Latest Devonian and Early Carboniferous Miospore Assemblages from Saudi Arabia
Published:January 01, 2000
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Geoffrey Clayton, Bernard Owens, Sa’id Al-Hajri, John Filatoff, 2000. "Latest Devonian and Early Carboniferous Miospore Assemblages from Saudi Arabia", Stratigraphic Palynology of the Palaeozoic of Saudi Arabia, Sa’id Al-Hajri, Bernard Owens
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Upper Devonian and Lower Carboniferous miospore assemblages are described from two wells in Saudi Arabia. Well Abu Safah-29 (ABSF-29), located in the Arabian Gulf, 530 kilometres northeast of Riyadh, penetrated Lower Carboniferous and Upper Devonian sediments. Well preserved but highly carbonised miospore assemblages represent the Visean, lower Tournaisian and Upper Devonian in this section. Reworked uppermost Devonian (Strunian) taxa are common in the Tournaisian assemblages. Well Harmaliyah-51 (HRML-51), located east of the Ghawar oil field in eastern Saudi Arabia (about 300 kilometres east of Riyadh), contains well preserved Strunian assemblages dominated quantitatively by Retispora lepidophyta, and deeper in the well, poor assemblages of Upper Devonian (pre-Strunian) age. Erosion or non-deposition has resulted in strata including the Devonian-Carboniferous boundary not being preserved in either of the well sections investigated.
Well preserved but carbonised miospores have been recovered from two Saudi wells, Abu Safah–29, drilled 530 kilometres (km) northeast of Riyadh in the Arabian Gulf, and Harmaliyah-51, 300 km east of Riyadh, to the east of the Ghawar oil field (Figure 1). The Early Carboniferous-latest Devonian section in the former well was extensively cored but only ditch cuttings samples were recovered from the latter. Preliminary results from Abu Safah-29 were published by Clayton (1995) together with data from wells ST-8 and Ar’ar-1 (ARAR-1) in the northwest of Saudi Arabia.
Taxa recorded are shown in Table 1. Selected taxa are illustrated in Plate 1 and their stratigraphic distribution shown in Figures 2 and 3. The depth given for ditch cuttings samples refers to the top of the interval sampled.
COMPOSITION OF THE MIOSPORE ASSEMBLAGES
Well Abu Safah-29
The sample positions and lithostratigraphy from Abu Safah-29 are shown in Figure 2. Only poorly preserved miospores were obtained from the Carboniferous Berwath Formation above core 13. Taxa recorded include Aratrisporites saharaensis, Prolycospora rugulosa, Spelaeotriletes arenaceus, Radiizonates genuinus and Vallatisporites agadesi. This assemblage is assigned to the RT Biozone of Visean age originally erected in Libya (Loboziak and Clayton, 1988).
Assemblages from 15,290.0-15,331.0 feet (ft) (core 13) and from a cuttings sample at 15,280.0 ft from the Jubah Formation all contain abundant Indotriradites explanatus. Retispora lepidophyta occurs in small numbers and is considered reworked. Convolutispora spp., Vallatisporites verrucosus and Perotrilites spp. are all common in this ‘Indotriradites explanatus Assemblage’.
Assemblages recovered from the upper part of the Jubah Formation at 15,539.0 ft and 15,589.0 ft, both from core 14, are dominated by Verruciretusispora famenensis, Cymbosporites sp. and Retusotriletes spp. They also include poorly preserved Ancyrospora spp. and Hystricosporites spp., normally with broken processes. Also present in these assemblages are Rugospora flexuosa and R. radiata but Retispora lepidophyta is absent. This assemblage is referred to as the ‘Verruciretusispora famenensis Assemblage’.
Assemblages from cuttings samples from depths 15,700.0-15,900.0 ft from the uppermost part of the Jubah Formation are all dominated (more than 90%) by Retispora lepidophyta. Other taxa include Densosporites spitsbergensis, Indotriradites explanatus, Perotrilites sp. and Vallatisporites pusillites. The genera Ancyrospora and Hystricosporites are present but too poorly preserved to permit identification to species level. Vallatisporites verrucosus was recorded at 15,730.0 ft and at 15,800.0 ft together with Verrucosisporites nitidus. These assemblages are collectively referred to as the ‘Retispora lepidophyta Assemblage’ (= ‘D0’ in Al-Hajri et al., 1999).
