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Alain Le Hérissé, 2000. "Characteristics of the Acritarch Recovery in the Early Silurian of Saudi Arabia", Stratigraphic Palynology of the Palaeozoic of Saudi Arabia, Sa’id Al-Hajri, Bernard Owens
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Silurian core samples from the central region of Saudi Arabia provide an excellent insight into the temporal evolution of acritarchs and other related forms. Based on palaeogeographical reconstructions, the area was situated in a temperate region on the eastern margin of Gondwana. The core samples have furnished the basis for site-to-site sequence correlations that made use of biostratigraphy. The composition of the microflora changes rapidly and distinctly, and the palynological composition of acritarchs and related forms reflect the nature of changes in the depositional environments immediately above the uppermost Ordovician. The uppermost Ordovician and lowermost Silurian interval is characterised by a low diversity of simple forms of acritarchs and some prasinophytes. The acritarchs recovery is marked by the appearance of innovative morphologies in the atavus-acinaces Zones above the “hot shales” interval, and distinct microfloral turnovers occurred later. Some similarities are evident between the Rhuddanian assemblages of Saudi Arabia and those of North America. By comparison with the acritarch biozonation established in the type areas of Great Britain, the precocious appearance of taxa such as Carminella, Leprotolypa gordonense, Visbysphaera, and others, occurs in this part of the northern Gondwanan margin. Characteristic Telychian species allow precise correlations with the Silurian sequences of southwestern Libya and Parnaiba Basin of Brazil. A readjustment of the age of the previously defined acritarch Biozones 6 and 7 (Le Hérissé et al., 1995) is proposed taking account of the details and stratigraphic precision revealed here by the study of additional samples.
This study attempts to integrate two different aspects of the acritarchs in Saudi Arabia (Figure 1). They are:
(1) the evolution of biodiversity (radiation and survivorship patterns) in acritarch assemblages and related forms (Prasinophyceae, Chlorophyceae, Zygnemataceae) in the Lower Silurian Qalibah Formation of Saudi Arabia (Figure 2); and
(2) their use and importance in biostratigraphic zonation and regional correlations, as regards other microfossil groups such as the Chitinozoa.
The palynomorph assemblages from the Qalibah Formation appear to be largely influenced by the periodic environmental changes recorded from the early to late Llandovery. This study has detected the environmental signals useful for interpreting the local distribution of palynomorphs, and has refined the biostratigraphy. As a result of this investigation, the similarities and differences that emerge from the study of the acritarch distribution can be compared with the assemblages described elsewhere from different palaeocontinents, palaeolatitudes, and palaeoenvironmental settings.
The Early Silurian Qalibah Formation of Saudi Arabia, has a potential as a petroleum source rock. It consists in ascending order of the Qusaiba and Sharawra members. The Qusaiba Member rests unconformably on the Upper Ordovician Quwarah Member of the Qasim Formation, or on the Sarah Formation. The latter represents a glacio-fluvial depositional phase preserved in palaeovalleys or as a marginal-marine facies. It is dated as Late Ordovician or earliest Silurian (Vaslet, 1990; Molyneux and Al-Hajri, this volume). Similar Late Ordovician-Early Silurian deposits are also present in southern Jordan (Powell et al., 1994). In Saudi Arabia, the Qusaiba Member consists mainly of organic-rich graptolitic shale. The shale facies was deposited across the margins of Gondwana as a result of the world-wide marine transgression that began in the latest Ordovician and was caused by deglaciation.
The model for the deposition of the Qusaiba Member is of great interest in this analysis. On a regional basis the Lower Silurian clastics represent a progradational sequence that was deposited on a broad continental shelf off eastern Gondwana. The Qusaiba accumulated as the offshore graptolite-bearing clays of an immense delta complex, and the siltstones and micaceous sandstones of the overlying Sharawra Member represent the pro-deltaic facies. The Qusaiba Member includes a basal, euxinic, black “hot shale” that is considered to be the source rock of the Palaeozoic hydrocarbons of Saudi Arabia. The laminated basal shales are overlain by silty micaceous shales, and by fine-grained sandstones that become predominant in the upper part (Mahmoud et al., 1992; Stump et al., 1995).
