation to be late Westphalian to Sakmarian in age, mainly by comparison with the Australian biozones of Kemp et al. (1977). Stephenson (1998) and Stephenson and Filatoff (2000a), working on subsurface core samples, suggested an Asselian-Sakmarian age for sediments in the upper part of the formation in the Amal-6 and –9 wells in southern Oman (Figure 1), mainly on the basis of the occurrence of taxa indicative of the Granulatisporites confluens Oppel Zone of Foster and Waterhouse (1988).
The Gharif Formation, Oman
The Gharif Formation (the upper formation of the Haushi Group; after Hughes Clarke, 1988) overlies the Al Khlata Formation both disconformably and conformably and is in turn overlain conformably by the marine carbonates and marginal marine to non-marine ‘red-bed’ clastics of the Khuff Formation (Osterloff et al., in press, b). The formation is informally subdivided by PDO into 3 members: the Lower, Middle and Upper Gharif members, using subsurface sections (Hughes Clarke, 1988; Guit et al., 1995). In South Oman, the lower part of the Lower Gharif member is a complex of fluvial and fluviodeltaic clastics succeeded by marginal marine clastics toward the top; while in North Oman similar lower clastics give way to bioclastic limestone, known locally as the Haushi Limestone. The surface equivalent of the Lower Gharif member was termed the Saiwan Formation by the Bureau de Recherches Géologiques et Minières (BRGM) (Dubreuilh et al., 1992; Platel et al., 1992; Roger et al., 1992). The Middle Gharif member is a sequence of marginal marine clastics overlain by lacustrine and fluvial units, capped by stacked palaeosols (the ‘Playa Shale’ sensu Guit et al., 1995), deposited in a semi-arid climate. The unit is barren of palynomorphs in its upper part (Osterloff et al., in press, b). Lying unconformably above the Middle Gharif member is the Upper Gharif member. The upper parts of this clastic unit contain abundant plant remains (Broutin et al., 1995) in the outcrops of the Northern Huqf area.
Age determinations for the Lower Gharif member come from palynology and macropaleontology. Miller and Furnish (1957) and Hudson and Sudbury (1959) suggested Mid Permian and Sakmarian ages respectively for fauna from the Haushi Limestone, while recent brachiopod evidence (Angiolini in Broutin et al., 1995; Angiolini et al., 1997) suggests a late Sakmarian age for the Haushi Limestone.
Palynological age determinations for the Gharif Formation are relatively few. Love (1994) assigned the Lower and Middle Gharif members to his Kingiacolpites subcircularis Assemblage, considered to be of Artinskian age, while Broutin et al. (1995) considered the palynomorphs of the ‘Gharif Paleoflora’ of the Upper Gharif member to be of Murgabian (Wordian) age. Recent palynological work (Stephenson and Osterloff, 2002) on the subsurface Lower Gharif member suggests a tentative Artinskian age, but this is inconsistent with the older Sakmarian age suggested for the surface Haushi Limestone, which overlies the palyniferous part of the Lower Gharif member in North Oman.
The Unayzah Formation, Saudi Arabia
The Unayzah Formation of central Saudi Arabia is, in part, coeval with the Al Khlata and Gharif formations (Stephenson and Filatoff, 2000a). Details of stratigraphical correlation of its upper parts with the Gharif Formation of Oman remain elusive (Sharland et al., 2001), partly because of different definitions for the upper boundary of the Unayzah Formation (see Stephenson and Filatoff, 2000b).
The Unayzah Formation rests on the ‘Hercynian’ unconformity and as such overlies lower Carboniferous to Precambrian rocks (Al-Laboun, 1987). The Khuff Formation lies disconformably on the Unayzah Formation (McGillivray and Husseini, 1992). The latter authors considered the Unayzah Formation to be present, at subsurface, as far east as the Arabian Gulf, and C. Heine (unpublished report, Saudi Aramco, 1998) considered it to be genetically related to the Al Khlata Formation and contiguous with the latter beneath the Rub’ Al-Khali (Figure 2).
At outcrop, the Unayzah Formation consists of a red bed sequence of poorly sorted conglomerate, sandstone, siltstone, mudstone, caliche and nodular anhydrite, which is regarded as having been deposited in coalescing alluvial fans, braided streams and playa lakes in mainly arid conditions (Senalp and Al-Duaiji, 1995). However there may have been more humid periods during the deposition of the Unayzah Formation, based on the flora of the Unayzah Plant Bed (El-Khayal and Wagner, 1985).
In the subsurface, the formation is divided into informal A, B and C members (Ferguson and Chambers, 1991; McGillivray and Husseini, 1992). The latter authors considered the upper member (the Unayzah A member) to be of alluvial or fluviatile origin, but C. Heine (unpublished report, Saudi Aramco, 1998) considered it to be of aeolian origin. The Unayzah B member is coarser and wholly of (?glacio) fluviatile origin (McGillivray and Husseini, 1992). Locally, a basal valley-fill member (the Unayzah C member) is also present in central Saudi Arabia.
Ages assigned to the Unayzah Formation units range from Late Carboniferous to Late Permian, depending on the definition used for the upper boundary of the unit. On the basis of plant macrofossils from the outcrop Unayzah Plant Bed, El-Khayal et al. (1980) suggested a Westphalian to Early Permian age but conceded that the age could extend to the Late Permian. El-Khayal and Wagner (1985) and Lemoigne (1981) suggested that the Unayzah Plant Bed fossils indicated Late Permian and ‘end of Middle to Late Permian’ ages respectively. Recent reappraisal of the Unayzah Plant Bed assemblage by Broutin et al. (personal communication in Vaslet et al., in press) indicates a possible Roadian to Wordian age.
No palynological evidence can be given for the age of the Unayzah Formation at outcrop since it (including the Unayzah Plant Bed) is barren; however, limited recovery has been achieved from the Unayzah Formation in the subsurface. Stephenson and Filatoff (2000b) recovered palynomorphs from the Hilwah-3 well in central Saudi Arabia, which penetrated the upper part of the Unayzah Formation (Unayzah A member) approximately 50 feet (15 m) below the pre-Khuff unconformity. The assemblages were considered to indicate an Artinskian to Kungurian age. The apparent discrepancy between the ages of the Unayzah Formation at outcrop and beds assigned to the Unayzah A member in the subsurface may mean that the two units are not directly comparable. Stephenson and Filatoff (2000a) also recovered palynomorphs from the Unayzah B member in the Jufarah-1 well (central Saudi Arabia). The assemblages recovered are similar to those of the upper parts of the Al Khlata Formation of Oman, principally in the occurrence of Granulatisporites confluens. The presence of this taxon suggests an Asselian to Sakmarian age.
The Khuff Formation, Oman and Saudi Arabia
As indicated in the section describing the Unayzah Formation, the lower boundary of the Khuff Formation in Saudi Arabia has not been consistently established, since the basal siliciclastic unit (the ‘basal Khuff clastics’) is included by some authors in the Unayzah Formation and by others in the Khuff Formation. Senalp and Al-Duaiji (1995) demonstrated a regionally correlatable unconformity marked by thick caliche, soil horizons and colour changes which they considered to be representative of fundamental changes in depositional environment. They suggested that this unconformity marked the onset of the marine conditions of the Mid- to Late Permian and therefore considered the overlying ‘basal Khuff clastics’ to be genetically part of the Khuff Formation. Broadly in agreement with El-Khayal and Wagner (1985) and Vaslet et al. (in press), they considered the sediments below this ‘pre-Khuff unconformity’ to be the upper part of the Unayzah Formation. In this study the definition of the base of the Khuff Formation is that used by Senalp and Al-Duaiji (1995) and Vaslet et al. (in press).
The Khuff Formation rests disconformably on the Unayzah Formation in parts of central Saudi Arabia (van der Eem, in Al-Jallal, 1995; Stephenson and Filatoff, 2000b) but over the Central Arabian Arch, where the Unayzah Formation is absent, the Khuff Formation unconformably overlies Proterozoic to Silurian rocks (Powers, 1968; Al-Aswad, 1997; Vaslet et al., in press). In Saudi Arabia, the Khuff Formation is conformably overlain by the Lower Triassic Sudair Shale; but in Oman, it is overlain unconformably by rocks of various Mesozoic ages (Osterloff et al., in press, b). The Khuff Formation crops out over a very wide area within Arabia and occurs in Saudi Arabia, Qatar, United Arab Emirates and Oman. Al-Jallal (1995) reported the thickness of the Khuff Formation to be 1,200 feet (366 m) in western Arabia, gradually thickening eastward to more than 5,000 feet (1,525 m) in Iran, where it is represented by the Dalan and Kangan formations. The Khuff Formation is composed of alternating sequences of carbonates and siliciclastic sediments. Siliciclastic sediments are common in the basal parts in central Saudi Arabia (Al-Jallal, 1995), though to the south these are often absent (Al-Jallal, 1995; Al-Aswad, 1997). The proportion of siliciclastics as a whole in the Khuff increases southward (Al-Aswad, 1997; Vaslet et al., in press).
