Ordovician Chitinozoans from Central Saudi Arabia
Published:January 01, 2000
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Florentin Paris, Jacques Verniers, Sa’id Al-Hajri, 2000. "Ordovician Chitinozoans from Central Saudi Arabia", Stratigraphic Palynology of the Palaeozoic of Saudi Arabia, Sa’id Al-Hajri, Bernard Owens
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Biostratigraphic investigations have been carried out on Ordovician chitinozoans mostly from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation in central Saudi Arabia. Among the 96 core samples processed from seven wells, about half of them (from wells Berri-84, Shedgum-239, Ain Dar-196 and Ain Dar-277) provided workable specimens, whereas the other wells, namely Ain Dar-281, Haradh-51 and Abu Jifan-25 were barren. Some chitinozoan species of Baltic affinities (e.g. Laufeldochitina striata) or of Laurentian affinities (e.g. Lagenochitina cf. pirum) are present, but most of the recovered chitinozoan species are of northern Gondwana affinities. These chitinozoans allow accurate correlation with the local chitinozoan biozones already established for northern Saudi Arabia. Precise correlation with some of the Ordovician chitinozoan biozones for the northern Gondwana regions can also be proposed (e.g. lower part of the pissotensis biozone). The available samples are not, however, sufficiently closely spaced for characterizing all the Ordovician chitinozoan biozones. It is not yet possible, therefore, to document eventual hiatuses in the Ordovician sedimentary succession of central Saudi Arabia. The investigated samples from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation are respectively dated as Ashgill, late Caradoc/earliest Ashgill, Caradoc and Llanvirn. Strata referred to the Sarah Formation are probably of topmost Ashgill age but may range into earliest Llandovery. Several new species have been observed. They are presently kept in open nomenclature until better preserved material is available for proposing well-documented diagnosis.
Ordovician strata crop out on the eastern border of the Arabian Precambrian Shield (Figure 1; Vaslet et al., 1987). They are also present in the sub-surface in northern, central and southern Saudi Arabia (see Stump et al., 1995 for references). The regional lithostratigraphy of the Ordovician System of central Arabia includes from the oldest to the youngest, the upper part of the Saq Formation, and the Hanadir, Kahfah, Ra’an and Quwarah members of the Qasim Formation, and the glaciogenic Sarah Formation (Vaslet, 1990; Al-Hajri, 1995).
Investigations into Saudi Arabian Ordovician chitinozoans are relatively recent and mainly concentrated in the northern part of the Kingdom (McClure, 1988; Al-Hajri, 1991, 1995) where the palynomorphs are well-preserved and abundant. Early Silurian chitinozoans, both from northern and central Saudi Arabia have, however, been extensively studied (McClure, 1988; Al-Hajri, 1991; Paris and Al-Hajri, 1995; Paris et al., 1995; Al-Hajri and Paris, 1998).
In central Saudi Arabia (Figure 1), the preservation of the Ordovician chitinozoans is moderate to poor (dark brown, opaque, and frequently broken vesicles). Productive samples occur in wells Berri-84 (BRRI-84), Shedgum-239 (SDGM-239), Ain Dar-196 (ANDR-196) and Ain Dar-277 (ANDR-277). Other samples from wells Ain Dar-281 (ANDR-281), Haradh-51 (HRDH-51) and Abu Jifan-25 (ABJF-25), regarded as Ordovician in age on lithological evidence, are barren. Finally, from a total of 96 processed samples from central Saudi Arabia, only 45 core samples yielded utilizable chitinozoans. In order to allow taxonomic comparison with better preserved material and to document regional biostratigraphic correlation, 46 additional fertile samples from northern Saudi Arabia have been processed from wells Kahf-1, and Tayma-1S, but not studied in great detail here.
For the processing of the samples and for Scanning Electron Microscope analyses the standard techniques described by Paris (1981) are adopted here. The terminology and the generic assignments of the observed chitinozoans correspond to the new classification proposed by Paris et al. (1999). Several new taxa have been observed and are kept in open nomenclature until better preserved material becomes available for a full taxonomic study. Furthermore, open nomenclature is also used for numerous other taxa because of their poor preservation. The illustrated specimens are stored in the Palaeontological Collections of the Natural History Museum, London, under the repository numbers FC 148 to FC 158.
The main biostratigraphical data for each investigated well are displayed in Figures 2 to 4. The abundance of chitinozoans in each processed core sample is expressed in the number of specimens per gram of rock. Due to the occurrence of numerous fragmented chitinozoans, this quantification must be regarded as fairly approximate. The chitinozoan biodiversity in each sample is also broadly quantified (5 classes of values: less than 3%, between 3 and 10%, between 10 and 20%, between 20 and 50% and more than 50%). The percentages are calculated exclusively using taxa identified both at species level, and in open nomenclature, i.e. on a variable fraction of the chitinozoans present depending on the preservation.