The periodic alternations between marine and continental palynomorph ratios throughout the Qusaiba Member are consistent with changes in the depositional regime. The fluvio-deltaic, marine- or riverdominated system was subject to frequent terrestrial discharges, and was influenced by climatic and environmental changes. Similar trends can be observed today in the recent evolution of deltaic systems, such as the huge Zaire deep-sea fan delta off West Africa (Droz et al., 1996; Le Hérissé, unpublished data). Modern delta systems provide comparative data that are important in understanding the distribution patterns of palynomorphs on ancient oceanic margins, together with factors controlling the sediment supply and accumulation in deep basins.
EVOLUTIONARY SUCCESSION OF ACRITARCHS IN THE LLANDOVERY
For this study, investigations have concentrated on the Early Silurian acritarchs recorded from selected boreholes with good chronostratigraphic control provided by chitinozoa in the work of Paris et al. (1995) and Paris and Al-Hajri (1995). The acritarch zonation is correlated with the Llandoverian chitinozoan zonation proposed in the two papers cited above. Unfortunately data on graptolites are scarce in Saudi Arabia. In the Appendix the reference to the standard graptolite zonation is only indicated for the discussion of correlations. The present study presents data and calibrations which complement the zonal scheme proposed by Le Hérissé et al. (1995).
Many boreholes were examined in order to establish a high-resolution chronostratigraphic framework, and to ascertain the evolutionary succession of the acritarchs. Nevertheless, it is necessary to be fully aware of discontinuities that exist in this type of sampling. The same is true for other groups of palynomorphs. The stratigraphic distribution of some species of acritarchs can be influenced by the evolution of the depositional sequence and emphasis is placed on species that are environmentally sensitive. Also, the stratigraphic distribution of acritarchs has been scrutinised against artefacts imposed by the sampling procedure or by the depositional environment.
The earliest Silurian acritarch assemblages of the acuminatus Zone: preliminary results from the Mukassir-1 borehole
The first documented Llandoverian chitinozoan biozone (Spinachitina fragilis Zone) (Verniers et al., 1995), encountered in Saudi Arabia (Paris et al., 1995), corresponds to the earliest Rhuddanian acuminatus Zone (Cocks and Rickards, 1988).
The S. fragilis biozone is present through the light-grey siltstones at the base of the Qusaiba Member of the Qalibah Formation within the “hot shale” horizon that coincides with dark graptolitic shales. The biozone has a very short time interval. Core 12 in Mukassir-1 (Figure 2) revealed only low diversity of acritarchs between 16,783.0 feet (ft) and 16,780.2 ft. The acritarchs are represented by a Muraticavea sp., some large and spinose (?) sphaeromorphic acritarchs and veryhachids. The assemblage is composed of quite simple forms and does not exhibit any particular innovative morphologies. The sphaeromorphic acritarchs could be referred to as prasinophytes and their abundance may indicate a marginal-marine environment. The veryhachids were interpreted as unspecialised surviving taxa (possibly related to a heterotrophic group), with no restricted ecological requirements and best adapted to eutrophic environments.
The spore-like palynomorphs and sphaeromorphic acritarchs are predominant in a transition interval from 16,777.4 ft to 16,774.16 ft, immediately below anoxic black shales (16,773.2-16,761.6 ft). The shales are rich in graptolites, chitinozoans and other organic material but do not contain acritarchs.
It is important to note that this zone is insufficiently investigated. Of interest would be a study of the silty interlayers and clays within the black graptolitic sediments.