The lowest siliciclastic unit of the Khuff Formation in central Saudi Arabia is informally termed the ‘basal Khuff clastics’ (Senalp and Al-Duaiji, 1995). The lowest sediments of the unit comprise grey-green, carbonaceous and calcareous shales. Overlying sandstones, interpreted as channel-fill, are incised into these marine sediments. Above these is another unit of grey-green shales interbedded with limestone and dolomite, and following this are massive limestones and dolomites that were assigned by Senalp and Al-Duaiji (1995) to the Huqayl Member. The sediments between the pre-Khuff unconformity and the massive limestones of the Huqayl Member were named the Ash-Shiqqah Member by Senalp and Al-Duaiji (1995). The latter authors suggested this term as a replacement for the informal term ‘basal Khuff clastics’. Vaslet et al. (in press) defined the Ash-Shiqqah Member as being slightly thicker, and include the first massive limestone and dolomites in the unit. Thus the ‘basal Khuff clastics’ of Senalp and Al-Duaiji (1995) correspond to the lower and middle parts of the Ash-Shiqqah Member sensu Vaslet et al. (in press), in central Saudi Arabia.
Ages for the ‘basal Khuff clastics’ in central Saudi Arabia, based on micro- and macrofloral evidence, are many and varied. No palynomorphs have been recovered from the unit at outcrop, but in the subsurface, the upper argillaceous part of the unit is richly palyniferous. Le Nindre et al. (1990) and Stephenson and Filatoff (2000b) suggested a Late Permian age for the ‘basal Khuff clastics’ in central Saudi Arabia, based on palynology. Recent work on outcrops in central Saudi Arabia (Vaslet et al., in press) suggests that the Ash-Shiqqah Member is Capitanian in age, while the overlying carbonates of the Huqayl Member are Wuchiapingian in age.
The lower part of the Khuff carbonates in Oman has been dated as Mid Permian (Kubergandian/Roadian) by Dickens (in Broutin et al., 1995). Later work, also on the lower part of the Khuff carbonates in Oman, based on brachiopods, bivalves, foraminifera and conodonts (Angiolini et al., 1998; Dickens, 1999) indicates a potentially wide range of ages. The conodonts of the section indicate a late Wordian to Capitanian age, the foraminifera indicate an Artinskian to early Wordian age and the brachiopods and bivalves both suggest a Roadian-Wordian age. On the basis of this data, Angiolini et al., (1998) suggested a compromise Wordian age for the unit.
PREVIOUS PALYNOSTRATIGRAPHICAL WORK IN THE REGION
Besems and Schuurman (1987) and Love (1994) have produced palynostratigraphical schemes for Oman (Figure 2). The former authors, working on outcrops of the Al Khlata Formation in east-Central Oman, recognised two distinct palynological assemblages. Assemblage Group A is dominated by zonate, trilete spores and is related by Besems and Schuurman (1987) to the Western Australian palynological Unit II of Balme in Kemp et al. (1977). Assemblage Group B is characterised by taeniate and non-taeniate bisaccate pollen and is related by Besems and Schuurman to Western Australian Unit III. Love (1994) described four palynological assemblages from the entire subsurface Haushi Group. The Potonieisporites Assemblage (late Westphalian to early Stephanian) is reportedly low in diversity containing simple trilete spores and monosaccate pollen such as Potonieisporites. Love suggested that the assemblage correlates with Unit I of Western Australia (Balme in Kemp et al., 1977). Love (1994) related his succeeding Microbaculispora Assemblage (mid- to late Stephanian) to Besems and Schuurman’s Assemblage Group A and his third assemblage, the Cycadopites cymbatus Assemblage (Asselian to Sakmarian) to Besems and Schuurman’s Assemblage Group B. Love considered his Kingiacolpites subcircularis Assemblage to be Artinskian in age. Based on the work of Love (1994), Penney and Osterloff (2002) have produced a local subdivision for South Oman sequences of the Al Khlata Formation that allows up to ten palynostratigraphical subdivisions.
No published palynostratigraphical schemes exist for the Permian of Saudi Arabia, though Stump and van der Eem (1995) illustrated taxa associated with Sakmarian-Artinskian, Kazanian and Tatarian sediments from the Wajid Outcrop Belt, southern Saudi Arabia (Figure 1). Generally in-house palynostratigraphical schemes are used by Saudi Aramco (e.g. Senalp and Al-Duaiji, 1995).
The work of Stephenson (1998), Stephenson and Osterloff (2002), and Stephenson and Filatoff (2000a) has helped to correlate Early Permian assemblages within Oman and Saudi Arabia, but also to correlate Arabian assemblages with those of other parts of the Permian post-glacial Gondwana realm, for example the Collie and Canning Basins of Australia and the Paraná and Chacoparana Basins of Argentina and Brazil. Stephenson (1998) and Stephenson and Filatoff (2000a) correlated the upper part of the Al Khlata Formation in Oman and the Unayzah B member in central Saudi Arabia with the Granulatisporites confluens Zone of Foster and Waterhouse (1988), suggesting an Asselian-Sakmarian age. Stephenson and Filatoff (2000a) suggested detailed correlation of the Arabian Lower Permian with the Lower Permian South American Cristatisporites Zone of Archangelsky and Gamerro (1979), Russo et al. (1980) and Vergel (1993).
CHRONOSTRATIGRAPHICAL CORRELATION AND PALYNOLOGY
Palynology is a well-established tool for high-resolution and regional correlation in Phanerozoic clastic sequences. In the Permian of the Arabian Peninsula, its use in high-resolution correlation between and within fields has been established (see Stephenson and Osterloff, 2002, for example) and from basin to basin within the region (Stephenson and Filatoff, 2000a). However the ability of palynology to correlate outside the region, and, crucially, to correlate with international stratigraphical stages, is hampered by a number of factors.
The Permian was a time of great phytogeographical provicialism, with the consequence that inter-regional correlation is difficult (Warrington, 1996). Even within single floral provinces, correlation is difficult due to differing taxonomic approaches and criteria used to establish zones (Utting and Piasecki, 1995). In the Indian part of the Gondwana province, for example, palynology has been mainly concerned with the correlation of coal seams that is achieved by comparisons of frequencies of palynomorph genera. In Australian and South American palynostratigraphy (Truswell, 1980), stratigraphical ranges of single palynomorph species are mainly used for correlation. Recently attempts have been made (Backhouse, 1991; Foster and Jones, 1994; Gomankov, 1994; Stephenson and McLean, 1999) to use distinctive palynomorph species which occur in several basins, to correlate distant sequences, for example in the Alps, Australia, southern Africa, Russia and the Salt Range, Pakistan. Radioisotopic dating and magnetostratigraphical methods accompanied by interdisciplinary biostratigraphical studies (including palynology) are also being employed in long-range correlation (Cazzulo-Klepzig et al., 2002; Melchor, 2000; Jin and Menning, 1996; Roberts et al., 1995). Similarly, methods that combine palynology with δ13C studies on kerogen from clastic rocks may form a valuable method in correlation of the Permian (Foster et al., 1998; Stephenson et al., 2002). Despite these advances, correlation with the new basal Permian stratotype at Aidaralash Creek, Kazakhstan (Davydov et al., 1998) cannot be achieved in Saudi Arabia and Oman because of the lack of taxa common to the two areas (Dunn, 2001).
Apart from difficulties that relate to palaeophytogeographic variation, correlation of Saudi Arabian and Omani sequences is also made difficult by lack of palynological data from type areas of the Permian chronostratigraphical scale (Jin et al., 1997). There is very little palynological data from the Middle Permian Guadalupian Series, because it is mainly represented by carbonates in its type area in the United States (Utting, 2001). Similarly the type area of the Upper Permian Lopingian Series (in south China) is represented by very little palynostratigraphical information in English, or is difficult to correlate to because of the prevalence of carbonates that are barren or yield poorly preserved assemblages of palynomorphs (Ouyang and Utting, 1990). Indirect correlation with the standard stages via palynologically productive marine sequences in the Canadian Arctic, Alaska, Greenland and Arctic Europe (Utting, 1994, 2001; Mangerud, 1994) is also difficult due to lack of common taxa (Stephenson and Filatoff, 2000b).