The studied wells (Berri-84, Ain Dar-277, Ain Dar-281, Shedgum-239, Ain Dar-196, Abu Jifan-25 and Haradh-51) are roughly located along a north-south line (Figure 1) from the Arabian Gulf (Berri-84), to the southeast of Riyadh (Haradh-51). Barren samples have been recovered respectively in the Saq and pre-Saq formations in well Haradh-51 (reddish to greenish shales and siltstones from cores 18 to 25), in strata referred to the lower part of the Kahfah Member in well Ain Dar-281 (core 16) and in reddish to greenish siltstones and sandstones in well Abu Jifan-25 (cores 6 and 11). However, it must be noted that core 1 of well Abu Jifan-25 has yielded wood debris, spores and pollen probably of Permian age.
Chitinozoan Assemblage in Well Shedgum-239
In this well, strata referred to the latest Ordovician on lithologic evidence occur between the Qusaiba ‘hot shale’ and the Quwarah Member of the Qasim Formation. The studied material is from core 8 (15,255.5 to 15,264.7 ft), which is located immediately below the base of the Qusaiba Member of the Qalibah Formation. Only three of the seven processed core samples provided chitinozoans (depth 15,255.5, 15,256.8 and 15,264.7 ft). The preservation is extremely poor and therefore firm identifications at species level are not possible. However, the composition of the chitinozoan assemblage is significantly different at 15,255.5 and 15,264.7 ft. The former assemblage is dominated by Conochitinidae (probably Belonechitina species with an eroded surface) associated with a few Cyathochitina sp. and possibly a Pterochitina sp. These Belonechitina are morphologically fairly close to those recorded in Assemblage 1 in well Ain Dar-196. The chitinozoan assemblage at depth 15,264.7 ft is practically composed exclusively of a Cyathochitina species with a short neck, and closely resembles the early Silurian Cyathochitina caputoi Costa. A few very small Belonechitina? sp. coexist with these Cyathochitina. Such assemblages, and the chitinozoan succession, display latest Ordovician and Early Silurian features as documented, for instance, in the Ordovician-Silurian boundary beds in Mauritania (Paris et al., 1998) where Cyathochitina spp. are dominant just above the ultimate glacial diamictites in beds yielding graptolites of the persculptus Zone (Underwood et al., 1998). However, due to the poor preservation, additional investigations are necessary before providing a definite age assignment for the chitinozoan Assemblage 1 from core 8 of well Shedgum-239.
Chitinozoan Assemblage in Well Berri-84
In the present study the youngest definite Ordovician chitinozoan bearing samples are from core 29 in well Berri-84 (Figure 2). The upper part of this core (not processed yet) corresponds to the Qusaiba ‘hot shale’ usually referred to the Lower Silurian (Paris et al., 1995). The lower part of this core belongs to the Upper Ordovician. However, a stratigraphic hiatus related to the topmost Ordovician glacial event (see references in Vaslet, 1990, and in Al-Hajri, 1995) may occur in this core.
The chitinozoans recorded in the five investigated samples (16,638.7 ft to 16,661.2 ft) are very poorly preserved (broken processes, opaque vesicle) and moderately abundant (12 to 120 specimens per gram of rock). The association of Armoricochitina nigerica (Bouché, 1995) (Plate 1: i), Acanthochitina sp. nov. 1 (Plate 1: j) and a Belonechitina form with random spines and rounded anti-apertural end (Plate 1: a, c) which may represent more than 50% of the identified taxa (e.g. 16,656.4 and 16,661.2 ft), is regarded as representative of Assemblage 2. Among the subordinate taxa are a few Calpichitina lenticularis (Bouché), rare Ancyrochitina longispina Achab, Ancyrochitininae with broken spines resembling A. merga Jenkins (Plate 1: d), a few doubtful Plectochitina sylvanica Jenkins (broken processes), large Euconochitina sp. 1. Some representatives of the genus Spinachitina may also be present but the crown of spines on the margin is not preserved. A peculiar form provisionally labelled Kalochitina sp. aff. hirsuta (Laufeld) (Plate 1: e) is represented at depths 16,658.0 and 16,661.2 ft. It is characterised by a flaring collarette and long slim lambda spines. It resembles specimens recorded by Elaouad-Debbaj (1984, Plate 1, Figures 2, 6 and 7) in the Upper Ktaoua Formation, in Morocco (Rawtheyan).