Early Rhuddanian acritarchs from the Belonechitina postrobusta chitinozoan Zone (an equivalent of the late acuminatus-atavus graptolite Zone)
The material studied and referred to the Belonechitina postrobusta chitinozoan Zone (an equivalent to the late acuminatus-early atavus interval Zone), consists of samples from core 12 in Mukassir-1, from the first sidewall cores of Nuayyim-2 (NYYM-2) (12,475.0 ft), and from the graptolitic black shales at the top of the Qusaiba “hot shale”. Samples contain a considerable number of permanent tetrads and poorly diversified acritarch assemblages, mainly sphaeromorphs.
The samples from the succeeding interval (12,431.0-12,400.0 ft) in Nuayyim-2 correspond to the major part of the atavus Zone (P. Legrand, personal communication, 1992) and reflect the first significant diversification among acritarchs. Eight genera and 15 species are represented. Several morphotypes of Evittia are present but they are morphologically simple and difficult to separate taxonomically. Among the acritarchs are large Polygonium, some Multiplicisphaeridium and Micrhrystridium spp. Sphaeromorph acritarchs and reticulate forms (possibly prasinophytes) are also recorded. Cymatiosphaera cf. C. densiseptaMiller and Eames 1982 and Vermiculatisphaera obscuraMiller and Eames 1982, two species in common with the Rhuddanian Medina Group of the Silurian of western New York State (Miller and Eames, 1982), are present.
This interval (equivalent to the late acuminatus-early atavus Zone) does not seem to be linked to any major global sea-level change following the eustatic curve established by Johnson (1996). However, the diversification of acritarchs, and also graptolites (Melchin et al., 1998), coincides with a facies change from graptolitic shales (the “hot shales”) to a more-silty facies.
Middle Rhuddanian acritarchs of the Lagenochitina nuayyimensis chitinozoa Zone
Strata referred to this part of the Rhuddanian are from cores Raghib 1 (RGHB-1) between 10,020.0 and 9,170.0 ft; Nuayyim-2 between 12,350.0 and 11,351.0 ft; Hawtah-1 (HWTH-1) between 9,468.0 and 6,536.0 ft; and Hilwah-1 (HLWH-1) between 8,790.0 and 6,466.5 ft, and correspond to the extension of Zones 1 and 2 as defined in the first phase of the project (Le Hérissé et al., 1995). The richness of the acritarch species increases significantly in this interval and more than 100 acritarch species are identified after a short interval dominated by terrestrial palynomorphs (cryptospores and trilete spores), palynodebris and sphaeromorphs (e.g. in Nuayyim-2, 12,350.0-12,011.0 ft).
In the overlying layers, the terrestrial palynomorphs remain relatively common, but the palynofacies are dominated by marine components. The netromorphs (Leiofusa and Eupoikilofusa) are abundant, and the Evittia, Micrhystridium, Multiplicisphaeridium and Veryhachium spp. are also well represented. These taxa are ubiquitous. Tunisphaeridium or Visbysphaera spp. that have a restricted distribution are also observed in these samples. Elektoriskos pogonius, Eupoikilofusa saetosa sp. nov., Neoveryhachium carminae constricta, Sol radians, and the coenobial form Synsphaeridium sp., are also characteristic and may indicate brackish-water conditions.
Late Rhuddanian acritarchs of the Angochitina qusaibaensis chitinozoa Zone
The radiation of acritarchs is continued in this zone. It is interrupted only in the upper part by a new pulse of terrestrial palynomorphs. The diversity of acritarchs has been studied in various boreholes; for example: Nuayyim-2 between 11,250.0 ft and 11,000.0 ft; Hilwah-1 between 6,690.0 ft and 6,400.0 ft; Hawtah-1 from 6,536.0 ft and 6,533.0 ft; and Raghib-1 between 9,170.0 ft and 8,700.0 ft. Analogies can also be drawn with isolated samples from Qalibah-1 (e.g. from 279.0-299.0 ft). Unfortunately, the acritarchs are badly preserved in Udaynan-1 (UDYN-1).