The Gondwana Permian-Carboniferous province, to which Saudi Arabia and Oman belong, is well known for an almost complete lack of macrofauna between the Namurian-Westphalian and late Asselian (Truswell, 1980; Jones and Truswell, 1992; Kemp et al., 1977). Hence dating in this section relies on palynomorphs, which as mentioned above, were at this time provincial. Above the upper Asselian, limited independent palaeontological evidence is available in Western Australia, from ammonoids (Leonova, 1998) and brachiopods (Archbold, 1999). This has allowed the palynological biozones of Backhouse (1991) to be correlated to the international stages. Elsewhere, for example South America and India (Césari and Gutiérrez, 2000; Truswell, 1980; Playford and Dino, 2000), there are extremely limited independent palaeontological controls on palynological biozonations, though recent radioisotopic dates (Cazzulo-Klepzig et al., 2002; Melchor, 2000) allow point correlations to the international scale.
At present there is no standard Tethyan section with an established palynostratigraphic succession, to which stratigraphically isolated assemblages can be correlated.
PRELIMINARY INTEGRATED PALYNOZONATION FOR OMAN AND SAUDI ARABIA
The palynozones described in this paper are illustrated in Figure 2, along with their proposed ages, and relationships to the main lithostratigraphical units in the region. Each biozone has the prefix OSPZ for ‘ Oman and Saudi Arabia Palynological Zone’. An alphanumeric notation has been chosen against recommendations to the contrary (Rawson et al., 2002) for the following reasons.
The numerical sequence indicates relative stratigraphical position, obviating the need for knowledge of the order of appearance of index forms or assemblages, a convenience for explorationists with non-biostratigraphic specialisations.
Alphanumeric codes are concise and thus suitable for inclusion on geological cross sections and logs.
Alphanumeric notation circumvents the problems caused by taxonomic revisions of zonal taxa (see Price, 1997).
Schematic ranges of selected palynomorphs are shown in Figure 3. In accordance with normal stratigraphic practice, ‘first occurrence’ in this account refers to first uphole occurrence. For the present, the biozones should be regarded as assemblage zones, unless specifically noted otherwise. Because of the predominance of palynologically barren sections, particularly in the upper parts of the sequence, many of the biozone boundaries have not been observed in rock sequences, hence the bases of some biozones cannot, as yet, be precisely defined. Similarly reference sections quoted for biozones here defined may not represent their full stratigraphic ranges. Plates 1-6 illustrate the palynomorphs that are characteristic of the palynozones. Ages suggested for biozones are tentative. Data, which support the preliminary palynozonation given here, come from palynological analysis of many hundreds of core, sidewall and cuttings samples from wells in Oman and Saudi Arabia provided by PDO and Saudi Aramco.
Appendix 1 gives the full author citations of selected taxa referred to in the text. The generic names of taxa recorded from the sequence are spelled out in full in the first instance, and then abbreviated thereafter.
Reference Section: 15,100 ft (4,602 m) - 15,050 ft (4,587 m), Mukassir-1 (MKSR-1) well, southern Saudi Arabia.
The palynological assemblages of the Al Khlata and Unayzah formations are affected by reworking due to large-scale glacial erosional and depositional processes; therefore stratigraphical ranges of palynomorphs have been difficult to establish. However OSPZ1 assemblages of the lower part of the Al Khlata Formation and equivalent beds in Saudi Arabia (Figure 2), seem to have the following characteristics:
small numbers of Angulisporites cf. splendidus (0.5-10% of assemblages) and Anapiculatisporites concinnus (1-3% of assemblages);
common Punctatisporites spp. and Retusotriletes spp. (up to 50% of assemblages);
common bilaterally and radially symmetrical monosaccate pollen (up to 25% of assemblages).
The base of the biozone cannot be defined due to absence of older rocks assignable to a stratigraphically lower biozone.
Other taxa characteristic of, but not necessarily confined to OSPZ1 are: Ahrensisporites cristatus, Ancistrospora inordinata, Ancistrospora verrucosa, Brevitriletes cornutus, Cadiospora cf. magna, Caheniasaccites ovatus, Calamospora cf. microrugosa, Cannanoropollis janakii, Cannanoropollis talchirensis, Cristatisporites spp., Cristatisporites cf. crassilabratus, Jayantisporites conatus, Lundbladispora braziliensis, Plicatipollenites malabarensis, Potonieisporites spp., Punctatisporites gretensis forma minor, Retusotriletes nigritellus, Spelaeotriletes triangulus and Vallatisporites arcuatus. Taxa or taxon groups that are absent include cheilocardioid spores such as Microbaculispora tentula and Horriditriletes spp.. The diversity of the assemblages tends to be low with Punctatisporites spp., Retusotriletes spp. and bilaterally and radially symmetrical monosaccate pollen being the most abundant taxa or taxon groups.
OSPZ1 is similar to eastern Australian Stage 1 and the Asperispora reticulatispinosus Biozone (Kemp et al., 1977; Jones and Truswell, 1992), because bilaterally and radially symmetrical monosaccate pollen, Punctatisporites spp. and Retusotriletes spp. are common in all. Asperispora reticulatispinosus is however, not present in OSPZ1. Love (1994) described his Potonieisporites Assemblage as being dominated by monosaccate pollen and simple trilete spores, as well as species of Anapiculatisporites; but also reported the rare presence of Horriditriletes spp. and Microbaculispora spp.. This indicates that OSPZ1 is similar, but the presence of the latter two taxa, suggests that OSPZ1 may be in part older than the Potonieisporites Assemblage. OSPZ1 correlates with the in-house PDO 2159 Palynozone (see Osterloff et al., in press, a).
Age of OSPZ1
The abundance of monosaccate pollen indicates a post-Namurian (Jones and Truswell, 1992; Clayton et al., 1977), possibly Stephanian age since such pollen are characteristically abundant in the type area of the Stephanian of Western Europe (Balme, 1980). Anapiculatisporites concinnus has been recorded from Carboniferous rocks in Spitzbergen (Playford, 1962); Canning Basin, Australia (Powis, 1979, 1984); Britain (Smith and Butterworth, 1967); Argentina (Azcuy, 1975; see Jones and Truswell, 1992) and Oman (Besems and Schuurman, 1987; see Stephenson and Filatoff, 2000a). Recent work suggests that its range in eastern Australia is Namurian to earliest Asselian (Oppel-zones A to D; Jones and Truswell, 1992) though independent palaeontological evidence for the upper extent of this range is lacking. Angulisporites splendidus, a taxon closely similar to that recovered from OSPZ1, has been recorded from independently dated rocks of Stephanian A-D age in Western Europe (Clayton et al., 1977, see also Cleal et al., 2003) and from rocks considered to be of Stephanian age in Iraq (Nader et al., 1994), though there is no independent palaeontological date for the latter occurrence. In conclusion, the co-occurrence of A. concinnus and A. cf. splendidus, and the abundance of monosaccate pollen suggest a Stephanian age for OSPZ1.
The absence from OSPZ1 of Microbaculispora tentula and other cheilocardioid spores such as Horriditriletes ramosus support a pre-Permian age or pre-eastern Australian Stage 2 age (see Powis 1979, 1984; Jones and Truswell, 1992). The age suggested for OSPZ1 should however be considered to be tentative in the absence of substantial macrofaunal or other independent dating evidence.
Reference Section: 5,179 ft (1,578 m) - 4,451 ft (1,357 m), Amal-9 well, southern Oman.
The base of this biozone is defined by the first occurrence of Horriditriletes spp. and M. tentula. Within OSPZ2, these taxa are common, constituting up to 20-30% of assemblages. Other taxa occurring first within OSPZ2 are: Brevitriletes leptoacaina, B. parmatus, Converrucosisporites sp. B, Cycadopites cymbatus, Pachytriletes densus and Verrucosisporites andersonii. Granulatisporites confluens, Microbaculispora grandegranulata, Converrucosisporites sp. A and Verrucosisporites cf. naumovae occur first in the upper part of OSPZ2, and C. cymbatus becomes common at this level. Other taxa characteristic of, but not necessarily confined to OSPZ2 are: Plicatipollenites malabarensis, Punctatisporites gretensis forma minor, Dibolisporites disfacies and Psomospora detecta. Due to reworking from OSPZ1, A. cf. splendidus and A. concinnus are sometimes recorded in OSPZ2.