Accurate stratigraphical assignment is difficult because no late Ashgillian index species have been firmly identified due to the poor preservation. Nevertheless, the occurrence of abundant A. nigerica supports an early to ‘middle’ Ashgill age, possibly the merga biozone of Paris (1990). This assemblage has several taxa in common with the late Ordovician chitinozoan assemblage recovered by Molyneux and Paris (1985) and Paris (1988) in eastern Libya.
Chitinozoan Assemblage in Well Ain Dar-277
This well is located northeast of Riyadh (Figure 1). It penetrated Ordovician strata, i.e. respectively the Ra’an, and Kahfah members of the Qasim Formation directly under the Permian unconformity. The available Ordovician core material (core 8) is suggested to be from the Ra’an Member.
The chitinozoans recovered in the 12 processed samples (14,274.6 to 14,331.5 ft) from core 8 (Figure 3) are dominated by Tanuchitina fistulosa (Taugourdeau and de Jekhowsky) (Plate 2: a, b, c) and a form of Euconochitina (Plate 2: m). The latter represents more than 50% of the chitinozoan assemblage at depth 14,274.6 ft, 14,303.1 and 14,316.5 (Figure 3). It resembles Euconochitina communis (Taugourdeau). These two taxa are regarded as characterising Assemblage 3. They are associated with subordinate taxa such as Calpichitina lenticularis (Bouché), a few Fungochitina fungiformis (Eisenack) (Plate 2: e, j) and Belonechitina sp. A with thick random spines close to its margin (Plate 2: d, g). Also in this assemblage is Tanuchitina sp. with a short carina (Plate 2: i) which is similar to that reported as Cingulochitina sp. from northwestern Saudi Arabia (Al-Hajri 1991, Plate 24: a to f), and several forms of Conochitina. (e.g. Conochitina sp. 1, Plate 2: h). A special mention is made to Velatachitina sp. nov. A (= new Genus 1, new sp. 1 in Al-Hajri 1991, Plate 19: a, b, c) because of its peculiar morphology (Plate 2: f, k) and its range in the Late Ordovician of central (e.g. Ain Dar-196) and northern Saudi Arabia (Al-Hajri, 1991).
Most of the taxa recorded in Assemblage 3 are not significant for chronostratigraphic purposes. However, the low abundance of Fungochitina fungiformis, the index species of the late Caradoc in Baltoscandia (Nõlvak and Grahn, 1993), together with the great abundance of Tanuchitina fistulosa suggests an age assignment close to the Caradoc-Ashgill boundary. Indeed, rare and poorly preserved specimens of Tanuchitina fistulosa have been tentatively identified as T. ontariensis Jansonius in the lower part of the Agadir-Tissinnt Member of the Lower Ktaoua Formation in Morocco (Elaouad-Debbaj, 1986). At this locality, T. fistulosa seems to coexist with the first Armoricochitina nigerica (Bouché). In northwestern Saudi Arabia, Tanuchitna fistulosa was chosen as the index species of the late Caradoc Armoricochitina n.sp. aff. fistulosa biozone (Al-Hajri, 1995). At that locality it was reported to disappear in the early part of the A. nigerica biozone. Therefore, because of its short range and its wide distribution in northern Gondwana localities, T. fistulosa can be selected as the index species of a new chitinozoan Biozone (Figure 5) within the open interval still existing between the robusta and the nigerica biozones in the northern Gondwana biozonation of Paris (1990).
Chitinozoan Assemblages in Well Ain Dar-196
This well is located close to Ain Dar-277 (Figure 1). It penetrates Ordovician strata, directly under the Permian unconformity, and terminates in the basement (cores 28 and 29). The investigated Ordovician succession (cores 23 to 26) includes from the youngest to the oldest: post-Ra’an strata (cores 23 and 24), lower part of the Ra’an and upper part of the Kahfah members (core 25) and the Hanadir Member of the Qasim Formation (core 26).
The preservation is moderately good and the abundance of the chitinozoans increases significantly from the Ra’an and Kahfah members (0? to 78 specimens per gram of rock) to the Hanadir Member with values ranging from 50 to 250 specimens per gram of rock in core 26. Three chitinozoan assemblages have been observed respectively in cores 24, 25 and 26 (Figure 4).
The first assemblage is monospecific and ranges from depth 13,711.8 ft to 13,731.4 ft in core 24, i.e. in strata overlying the Ra’an Member of the Qasim Formation. The index species is Belonechitina sp. 1 (Plate 1: g, h, k), a form with densely distributed minute spines, and resembling Belonechitina postrobusta (Nestor). A Late Ordovician-lowermost Silurian age is likely based on the age assignment of the underlying assemblage and with chitinozoans of Assemblage 1 recovered in well Shedgum-239 (see above).