This zone is equivalent to acritarch Zone 3 (Le Hérissé et al., 1995), with good index species such as Disparifusa horrida comb. nov., a finely punctate Leiofusa, and Multiplicisphaeridium circumscriptum or Veryhachium strangulatum. Tunisphaeridium species are also abundant, and it is a characteristic common with the Middle Silurian Clinton Group and the Maplewood Shale Formation in western New York State (Deunff and Evitt, 1968). The Maplewood Shale Formation was attributed to the sedgwickii Zone (Aeronian) at that time, but Cramer and Diez (1972) have discussed the age of this formation and suggested that it is equivalent with the top of graptolite zone 18 (cyphus Zone). Their work has furnished another tool for correlation with the Llandoverian of western New York State.
The relatively high proportion of terrestrial material that has been observed in the upper part of the A. qusaibaensis Zone (Nuayyim-2, between 10,900.0 ft and 10,400.0 ft) could be interpreted as a response to shoreline fluctuations. The occurrence of teratological cases is also a characteristic of the zone.
Duvernaysphaera aranaides found in this zone, shows a precocious appearance when compared with material from Great Britain (Biozone 3b, upper Aeronian; Hill and Dorning, 1984).
Acritarchs from the Aeronian in the C. alargada-P. paraguayensis chitinozoa Zone
Sources of data on acritarch assemblages and diversity in this zone are from Nuayyim-2 (9,741.0-9,643.0 ft), Udaynan-1 (17,417.2-17,300.0 ft), and core 4 of Raghib-1 (between 8,166.7 ft and 8,136.0 ft). Comparing core with core, acritarch assemblages are less homogeneous than chitinozoans within this interval. By reference to the table of correlations proposed by Paris et al. (1995) for the chitinozoa, core 19 in Udaynan-1 and core 4 in Raghib-1 are not considered to be at the same biostratigraphic level. It is probable that core 4 from Raghib-1 represents the lower and/or middle part of the Aeronian from the appearance of novel morphological features that allowed recognition of Biozone 4. Biozone 4 of Le Hérissé et al. (1995) is characterized by the persistence of taxa from the preceding Zone and the occurrence of Tylotopalla caelamenicutis, Leprotolypa species and Multiplicisphaeridium mingusi. In the overlying interval in Nuayyim-2 and Udaynan-1, the palynomorph assemblages change markedly and demonstrate a predominance of cryptospores. The acritarchs, by contrast, exhibit low diversity. The influx of terrestrial material in this interval could suggest deltaic progradation onto a shallow-marine shelf in the middle or late Aeronian. Alternatively, it could correspond to the significant regression suggested by Johnson (1996) in the early sedgwickii Zone that was also a time of dramatically reduced diversity for the graptoloids (Melchin et al., 1998).
Early Telychian acritarch assemblages in the S. solitudina-A. hemeri chitinozoa Zone
The data obtained in Kahf-1 from between 5,792.0 ft and 5,654.0 ft have been used to define acritarch Biozone 5 that is characteristic of the lowermost Telychian. However, it is possible that the assemblage from samples at 5,792.0 ft may be indicative of the uppermost Aeronian. This is because of the incoming of the Dactylofusa group and correlations with Great Britain where D. estillis is characteristic of the upper Aeronian (Hill and Dorning, 1984). Biozone 5 of Le Hérissé et al. (1995), illustrates an interval of radiation with a significant degree of diversification. Important species such as Ammonidium palmitella, Cymbosphaeridium sp. 1, Dactylofusa maranhensis (abundant), and Tylotopalla caelamenicutis are recorded. Higher, at 5,654.0 ft Circinatisphaera aenigma and Pteroverricatus zonocylindricus have been observed. The interval studied in Kahf-1 could be equivalent to the lowermost Telychian Sp. guerichi graptolitic Zone, distinguished elsewhere as occurring just before the turriculatus-crispus Zone (Melchin et al., 1998, Figure 2, p. 167), although the characteristic graptolites of the Sp. guerichi Zone have not been found here.