The diversity of the assemblages is high; cheilocardioid spores such as M. grandegranulata, M. tentula, Horriditriletes tereteangulatus, H. uruguaiensis and H. ramosus are common as are radially symmetrical monosaccate pollen such as P. malabarensis and Cannanoropollis janakii, and colpate pollen such as C. cymbatus. Non-taeniate and taeniate bisaccate pollen such as Alisporites indarraensis, Complexisporites polymorphus, Protohaploxypinus limpidus, P. amplus, Striatoabieites multistriatus and Strotersporites indicus are also present, but they are not common.
OSPZ2 is associated with the upper parts of the Al Khlata Formation, and the Unayzah B member (Figure 2). The lower part of OSPZ2 correlates with the Microbaculispora Assemblage Zone of Love (1994) because this part contains common Microbaculispora spp., as well as a large number of other co-occurring taxa. The upper part of OSPZ2 correlates with the Cycadopites cymbatus Assemblage Zone of Love (1994), because C. cymbatus is common in both. OSPZ2 also correlates with the in-house PDO 2165 and 2141 palynozones (Figure 2; see Osterloff et al., in press, a).
The base of OSPZ2 coincides with that of eastern Australian Stage 2 (sensuPowis, 1984) because both are defined by the first occurrence of M. tentula and Horriditriletes spp.. The two biozones are also similar in composition, with a number of taxa in common including, in addition to M. tentula and Horriditriletes spp.: Leiotriletes virkkii, B. cornutus, C. cymbatus and S. multistriatus. The base of OSPZ2 also coincides with that of the Microbaculispora tentula Oppel-zone of Jones and Truswell (1992), established in the Joe Joe Group, Queensland, Australia. This is because the base of the M. tentula Oppel-zone is defined by the first occurrence of M. tentula; and because the first appearance of Horriditriletes ramosus is coincident with that event in the Joe Joe Group (Jones and Truswell, 1992; fig. 6). The M. tentula Oppel-zone differs in quantitative terms from OSPZ2 in containing more abundant Punctatisporites spp. and Calamospora spp.; and in being less diverse, certainly than upper parts of OSPZ2.
Backhouse (1991, 1993) reported that G. confluens occurs first toward the top of Australian Stage 2, and this order of first occurrence is also evident in Oman and Saudi Arabia, where G. confluens occurs stratigraphically well above the first occurrences of M. tentula and H. ramosus. Hence some of the upper part of OSPZ2 correlates to ‘Stage 2 with G. confluens’ (sensuBackhouse 1991, 1993), and to the G. confluens Oppel Zone of Foster and Waterhouse (1988) (see Stephenson, 1998 and Stephenson and Filatoff, 2000a for details). The presence of large, heavily ornamented cheilocardioid spores such as Converrucosisporites sp. A and V. cf. naumovae in the highest parts of OSPZ2 may suggest upward extension into the age equivalent of the Pseudoreticulatispora pseudoreticulata Biozone of Backhouse (1991) (lower part of APP2, Price 1997). This highest section of OSPZ2 also correlates with a field-scale biozone established in central Oman, the Converrucosisporites sp. A - Microbaculispora grandegranulata Biozone of Stephenson and Osterloff (2002). The underlying field-scale Microbaculispora tentula Biozone of Stephenson and Osterloff (2002), established in the top 20-30 m of the Al Khlata Formation, corresponds to the upper parts of OSPZ2, but not the highest part.
Age of OSPZ2
The assemblages of the upper part of OSPZ2 are similar to those of the Pseudoreticulatispora (= Granulatisporites) confluens and P. pseudoreticulata biozones of Backhouse (1991) because they contain Alisporites spp., B. cornutus, B. levis, Calamospora spp., C. cymbatus, G. confluens, H. ramosus, H. tereteangulatus, Leiotriletes directus, Limitisporites rectus, M. tentula, Plicatipollenites spp., Potonieisporites novicus, P. amplus, P. limpidus, P. gretensis forma minor, Scheuringipollenites ovatus (as Sulcatisporites ovatus) and various large heavily ornamented cheilocardioid spores such as Converrucosisporites sp. A and V. cf. naumovae. Recent work by Archbold (1999), in which the palynological zones of Backhouse (1991) are linked with the Russian Stages via faunal evidence, suggests that the range of G. confluens spans the equivalent of the Asselian-Sakmarian boundary in Western Australia, while P. pseudoreticulata occurs first in the Late Sakmarian equivalent in Western Australia. Although G. confluens is undoubtedly present in OSPZ2, notably absent is P. pseudoreticulata. The presence, however, of V. cf. naumovae and Converrucosisporites sp. A (similar in morphology to P. pseudoreticulata) in OSPZ2 suggests similarities to the P. pseudoreticulata Biozone. Correlation of the lower parts of OSPZ2 with international stages is not possible because Western Australian sequences lack fauna older than late Asselian. It seems reasonable to suggest however, that by superposition, the lower parts of OSPZ2 are Asselian in age. As discussed earlier, correlation with the basal Permian stratotype at Aidaralash Creek, Kazakhstan is not possible due to lack of common taxa, thus the position of the Carboniferous – Permian boundary cannot be determined in Arabian sequences, by palynological means.
Reference Section: 942 m - 885 m, Thuleilat-42 (TL-42) well, southern Oman.
This biozone is subdivided into three sub-biozones following high-resolution biozonation of the Lower Gharif member and Rahab member of Oman (Stephenson and Osterloff, 2002). For the purposes of this paper, the three sub-biozones are denoted in ascending stratigraphic order: OSPZ3a, OSPZ3b and OSPZ3c (respectively the Alisporites indarraensis, Ulanisphaeridium omanensis and Cyclogranisporites pox biozones of Stephenson and Osterloff, 2002). Compared with the other biozones described here, the three sub-biozones of OSPZ3 are smaller in scale and somewhat localised in geographical extent, so they may not be recognisable throughout Arabia either due to palaeophytogeographical variation or hiatus. The base of OSPZ3 is defined by the abrupt increase of the small non-taeniate bisaccate pollen A. indarraensis from approximately 10 to 50 or 60% of assemblages. This increase is accompanied by an increase in coarsely ornamented forms of Cristatisporites (to a maximum of approximately 4% of assemblages). Other taxa that occur first consistently in OSPZ3 are the taeniate bisaccate pollen Striatopodocarpites cancellatus and S. fusus, and the taeniate ‘circumstriate’ pollen taxa such as Circumstriatites talchirensis and Striasulcites tectus. Kingiacolpites subcircularis is common throughout this biozone occasionally reaching 50% of assemblages, but more typically 5-10% of assemblages. A. indarraensis has high abundances, usually greater than 50% of assemblages, throughout the biozone. OSPZ3 is associated with the Lower Gharif member; the biozone correlates with the lower parts of the Kingiacolpites subcircularis Assemblage of Love (1994) since K. subcircularis, Striatopodocarpites spp., and other taeniate and non-taeniate bisaccate pollen are common in both.
Age of OSPZ3
The Striatopodocarpites fusus/cancellatus complex generally appears close to the base of OSPZ3, though similar taxa, normally with distinct central monolete marks, occur earlier, making exact correlation with the S. fusus Biozone of Backhouse (1991) difficult. However, similarity in other quantitative and qualitative characteristics with the S. fusus Biozone is apparent in OSPZ3 with S. fusus, S. cancellatus and Weylandites spp. co-occurring amongst many other taxa that persist from the biozones below. The overall quantitative and qualitative similarity therefore suggests that OSPZ3 may be Artinskian in age, based on comparison with faunally calibrated palynological biozones in Western Australia (see Archbold, 1999, 2001).
The age suggested by palynology is inconsistent with that suggested for the outcropping Haushi Limestone, which overlies the clastics of the Lower Gharif member. Recent brachiopod evidence (Angiolini, in Broutin et al., 1995; Angiolini et al., 1997) suggests a late Sakmarian age for the Haushi Limestone. Further work is required to understand this discrepancy.