AGE AND CORRELATION OF THE ASSEMBLAGES
Verruciretusispora famenensis Assemblage
This assemblage can be correlated with similar assemblages in North Africa, for example, ‘Palynozone 10’ of Grignani et al. (1992) from the Al Kufrah Basin of Libya. It is in part the correlative of the Diducites versabilis-Grandispora cornuta (VCo) Biozone of Europe but differs significantly from this in that typical taxa including Diducites versabilis and Grandispora cornuta have not yet been recorded from the Abu Safah-29 samples. Perotrilites sp. in the present study may prove to be synonymous with Diaphanospora rugosa Naumova, a common constituent of VCo assemblages in Brazil (Loboziak et al., 1997) but the former taxon may be somewhat different in terms of the relative size and the totally laevigate nature of its exoexine. Further work is in progress on this part of the succession.
Retispora lepidophyta Assemblage
These assemblages can be confidently assigned to the uppermost Devonian (Fa2d). Similar assemblages have been described from many sections in North Africa, see for example Coquel and Latreche (1989). The first appearance of Indotriradites explanatus after the first appearance of Retispora lepidophyta seems to be a consistent feature of the northern Gondwanan miospore succession and may be tentatively correlated with the base of the Retispora lepidophyta-Indotriradites explanatus (LE) Biozone of Western Europe.
The first appearance of Verrucosisporites nitidus defines the base of the uppermost Devonian Retispora lepidophyta-Verrucosisporites nitidus (LN) Biozone in Western Europe, with Vallatisporites verrucosus typically appearing at the same level. The first appearances of these two taxa, however, are less certain in northern Gondwanan sections. Unfortunately, all the assemblages from Harmaliyah-51 are from cuttings samples and the extent of caving could be considerable. While it appears likely that at least the highest assemblages are equivalent in age to the LE and possibly LN Biozones of Europe, the age of the lower assemblages could be older.
Indotriradites explanatus Assemblage
These assemblages contain several taxa that persist from the uppermost Devonian, including Indotriradites explanatus, Verrucosisporites nitidus, Verrucosisporites verrucosus and Retusotriletes spp. Retispora lepidophyta occurs in small numbers, as it does in many Saudi Arabia assemblages through the whole Carboniferous section. These specimens are considered reworked. This assemblage is considered to be Tournaisian (probably early Tournaisian) in age and is similar in composition to ‘Palynozone 12’ of Grignani et al. (1992), described from a short (31-metre) interval in a single well from the Al Kufrah Basin of Libya.
The absence of the ‘R. lepidophyta Assemblage’ in Abu Safah-29 suggests either non-deposition or removal by erosion of the uppermost Devonian (‘Strunian’) in this area. The common records of reworked specimens of R. lepidophyta throughout the Carboniferous section probably support the latter explanation.
The poorly preserved assemblages from the upper part of the Carboniferous section in Abu Safah-29 include Aratrisporites saharaensis, Prolycospora rugulosa, Spelaeotriletes arenaceus, Radiizonates genuinus and Vallatisporites agadesi but lack monosaccate pollen. These resemble much more diverse assemblages from Saudi wells Ar’ar-1 and ST-8 and can be assigned to the late Visean/early Namurian RT Biozone of Libya (Clayton, 1995). There is no evidence in Abu Safah-29 of assemblages that could be assigned to the late Tournaisian/Visean VP, PO or OA Biozones of Libya.
Taxa that characterise the upper Tournaisian/Visean of North Africa and the Middle East include Aratrisporites saharaensis, Radiizonates genuinus and Vallatisporites agadesi. None of these are present in the ‘Indotriradites explanatus Assemblage’ from Abu Safah-29. The implications of this are far-reaching in terms of proving the existence of a non-sequence or hiatus separating the Devonian and Carboniferous successions in many parts of the region.