Assemblage 3A occurs in the upper half of core 25 (14,100.6 ft to 14,1110.7 and possibly to 14,116.9 ft), i.e. in the lower part of the Ra’an Member of the Qasim Formation. The diagnostic species are Tanuchitina fistulosa (Taugourdeau and de Jekhowsky) (Plate 3: b) and Fungochitina fungiformis (Eisenack) (Plate 3: h). The accompanying taxa are Calpichitina lenticularis (represented by thin-walled individuals), Tanuchitina sp., Conochitina sp. A corresponding to a very large stubby form (Plate 3: d), and Conochitina sp. 2 (Plate 3: a). A few Desmochitina e.g. minor and representatives of the genus Euconochitina are also present in Assemblage 3A. Velatachitina sp. nov. A is only recorded at depth 14,106.2 ft. None of these taxa gives a firm chronostratigraphic assignment, but the association of some species (e.g. T. fistulosa, Velatachitina sp. nov. A) with Acanthochitina barbata Eisenack in other Saudi Arabian wells suggests a late Caradoc-early Ashgill age for this assemblage. Fungochitina fungiformis, however, is the index species of the late Caradoc chitinozoan biozone in Baltoscandia (Nõlvak and Grahn, 1993).
Assemblage 3B corresponds to the lower part of core 25 (14,129.6 ft to 14,133.0 ft). It does not contain diagnostic taxa. However, it is separated here from Assemblage 3A because T. fistulosa seems absent in samples at depth 14,129.6 ft and 14,129.6 ft. Poor preservation prevents firm identification of the Tanuchitina recorded at 14,133.0 ft. This assemblage is probably of the same age as Assemblage 3A.
Assemblage 4 ranges at least in the whole core 26 from 14,773.2 ft to 14,832.2 ft, i.e. in the Hanadir Member of the Qasim Formation. The diagnostic taxa are Linochitina pissotensis Paris (Plate 3: i; Plate 4: k), abundant throughout the assemblage (more than 50% of the species identified at depth 14,773.2, 14,789.0 and 14,828.7 ft) and Laufeldochitina clavata (Jenkins) (Plate 4: a, b), which is also very abundant in core 26 (more than 50% of the identified species at depth 14,780.3 and 14,801.8 ft). The absence of the latter species in the younger sample (14,773.2 ft) of core 26 must be noted. It possibly indicates the last occurrence of L. clavata. However, in that case the disappearance down there of L. clavata, would be much more dramatic than usual when its relative abundance decreases slowly. Beside these two stratigraphically important and abundant species are subordinate ones (Figure 4). Some are important at the level of inter-plate correlation, e.g., Pterochitina retracta Eisenack (Plate 3: j) and Laufeldochitina striata (Eisenack) (Plate 3: f, g, k), both recorded in Baltoscandia (Nõlvak and Grahn, 1993). Others such as Lagenochitina cf. pirum Achab (Plate 4: g, h, i) and Cyathochitina cf. jenkinsi Neville seem very close to species recorded by
Neville (1974) in Nova Scotia and reported by Achab (1988) in Québec. The occurrence of Saharochitina jaglini (Oulebsir and Paris) in Assemblage 4 is also of interest as this species does not extend higher than the lowest part of the pissotensis biozone in Algeria (Oulebsir, 1992; Oulebsir and Paris, 1995).
The range of these different taxa suggests that core 26 is close to the boundary between the clavata and the pissotensis biozones, i.e. close to the upper limit of the Abereiddian (Paris, 1990; Oulebsir and Paris, 1995). This means that the corresponding part of the Hanadir Member is of late Abereiddian age and is in agreement with the joint occurrence of L. pissotensis and of the graptolite Didymograptus murchisoni recently documented in northwestern Saudi Arabia (Al-Hajri, 1995) and in Spain (Gutiérrez-Marco et al., 1996). However, it must be stressed that the base of the Llandeilian, and therefore the top of the Abereiddian, corresponds to the first occurrence of G. teretiusculus (see Fortey et al., 1995) and not to the last occurrence of D. murchisoni as this graptolite may range fairly high within the teretiusculus biozone (see discussion in Melou et al., 1999).
The chitinozoans recovered in several wells from central Saudi Arabia allow local biostratigraphic subdivisions and tentative chronostratigraphic assignment (Figure 5).