OSPZ3a: The base of this sub-biozone is marked by the most distinct palynological discontinuity in the Lower Permian section, which corresponds closely to the transition between the Rahab member and the Lower Gharif member; it may also be linked with large-scale climatic change (Stephenson and Osterloff, 2002; Stephenson et al., 2002). The reference section for the sub-biozone is Rahab-2 (RA-2) well, southern Oman, between 964 m and 935 m. As indicated above, the base of this sub-biozone, which corresponds to the base of OSPZ3, is marked by an increase in the abundance of A. indarraensis and the consistent occurrence of taeniate bisaccate pollen such as S. fusus and S. cancellatus. The monocolpate pollen grain K. subcircularis may also become common at this level. Other taxa or taxon groups characteristic of, but not necessarily confined to OSPZ3a are: B. cornutus, C. janakii, C. ovatus, C. polymorphus, Cristatisporites spp., Hamiapollenites fusiformis, Indotriradites apiculatus, Lahirites karanpuraensis and Limitisporites rotundus. Granulatisporites confluens, H. ramosus and H. tereteangulatus are absent or extremely rare; their occurrences may be due to reworking.
OSPZ3b: The base of the sub-biozone is marked by the first appearance of the small, but distinctive acritarch Ulanisphaeridium omanensis. The reference section for the sub-biozone is Qaharir-2 (Q-2) well, southern Oman, between 4,392.8 ft (1,339 m) and 4,354.8 ft (1,327 m). In Oman this taxon ranges over an interval of about 10-12 m, toward the middle of the Lower Gharif member, and reaches abundances of up to 19% of assemblages, but abundances of 1-2% are more typical. In southern Oman wells, the first occurrence of U. omanensis is accompanied by that of Lundbladispora gracilis but there are no other significant differences between this biozone and the preceding one. The presence of U. omanensis appears to be geographically restricted, in that in numerous wells in Saudi Arabia, at a probable equivalent stratigraphical level, it does not occur. This suggests either that its occurrence is controlled by local palaeotopography or distance from a transgressing sea, that its absence is due to missing section (unconformity), or that oxidation has removed palynomorphs at this level in Saudi Arabia. For the moment it would appear that this sub-biozone can only be recognised in Oman.
OSPZ3c: The base of the Cyclogranisporites pox Biozone of Stephenson & Osterloff (2002) is defined by the first occurrence of the small, distinctive spore C. pox. In the Thuleilat and Qaharir fields, south Oman, the C. pox Biozone is useful in recognising the upper part of the Lower Gharif member. However in other parts of Oman, C. pox occurs earlier or is absent in sections of equivalent age (Stephenson and Osterloff, 2002). Recent work from a wider area within Oman has confirmed that assemblages from the upper part of the Lower Gharif member are distinct, in that Corisaccites alutas, Caheniasaccites ovatus, Divarisaccus sp. A, Florinites flaccidus and Vesicaspora spp. occur in significant numbers (1-5% of assemblages) at this level. Divarisaccus sp. A (of Stephenson and Osterloff, 2002) is geographically widespread and distinctive in appearance. When present, it first occurs at a fairly consistent stratigraphic level, approximating to the base of the C. pox Biozone, the exception being some extremely rare and questionable earlier occurrences in Thuleilat-42 (TL-42). It is therefore proposed, that pending further investigation, the first occurrence of Divarisaccus sp. A should mark the base of OSPZ3c. Taxa characteristic of, but not necessarily confined to OSPZ3c include: S. fusus, S. multistriatus and S. ovatus. The reference section for OSPZ3c is Qaharir-2 well, southern Oman, between approximately 1325 m and approximately 1316 m.
This biozone is associated with the Middle Gharif member and the middle to upper parts of the subsurface Unayzah Formation. Both units are dominantly palynologically barren so only a relatively small number of samples have contributed to our knowledge of OSPZ4 assemblages. The chief distinguishing characteristic of the biozone is the common occurrence of Barakarites rotatus, K. subcircularis and P. limpidus, though C. alutas, F. flaccidus, P. malabarensis, P. amplus, Plicatipollenites spp., Striatopodocarpites spp., S. tectus, Strotersporites spp., Vesicaspora spp., Vittatina costabilis and Vittatina spp. are also present. All these taxa are present in biozones below, including (in small numbers) B. rotatus, so this biozone is not strongly differentiated from OSPZ3. Although similar to OSPZ3, OSPZ4 is quite distinct from OSPZ5 since Cedripites sp. B, Densipollenites indicus, Distriatitesinsolitus, Distriatites sp. A, Indotriradites sp. C, Lueckisporites virkkiae and Playfordiaspora cancellosa do not occur in OSPZ4. Overall the assemblages of OSPZ4 are unusual in being dominated by saccate and colpate pollen, with very few spores. No rock sequence has been observed that preserves the boundary between OSPZ3 and OSPZ4, hence the base of OSPZ4 cannot, at present, be precisely defined; indeed most occurrences of OSPZ4 assemblages are stratigraphically and geographically isolated, particularly in Saudi Arabia. For these reasons a reference section for the biozone is not suggested for the present.
No previously published assemblages are exactly comparable with OSPZ4. Stephenson and Filatoff (2000b) described assemblages from Hilwah-3 (HLWH-3) well in central Saudi Arabia, which contained common Circumstriatites talchirensis, K. subcircularis, P. limpidus and S. tectus as well as smaller numbers of B. rotatus, C. polymorphus, Caheniasaccites ovatus, V. costabilis, and extremely rare spores. Broadly this composition is the basis for the in-house Saudi Aramco P3 Biozone (Figure 2), recovered from a small number of short, stratigraphically and geographically isolated, well sections in Saudi Arabia. The Hilwah-3, and other P3 assemblages are similar to OSPZ4, but the high numbers of Circumstriatites talchirensis, K. subcircularis and S. tectus and are also similar to OSPZ3b and OSPZ3c assemblages; hence, pending further work, the P3 assemblages are correlated with the OSPZ3b to OSPZ4 interval.
Age of OSPZ4
The previous discussion of assemblages from OSPZ1 to OSPZ3 shows that they are similar to those of coeval sequences in other former Gondwana countries, and that the chronostratigraphical ages assigned to them are partly gained from correlation with faunally-calibrated palynological biozones in Western Australia. At the level of OSPZ4, however, such correlation becomes difficult because:
significant differences are evident between Western Australian and Arabian assemblages;
other Gondwana palynological biozonations for South American and Indian sequences - with which Arabia has slightly greater affinity - are poorly constrained chronostratigraphically;
no direct macrofossil evidence or age diagnostic palynomorphs are present.
For these reasons the age of OSPZ4 is poorly constrained. However, notwithstanding the discrepancy between the ages suggested for sediments of the Haushi Limestone and below, noted in earlier discussion, a lower age limit for OSPZ4 must lie within the early Artinskian or late Sakmarian since the Middle Gharif member, from which OSPZ4 assemblages are recovered, overlies the Haushi Limestone. Similarly the absolute upper age limit of OSPZ4 must lie within the Wordian since the Khuff Formation, which overlies the Gharif Formation in Oman, is of that age.
The presence of B. rotatus may allow a slightly more precise age assignment. This taxon has an approximate range of late Early Permian to Late Permian (Balme and Playford, 1967; Segroves, 1969; MacRae, 1988; Lindström 1996) and recent work by Backhouse (1991) and Archbold (1999) suggests that its first appearance is in the ‘mid-Artinskian’ in faunally dated sequences in Western Australia, however specimens similar to B. rotatus have been recovered from the upper parts of the Al Khlata Formation (Stephenson and Filatoff, 2000a; R. Penney oral comm., 2002), so the value of this taxon in age dating may be limited. For the present a tentative maximum age range of ‘mid-Artinskian’ to Kungurian is suggested for OSPZ4.
Reference Section: 9,062.5 ft (2, 762 m) - 9,029.5 ft (2,752 m), Saih Rawl–8 (SR-8) well, north central Oman.
Assemblages assigned to this biozone occur in many wells in Oman, but have so far been recovered from one well only in Saudi Arabia (Rawakib-1, RWKB-1), in the extreme southeast of that country, close to the border with Oman. As with the boundary between OSPZ3 and OSPZ4, no rock sequence yet observed preserves the boundary between OSPZ4 and OSPZ5, hence the base of OSPZ5 cannot, as yet, be precisely defined. However the assemblages of OSPZ5 are quite distinct from those of the preceding biozone, being dominated by taxa such as Cedripites sp. B, Distriatites sp. A and D. insolitus, none of which occur in OSPZ4. Other taxa that occur first in OSPZ5 are: Densipollenites indicus, Platysaccus cf. queenslandi, P. cancellosa, Pteruchipollenites owensii and T. opaqua. Taxa characteristic of, but not necessarily confined to OSPZ5 are: A. indarraensis, Alisporites cf. nuthallensis, Brevitriletes cf. hennellyi, C. alutas, H. fusiformis, Hamiapollenites spp., Indotriradites sp. C, K. subcircularis, Laevigatosporites cf. callosus, P. novicus, P. amplus, P limpidus, Protohaploxypinus cf. microcorpus, ?Reduviasporonites chalastus, Striatoabieites cf. multistriatus, S. ovatus, V. costabilis and Weylandites lucifer. A few specimens of Lueckisporites virkkiae have been recorded from assemblages in two wells in Oman that are assignable to OSPZ5, but the taxon is not present in most OSPZ5 assemblages.