In neighbouring northeast Syria, Ravn et al. (1994) described a ‘Middle Assemblage’ containing the above ‘Visean’ taxa from strata immediately overlying rocks containing a latest Devonian ‘Lower Assemblage’ including Retispora lepidophyta, with no sharp break indicated by the electric logs. These authors tentatively suggested a Tournaisian rather than Visean age for the assemblages with no discontinuity separating the Devonian and Carboniferous sequences. The ‘I. explanatus Assemblage’ from Abu Safah-29 is clearly younger than the ‘Lower Assemblage’ of Ravn et al. (1994) but older than the ‘Middle Assemblage’. This strongly suggests a disconformable relationship between the Devonian and Carboniferous with at least part of the Tournaisian Series not represented. A more significant discontinuity separates the late Visean or early Namurian RT Biozone assemblages from the underlying (?early) Tournaisian I. explanatus Assemblage.
Miospore Taxa Recorded
Aratrisporites saharaensis Loboziak, Clayton and Owens, 1986
Auroraspora macra Sullivan, 1968
Calamospora liquida Kosanke, 1950
Calamospora pallida (Loose) Schopf, Wilson and Bentall, 1944
Calamospora parva Guennel, 1958
Convolutispora caliginosa Clayton and Keegan, 1982
Convolutispora flexuosa forma minor Hacquebard, 1957
Convolutispora major (Kedo) Turnau, 1978
Convolutispora vermiformis Hughes and Playford, 1961
Cordylosporites marciae Playford and Satterthwait, 1985
Cristatisporites menendezii (Menendez and Azcuy) Playford, 1978
Densosporites claytonii Ravn, McPhilemy, Rutherford, Talli and Bahra, 1994
Densosporites spitsbergensis Playford, 1963
Diducites mucronatus (Kedo) emended Van Veen, 1981
Dictyotriletes cf. trivialis Naumova ex Kedo, 1963
Emphanisporites hibernicus Clayton, Higgs and Keegan, 1977
Emphanisporites rotatus McGregor emended McGregor, 1973
Indotriradites explanatus (Luber) Playford, 1991
Knoxisporites literatus (Waltz) Playford, 1963
Perotrilites magnus Hughes and Playford, 1961
Perotrilites perinatus Hughes and Playford, 1961
Prolycospora rugulosa (Butterworth and Spinner) Turnau, 1978
Punctatisporites irrasus Hacquebard, 1957
Punctatisporites minutus Kosanke, 1950
Radiizonates genuinus (Jushko) Loboziak and Alpern, 1978
Raistrickia variabilis Dolby and Neves, 1970
Retispora lepidophyta (Kedo) Playford, 1976
Retusotriletes crassus Clayton, Johnston, Sevastopulo and Smith, 1980
Retusotriletes incohatus Sullivan, 1964
Retusotriletes leptocentrum Higgs, 1975
Retusotriletes planus Dolby and Neves, 1970
Retusotriletes triangulatus (Streel) Streel, 1967
Rugospora flexuosa (Jushko) Streel, 1974
Rugospora radiata (Jushko) Byvsheva, 1985
Spelaeotriletes arenaceus Neves and Owens, 1966
Tumulispora malevkensis (Kedo) Turnau, 1978
Vallatisporites agadesi Loboziak and Alpern, 1978
Vallatisporites pusillites (Kedo) Dolby and Neves emended Byvsheva, 1985
Vallatisporites vallatus Hacquebard, 1957
Vallatisporites verrucosus Hacquebard, 1957
Verruciretusispora famenensis (Kedo) Owens, 1971
Verrucosisporites nitidus (Naumova) Playford, 1964
The authors acknowledge with gratitude the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company for permission to publish this study.
ABOUT THE AUTHORS
Geoffrey Clayton is Associate Professor of Geology at Trinity College, Dublin, and was President of the Commission Internationale de Microflore du Paléozoïque. He is involved in palynology and organic maturation projects in Saudi Arabia, the USA and Europe. Geoffrey received his PhD in Geology from the University of Sheffield in 1972.
Bernard Owens (see page 17)
Sa’id Al-Hajri (see page 17)
John Filatoff (see page 167)