Assemblage 1 containing the youngest chitinozoan recovered during this study, i.e. from post-Quwarah to pre-Qusaiba strata (Sarah Formation), is represented in well Shedgum-239 where core 8 possibly extends into the lower Rhuddanian. A similar conclusion is tentatively proposed for core 24 of well Ain Dar-196 with the Belonechitina sp. 1 assemblage. However, it must be stressed that Spinachitina fragilis (Nestor), the chitinozoan index species for the base of the Rhuddanian (Verniers et al., 1995), has not been identified in these two wells. Assemblage 2, with numerous A. nigerica is clearly of Ashgill age. It is documented in core 29 of well Berri-84. Assemblage 3, with T. fistulosa, is represented both in core 8 of well Ain Dar-277 and in core 25 of well Ain Dar-196, i.e. in the Ra’an Member. It is of late Caradoc, and possibly earliest Ashgill age. Assemblage 4 is present in core 26 of well Ain Dar-196, i.e from the middle part of the Hanadir Member. It corresponds to the Llanvirn and more precisely to the late Abereiddian-earliest Llandeilian.
The three last assemblages (Assemblages 2, 3 and 4) are respectively correlated (Figure 5) with the A. merga/A. nigerica, the A. n. sp. aff. fistulosa and L. pissotensis of the Saudi Arabian chitinozoan biozones proposed by Al-Hajri (1995, Figure 5) on material mostly from northern Saudi Arabia, especially from wells Tayma-1 and Kahf-1 (Al-Hajri, 1995). It must be noted that Assemblage 1 has not been identified yet in these two wells (Paris unpublished, 1998) and therefore, additional investigations are needed to clarify the problem of the age of the first post-glacial deposits in central Saudi Arabia.
List of recorded Chitinozoan taxa
Acanthochitina sp. nov. 1
Ancyrochitina longispina Achab, 1978
Armoricochitina nigerica Bouché, 1965
Belonechitina micracantha Eisenack, 1931
Belonechitina sp. 1
Belonechitina sp. 2
Belonechitina sp. 3
Belonechitina sp. 4
Belonechitina sp. A
Calpichitina lenticularis Bouché, 1965
Conochitina elegans Eisenack, 1931
Conochitina minnesotensis Stauffer, 1933
Conochitina sp. 1
Conochitina sp. 2
Conochitina sp. 3
Conochitina sp. 4
Conochitina sp. A
Conochitina sp. indet
Cyathochitina campanulaeformis Eisenack, 1931
Cyathochitina cf. jenkinsi Neville, 1974
Desmochitina erinacea Eisenack, 1931
Desmochitina minor Eisenack, 1931
Euconochitina cf. communis Taugourdeau, 1961
Euconochitina sp. 1
Euconochitina sp. 2
Fungochitina fungiformis Eisenack, 1931
Kalochitina sp. aff. hirsuta Laufeld, 1967
Lagenochitina cf. pirum Achab, 1982
Lagenochitina sp. 1
Laufeldochitina clavata Jenkins, 1967
Laufeldochitina striata Eisenack, 1937
Linochitina pissotensis Paris, 1981
Plectochitina sylvanica Jenkins, 1970
Pterochitina retracta Eisenack, 1955
Rhabdochitina gracilis Eisenack, 1962
Saharochitina jaglini Oulebsir and Paris, 1993
Tanuchitina fistulosa Taugourdeau and de Jekhowsky, 1960
Velatachitina sp. nov. A
The authors are grateful to Saudi Aramco and the Ministry of Petroleum and Mineral Resources, Saudi Arabia, for granting permission to publish this study, and for providing the rock samples. They acknowledge Danièle Bernard (University of Rennes) and Sabine Van Cauwenberghe (University of Gent) for processing the samples. Danièle Bernard also prepared the digitized plates. The authors thank Dr. Bernard Owens (British Geological Survey, Nottingham) and the reviewers for their valuable comments and English improvement.
ABOUT THE AUTHORS
Florentin Paris is Director of Research at the French CNRS at Géosciences-Rennes (France). He received his doctoral thesis at Rennes University in 1980 and works on Ordovician to Devonian biostratigraphy, palaeoenvironments and palaeogeography, using chitinozoans. Florentin recently initiated investigations on Early Palaeozoic biodiversition events (co-leader of IGCP no. 410). His research focuses on northern Gondwana regions, South America, China and northern Europe. He is a member of the International Sub-commissions on Stratigraphy and is councillor of IFPS.
Jacques Verniers is Research Director in the field of Palaeontology at the University of Gent from where he obtained his first degree in 1971 and his PhD in 1976. Jacques has worked extensively on the Ordovician and Silurian in Belgium and also has experience of working on Permian and Triassic spores and pollen from the Mozambique coal basins.
Sa’id Al-Hajri (see page 17)