OSPZ5 is associated with sediments of fluvial origin in the lower and middle parts of the Upper Gharif member, higher parts of that unit, including red-beds, being palynologically barren, at least in the subsurface. OSPZ5 assemblages are variable in diversity, some are dominated by species of Distriatites or Hamiapollenites, while others contain the former and a wide range of pollen and spore taxa.
The palynological changeover between OSPZ4 and OSPZ5 is probably the greatest recorded in the Permian palynological succession in Oman and Saudi Arabia. Although there is no palynological evidence from barren sections immediately below the Middle-Upper Gharif boundary, sedimentological evidence from the boundary itself indicates abrupt change in environment of deposition, accompanied by changes in chemostratigraphy and heavy mineral assemblages. In addition there is evidence for local incision and palaeosol development in the upper part of the Middle Gharif member (Osterloff et al., in press, b). This and palynological evidence of changes between OSPZ4 and OSPZ5 therefore suggests the presence of a hiatus between the Middle and Upper Gharif members.
Age of OSPZ5
The age of OSPZ5 is poorly constrained. This is because most of its taxa are undescribed and hence cannot be related to international stages. Though well established palynological taxa do occur in the OSPZ5 assemblages, one of these, P. cancellosa, is present in rocks much older than its previous records from Australia and India suggest, based on macropalaeontological dating of the overlying Khuff Formation in Oman. Thus, doubt surrounds the stratigraphical value of this taxon; this difficulty will be discussed later in the paper. Another age-diagnostic taxon, L. virkkiae, is extremely rare, occurring in only two assemblages. Its presence may indicate an early Kazanian (?early Wordian) lower age limit for OSPZ5 (Utting et al. 1997), but this date should be regarded as tentative, because of the rarity of the taxon.
Although palynology is unable to date OSPZ5 precisely, the age of the Khuff Formation in Oman, which immediately overlies the Upper Gharif member, provides an upper age limit. The Khuff Formation is dated by macro- and microfaunal data as Wordian (Angiolini et al., 1998). A tentative Roadian or earliest Wordian age is therefore suggested for OSPZ5.
Reference Section: 7,889 ft (2,405 m) - 7,811.2 ft (2,381 m), Dilam-1 (DILM-1) well, central Saudi Arabia.
The assemblages assigned to OSPZ6 are well known in central Saudi Arabia, where they are recovered from the ‘basal Khuff clastics’. OSPZ6 assemblages have not, as yet, been recovered from Oman. The key taxa associated with this biozone are, Camptotriletes warchianus, ?Florinites balmei, Pyramidosporites cyathodes, Protohaploxypinus uttingii and Triplexisporites cf. playfordii. Alisporites nuthallensis, Cedripites spp., C. alutas, D. indicus, D. insolitus, Klausipollenites cf. schaubergeri, Laevigatosporites callosus, L. virkkiae ‘norm A’, L. virkkiae, P. cancellosa, P. microcorpus, Taeniaesporites cf. pellucidus, T. opaqua, Reduviasporonites chalastus and W. lucifer are also present. As with the boundary between OSPZ4 and OSPZ5, no rock sequence yet observed preserves the boundary between OSPZ5 and OSPZ6, hence the base of OSPZ6 cannot, as yet, be precisely defined. The main difference between OSPZ5 and OSPZ6 is that OSPZ6 contains C. warchianus, ?F. balmei, P. cyathodes, P. uttingii and T. cf. playfordii, but lacks Cedripites sp. B, Distriatites sp. A and Indotriradites sp. C. The diversity of OSPZ6 assemblages is high; although dominated by taeniate and non-taeniate bisaccate pollen, a large number of spore taxa occur and the probable alga Reduviasporonites chalastus (see Foster et al., 2002) is often abundant.
Age of OSPZ6
Palynological evidence for the age of OSPZ6 comes from the studies of Le Nindre et al. (1990) and Stephenson and Filatoff (2000b). Le Nindre et al. (1990) studied assemblages from the ‘basal Khuff clastics’ (locally equivalent to the Ash-Shiqqah member; see Vaslet et al., in press) between 685.07 and 657m in SHD-1 Borehole, central Saudi Arabia recording taxa that include: Alisporites tenuicorpus, Cedripites priscus, Klausipollenites schaubergeri, L. virkkiae, P. amplus, Plicatipollenites gondwanensis and Taeniaesporites noviaulensis. On this basis a Late Permian age was suggested.
Stephenson and Filatoff (2000b) recorded the following taxa from the ‘basal Khuff clastics’ in wells Dilam-1 (DILM-1), Nuayyim-2 (nYYM-2) and Haradh-51 (HRDH-51), central Saudi Arabia: Cedripites spp., C. alutas, D. indicus, D. insolitus, Klausipollenites cf. schaubergeri, Laevigatosporites callosus, L. virkkiae ‘norm A’, P. cancellosa, P. microcorpus, Taeniaesporites cf. pellucidus, T. playfordii, T. opaqua, Tympanicysta stoschiana (=Reduviasporonites chalastus) and W. lucifer. On this basis Stephenson and Filatoff (2000b) suggested a Tatarian or younger (possibly Changhsingian) age for the ‘basal Khuff clastics’.
Many of the taxa recorded by Stephenson and Filatoff (2000b) and Le Nindre et al. (1990) are well known from Permian-Triassic sequences worldwide but few have been recorded from sequences that can be dated reliably using independent marine palaeontology (see later discussion). Of the taxa recorded a few may have age significance. Lueckisporites virkkiae is probably the most reliable age determinator. This taxon occurs first in the lower part of the Kazanian (Wordian) in its type area in the Russian Platform (Utting et al., 1997); in Europe, where independent palaeontological evidence is available, its last occurrence is close to the equivalent of the Permian-Triassic boundary (Visscher and Brugman, 1986). Independent confirmation of a Wordian first occurrence for L. virkkiae in the Gondwana palaeophytogeographic province comes from radioistopic dating of the Argentinian Striatites Biozone (Vergel, 1993; Archangelsky and Vergel, 1996) at the base of which that taxon makes its first appearance. Melchor (2000) reported a radioisotopic date of 266.3 ± 0.8 Ma (Wordian according to Wardlaw and Schiappa, 2001) for the base of the Striatites Biozone. Cedripites priscus was reported by Gomankov et al. (1998) to occur first in the upper parts of the Tatarian Stage on the northern Russian Platform, while R. chalastus appears to have a range of Capitanian to Griesbachian (Foster et al., 2002). On the basis of these occurrences, a maximum correlative range extending from the upper parts of the Tatarian Stage (?mid-Capitanian; Jin et al., 1997) to approximately the Permian-Triassic boundary could be suggested for OSPZ6.
The Changhsingian age for the ‘basal Khuff clastics’ tentatively suggested by Stephenson and Filatoff (2000b) was based partly on the occurrence of P. cancellosa, P. microcorpus and T. playfordii. Recent examination of specimens previously assigned to the latter taxon indicates that they may be better accommodated in T. cf. playfordii, pending comparative examination. However the presence of P. cancellosa and P. microcorpus can be confirmed. Playfordiaspora cancellosa, P. microcorpus and T. playfordii are considered in eastern Australia to be Changhsingian markers (Foster et al., 1998), though clearly the first occurrence of P. cancellosa might be inferred to be diachronous, since earlier, Foster and Jones (1994) suggested that in the European Alps, P. cancellosa had a first occurrence in the equivalent of the basal Tatarian. A reappraisal of these age calibrations is required based on recent Wordian dates for the Oman Khuff Formation (Dickens, 1999; Angiolini et al., 1998). This is because, in the subsurface, the latter formation overlies the Upper Gharif member, which yields undoubted specimens of P. cancellosa and P. microcorpus. The ‘Gharif Paleoflora’ beds, which underlie the Khuff Formation at the Northern Huqf outcrop area, also yield P. cancellosa (as P. crenulata) and P. microcorpus (see Broutin et al., 1995).
As suggested above, specimens similar to T. playfordii as well as P. cancellosa and P. microcorpus, also occur in the ‘basal Khuff clastics’ in central Saudi Arabia, where the unit is equivalent to the Ash-Shiqqah member, and is overlain by the Huqayl member. Vaslet et al. (in press) consider the Ash-Shiqqah member to be Capitanian in age, and the Huqayl member to be Wuchiapingian in age, based on foraminifera. These dates for associated sediments suggest that at least P. microcorpus and P. cancellosa, and probably T. playfordii, occur earlier in Oman and Saudi Arabia than in eastern Australia, or that some Permian – Triassic sequences in eastern Australia are wrongly dated.
The youngest Permian of eastern Australia is wholly non-marine (Foster et al., 1998), and hence independent age dates cannot be suggested for the eastern Australian sequences containing P. microcorpus, P. cancellosa and T. playfordii, (see Foster and Archbold, 2001). In Australia the reasoning behind assignment to a Late Permian age is as follows. The P. cancellosa, P. microcorpus and T. playfordii biozones are underlain by the eastern Australian Upper Stage 5 Palynozone (APP5; Price, 1997) from which have been recorded two faunas allowing an independent age date. One is the record of a single specimen of the ammonoid Cyclolobus persulcatus, from the Cherrabum Member of the Hardman Formation, in the Canning Basin, Western Australia (Glenister et al., 1990, see also Foster and Archbold, 2001), dated as ‘post Guadalupian’ by Glenister et al. (1990) and ‘Capitanian – Dzhulfian’ by Leonova (1998). The other is a record of ‘late Tatarian’ conchostracans in the Newcastle Coal Measures (Jones and Chen, 2000). Because the P. cancellosa, P. microcorpus and T. playfordii biozones are stratigraphically much higher than these two faunas, a latest Permian age has always been assumed by Australian workers (C. B. Foster, oral communication, 2001).
In addition, the presence of P. cancellosa, P. microcorpus and T. playfordii, solely in the Chhidru Formation (Changhsingian) of the Salt Range, Pakistan (Balme, 1970; and written communication to MHS 1998; note that range of P. cancellosa shown in fig. 16 of Balme (1970) results from drafting error) has been used by Australian workers to support a latest Permian age for the first occurrences of these taxa in eastern Australia (Foster, 1979, 1982). However, the sampling regime of Balme (1970) was not of a high enough resolution to ensure a detailed palynological succession for the Salt Range sequence; samples were studied from the Chhidru Formation, and the underlying Wargal and Amb formations (Capitanian-Wuchiapingian and Wordian respectively, see Wardlaw and Pogue, 1995), but only five samples were taken from the lower two formations. It is thus possible that the stratigraphic ranges of P. cancellosa, P. microcorpus and T. playfordii extend lower in the Salt Range than Balme (1970) originally indicated.
In conclusion, although P. cancellosa, P. microcorpus and probably T. playfordii occur in Middle Permian (Wordian – Capitanian) rocks in Arabia, at present it is not possible to be sure whether they occur early in Arabia or whether their reported Late Permian occurrences in eastern Australia are erroneous because the eastern Australian sequence as a whole is wrongly dated. What is certain is that the presence of P. cancellosa, P. microcorpus and possibly T. playfordii should not be taken as evidence of Changhsingian rocks. Further work on the occurrences of these taxa is required in other independently dated sections to be certain of their true age ranges. As discussed earlier, other macro- and microfossils of the ‘basal Khuff clastics’ do not allow a precise date for OSPZ6 and palynology can only suggest a late Tatarian to approximately end-Permian age range. For the present the Capitanian age suggested for the Ash-Shiqqah member (Vaslet et al., in press) is adopted.
Stratigraphic relationship between OSPZ5 and OSPZ6
The relative ages of OSPZ5 and OSPZ6 are difficult to determine by palynological means. Many taxa from OSPZ5, recovered from the lower and middle parts of the Upper Gharif member, are present in OSPZ6, which is associated with the central Saudi Arabian ‘basal Khuff clastics’, for example, D. insolitus, P. cancellosa and R. chalastus, but others e.g. T. cf. playfordii, are only present in OSPZ6. Similarly Cedripites sp. B, Distriatites sp. A and Indotriradites sp. C are present only in OSPZ5. The single published palynological account of higher parts of the Upper Gharif member in Oman (the ‘Gharif Paleoflora’; Broutin et al., 1995), and other unpublished consultancy data, do not contain enough information to ascertain whether there are significant palynological changes within the Upper Gharif member above the lower and middle parts, or if the section preserves the boundary between OSPZ5 and OSPZ6. There is also uncertainty over the relative positions of the subsurface Upper Gharif member and the outcrop ‘Gharif Paleoflora’ beds. Despite this uncertainty, the Capitanian age suggested for the Ash-Shiqqah member (Vaslet et al., in press), indicates that OSPZ6 may be significantly younger than the Wordian or pre-Wordian OSPZ5 assemblages. However, we recommend further study to establish this with certainty.
DISCUSSION AND RECOMMENDATIONS FOR FUTURE WORK
This work has illustrated some progress in integrating and refining the palynological succession in the Permian of Oman and Saudi Arabia. However, equally clear is the absence of palynological data for crucial sections in the middle and upper parts of the Gharif Formation in Oman, and their equivalent in Saudi Arabia. These barren sections are unlikely to be present in all areas of the Arabian Peninsula and other parts of the Middle East and Tethyan region, and hence it is vital that palynological information be gathered from such areas to allow the Permian palynological succession to be fully understood.
The Permian sequence of the Salt Range of western Pakistan is an ideal candidate for elucidating the Tethyan Permian palynological succession. The stratigraphy of the sequence is very well known and can be correlated by abundant micro- and macrofaunas to the Mid and Late Permian standard sections in the USA and China (see for example Wardlaw and Pogue, 1995). Unfortunately only one major palynological study, that of Balme (1970), has been carried out there. Balme (1970) studied samples from the Chhidru Formation, and the underlying Wargal and Amb formations, but sampled each formation only sparsely. Resampling of the whole sequence is required to understand better the crucial Middle and Upper Permian sequences of Oman and Saudi Arabia.
The palynological succession for the Permian of Oman and Saudi Arabia has been refined and integrated following extensive consultancy work integrating the data from the two major oil companies operating in this region. The succession has been subdivided into six major biozones covering the dominantly siliciclastic sequence of the Permian. The biozones are presented here as a framework on which to build. Most are incompletely defined in that their bases or tops are not defined, due to barrenness or lack of appropriate section. Similarly, the ages of some of the biozones are poorly constrained either because they lack age diagnostic taxa, or are defined by taxa that are as yet undescribed and hence cannot be related to international stratigraphical standards. Dating of the biozones has been considerably helped by new macrofaunal information from carbonate units in the Early and Middle Permian of Oman, but this is unlikely to allow precise dating of the intervening palynological biozones. To ensure such dating, new sections in the Middle East or wider Tethyan region, with well-established chronostratigraphy, for example the Permian sequence of the Salt Range, Pakistan, should be palynologically studied with a view to relating palynological assemblages to precisely dated rock units.
The authors are grateful to the managements of Petroleum Development Oman, the Saudi Arabian Oil Company, the Saudi Arabian Oil Ministry and the Oman Ministry of Oil and Gas for permission to publish this paper. M. H. Stephenson publishes with the permission of the Director, British Geological Survey. Moujahed Al-Husseini, Alan Heward, Jan Schreurs, Roger Price, Randall Penney and Jim Riding are thanked for reviewing and improving the paper. The design and drafting of the final graphics was by Gulf PetroLink.
APPENDIX 1: TAXON LIST
?Cadiospora magna Kosanke 1950
?Florinites balmei Stephenson and Filatoff, 2000
Ahrensisporites cristatus Playford and Powis, 1979
Alisporites cf. nuthallensis Clarke, 1965
Alisporites indarraensisSegroves, 1969
Alisporites tenuicorpusBalme, 1970
Anapiculatisporites concinnusPlayford, 1962
Ancistrospora inordinata Menéndez and Azcuy, 1972
Ancistrospora verrucosa Menéndez and Azcuy, 1972
Angulisporites cf. splendidus Bharadwaj, 1954
Barakarites rotatus (Balme and Hennelly) Bharadwaj and Tiwari, 1964
Brevitriletes cf. hennellyi Foster, 1975
Brevitriletes cornutus (Balme and Hennelly) Backhouse, 1991
Brevitriletes leptoacainaJones and Truswell, 1992
Brevitriletes levis (Balme and Hennelly) Bharadwaj and Srivastava, 1969
Brevitriletes parmatus (Balme and Hennelly) Backhouse, 1991
Cadiospora cf. magna Kosanke, 1950
Caheniasaccites ovatus Bose and Kar, 1966
Calamospora cf. microrugosa (Ibrahim) Schopf et al., 1944
Camptotriletes warchianusBalme, 1970
Cannanoropollis janakii Potonié and Sah, 1960
Cannanoropollis talchirensis (Lele and Makada) Stephenson and Osterloff, 2002
Cedripites priscusBalme, 1970
Cedripites sp. B (of consultancy reports) cf. Corisaccites alutas Venkatachala and Kar, 1966
Circumstriatites talchirensis Lele and Makada, 1972
Complexisporites polymorphus Jizba, 1962
Converrucosisporites sp. A (of Stephenson and Osterloff, 2002)
Converrucosisporites sp. B (of consultancy reports)
Corisaccites alutas Venkatachala and Kar, 1966
Corisaccites cf. alutas Venkatachala and Kar, 1966
Cristatisporites cf. crassilabratusArchangelsky and Gamerro, 1979
Cycadopites cymbatus (Balme and Hennelly) Segroves, 1970
Cyclogranisporites poxStephenson and Osterloff, 2002
Densipollenites indicus Bharadwaj, 1962
Dibolisporites disfaciesJones and Truswell, 1992
Distriatites insolitus Bharadwaj and Salujah, 1964
Distriatites sp. A (of consultancy reports) Divarisaccus sp. A (of Stephenson and Osterloff, 2002)
Florinites flaccidus Menéndez and Azcuy, 1973
Granulatisporites confluensArchangelsky and Gamerro, 1979
Hamiapollenites fusiformis Marques-Toigo, 1974
Horriditriletes ramosus (Balme and Hennelly) Bharadwaj and Salujah, 1964
Horriditriletes tereteangulatus (Balme and Hennelly) Backhouse, 1991
Horriditriletes uruguaiensis (Marques-Toigo) Archangelsky and Gamerro, 1979
Indotriradites apiculatusStephenson and Osterloff, 2002
Indotriradites sp. C (of consultancy reports)
Jayantisporites cf. conatus Lele and Makada, 1972
Kingiacolpites subcircularis Tiwari and Moiz, 1971
Klausipollenites cf. schaubergeri (Potonié and Klaus) Jansonius, 1962
Klausipollenites schaubergeri (Potonié and Klaus) Jansonius, 1962
Laevigatosporites callosusBalme, 1970
Laevigatosporites cf. callosusBalme, 1970
Lahirites karanpuraensis Bharadwaj and Dwivedi, 1981
Leiotriletes directus Balme and Hennelly, 1956
Leiotriletes virkkii Tiwari, 1965
Limitisporites rectus Leschik, 1956
Limitisporites rotundus Stapleton 1977
Lophotriletes sparsus Singh, 1964
Lueckisporites virkkiae ‘Norm A’ (see Visscher 1971)
Lueckisporites virkkiae Potonié and Klaus emend. Clarke, 1965
Lundbladispora braziliensis (Pant and Srivastava) Marques-Toigo and Pons, 1976
Lundbladispora gracilisStephenson and Osterloff, 2002
Microbaculispora grandegranulata Anderson, 1977
Microbaculispora tentula Tiwari, 1965
Pachytriletes densus Bose and Kar, 1966
Peroaletes khuffensis Stephenson and Filatoff, 2000
Platysaccus cf. queenslandi de Jersey, 1962
Playfordiaspora cancellosa (Playford and Dettmann) Maheshwari and Banerji, 1975
Plicatipollenites gondwanensis (Balme and Hennelly) Lele, 1964
Plicatipollenites malabarensis (Potonié and Sah) Foster, 1975
Potonieisporites novicus Bharadwaj, 1954
Protohaploxypinus amplus (Balme and Hennelly) Hart, 1964
Protohaploxypinus cf. microcorpus (Schaarschmidt) Clarke, 1965
Protohaploxypinus limpidus (Balme and Hennelly) Balme and Playford, 1967
Protohaploxypinus microcorpus (Schaarschmidt) Clarke, 1965
Protohaploxypinus uttingii Stephenson and Filatoff, 2000
Pseudoreticulatispora pseudoreticulata (Balme and Hennelly) Bharadwaj and Srivastava, 1969
Psomospora detecta Playford and Helby, 1968
Pteruchipollenites owensii Stephenson and Filatoff, 2000
Punctatisporites gretensis forma minor Hart, 1965
Pyramidosporites cyathodes Segroves, 1967
Reduviasporonites chalastus (Foster) Elsik, 1999
Retusotriletes nigritellus (Luber and Valts) Foster, 1979
Spelaeotriletes triangulus Neves and Owens, 1966
Striasulcites tectus Venkatachala and Kar, 1968
Striatoabieites cf. multistriatus (Balme and Hennelly) Hart, 1964
Striatoabieites multistriatus (Balme and Hennelly) Hart, 1964
Striatopodocarpites cancellatus (Balme and Hennelly) Hart, 1965
Striatopodocarpites fusus (Balme and Hennelly) Potonié, 1958
Strotersporites indicus Tiwari, 1965
Sulcatisporites ovatus (Balme and Hennelly) Bharadwaj, 1962
Sulcatisporites sp. A (of consultancy reports)
Taeniaesporites cf. pellucidus (Goubin) Balme, 1970
Taeniaesporites noviaulensis Leschik, 1956
Thymospora opaqua Singh, 1964
Triplexisporites cf. playfordii (de Jersey and Hamilton) Foster, 1979
Ulanisphaeridium omanensisStephenson and Osterloff, 2002
Vallatisporites arcuatus (Marques-Toigo) Archangelsky and Gamerro, 1979
Verrucosisporites andersonii Backhouse, 1988
Verrucosisporites cf. naumovae Hart, 1963
Vittatina costabilis Wilson, 1962
Weylandites lucifer (Bharadwaj and Salujah) Foster, 1975
ABOUT THE AUTHORS
Mike Stephenson is presently a stratigrapher with the British Geological Survey (BGS) in Nottingham, England. His education includes a BSc in Geology from Imperial College, an MSc and PhD in Palynology from the University of Sheffield, various postgraduate teaching qualifications and post graduate studies in development studies at the University of London. Mike’s scientific work is concerned mainly with the Palaeozoic stratigraphy of Arabia and he has published a number of papers on the region as well as working extensively as a consultant to oil companies in the area. Mike is also involved in computing applications in stratigraphy, and the palynostratigraphy, sedimentology and palaeoecology of Palaeozoic sequences from onshore and offshore northwest Europe, Australia and Africa. He has publications on Palaeozoic sequences in Ireland, Scotland and southern Africa. Mike spent much of the first part of his career in southern Africa working in education, development and publishing. At BGS he has kept his interest in development and education and has worked extensively with BGS on various development contracts for the World Bank and the Asian Development Bank in southeast Asia, central Asia and the Pacific. Mike is a member of the AASP, PESGB and TMS, and is currently Secretary General of the Commission Internationale de Microflore du Paléozoique.
Peter Osterloff is currently Team Leader, Stratigraphy with Sarawak Shell Berhad (SSB), Miri, Sarawak. Since receiving a PhD in 1993 in Carboniferous Palynostratigraphy from the University of Sheffield, England, Peter has worked at Shell Expro from 1982-1997 as a Carboniferous Palynostratigrapher as well as Stratigrapher for the Atlantic Margin and Central North Sea Tertiary. Transferred to Petroleum Development Oman (PDO) in 1997, he worked six years with PDO as a Senior (Bio)stratigrapher working on all aspects of the stratigraphic column. Peter relocated in March 2003 to SSB, swapping the Palaeozoic and Proterozoic of Oman for the Neogene of Offshore NW Borneo.Peter.
John Filatoff is a Geological Consultant with Saudi Aramco and Team Leader for the Palynology Support Group within the Geological Technical Services Division. His activities, since joining the company in 1992, focused initially on Tertiary, Red Sea exploration and, since 1994, on Palaeozoic deep-gas exploration in central and eastern Saudi Arabia. John has 30 years of oil industry experience, mostly in Australia, but also in Iran, Venezuela, and now Saudi Arabia. He has BSc and PhD degrees in Geology from the Universities of Queensland and Western Australia, respectively. John is a member of the AASP, Dhahran Geoscience Society, CIMP and GSA.