The S. solitudina-A. hemeri chitinozoa Zone was defined in Udaynan-1 (16,678.0-16,550.0 ft) and Tinat-2 (16,872.0-16,752.0 ft). However, it is difficult to be sure of the exact coverage of the material and its correlations with the early Telychian succession elsewhere, for the reasons discussed above (i.e. the absence of independent dating by means of graptolites). It is suggested here that this material is stratigraphically higher than that studied in Kahf-1. The acritarch diversity is quite low and is similar to that established for graptoloids in the utilis Subzone (boundary between the guerichi and turriculatus Zones) (Loydell, 1994). The acritarch association consists of Buedingiisphaeridium, Evittia (small forms), Tylotopalla (with good representation of T. caelamenicutis), Veryhachium spp., and prasinophytes (e.g. Pteropsermopsis sp. 1). Also observed is the re-emergence of species such as C. mapplewoodensis and N. carminae constricta (Lazarus taxa). The dwarfism phenomena supposed for some species such as the Evittia species is interesting since it appears to be in accordance with a ‘Lilliput effect’ recorded for graptoloids immediately after the utilis Subzone (Loydell, 1994). These conclusions need verification by studying more material. Analogies of behaviour between graptoloids and acritarchs are expected as similar factors controlled their distribution and diversity.
Middle to late Telychian sensu lato: equivalent to the A. hemeri and A. macclurei chitinozoa Zones
New material was investigated and some comparisons were established with material from Jordan (Keegan et al., 1990), Brazil and Libya. The arguments relating to new chitinozoa dates for the Sharawra Member (Al-Hajri and Paris, 1998) have been taken into account in reconsidering the ages attributed to Biozones 6 and 7 (Le Hérissé et al., 1995).
The material of acritarch Zones 6 and 7 occurs in two main intervals in the Kahf-1 borehole (4,957.6-4,948.0 ft, and 4,921.2-4,900.2 ft). It is considered to be representative of this interval for the most part, contrary to the conclusions of Le Hérissé et al. (1995).
The equivalent of Biozone 6 in Kahf-1 (4,957.6-4,948.0 ft) is present in Uthmaniyah-557 (UTMN-557) between 13,367.1 ft and 13,258.9 ft. The material is relatively well-preserved and indicates a major interval of diversification of acritarchs above a detritical layer rich in cryptospores, trilete miospores and heavy minerals. This interval has abundant P. deichai, A. hemeri and B. arabiensis, that could indicate the boundary between the S. solitudina /A. hemeri Zone and the A. hemeri Zone. Important new occurrences of acritarchs are as follows: Ammonidium microcladum, Crassiangulina cf. tesselita, Helosphaeridium cf. echiniformis, Pteroverricatus occuliformis sp. nov., Multiplicisphaeridium breviculum sp. nov., Veryhachium? owensii Al-Ruwaili (this volume) or Villosacapsula cf. rosendae. An interesting verrucate variant of Circinatisphaera aenigma also appears in this interval. The Visbysphaera species are also more common and T. caelamenicutis stays well represented. It would seem paradoxical that a spinose veryhachid like V. cf. rosendae occurs at this position, since this morphology is more characteristic of the Lower Devonian (Siegenian-Emsian). However, in this context, Martin (1969) noted a few specimens assigned to V. rosendae in the ‘Tarannon’ of Belgium (turriculatus to crenulata Zones).
C. cf. tesselita is a diagnostic component of Biozone 6 but it is interesting to note the association of this species with Crassiangulina (=Antruejadina) grotesca comb. nov. at 5,450.0 ft (Al-Ruwaili, personal communication, 1999), between Biozones 5 and 6 in a lower part of the core. Some Crassiangulina spp. have also been reported from various areas of the northern Gondwana margin and in Baltica. The two species cited above occur in the Tiangua Formation of the Parnaiba Basin of Brazil. Crassiangulina (as Antruejadina) grotesca is also a component of Zone JS.3, defined in the Llandovery (upper?) of Jordan (Keegan et al., 1990). The Brazilian material has also in common Pteroverricatus zonocylindricus, Veryhachium? owensii and a good representation of the Dactylofusa spp., Crassiangulina cf. tessellita and occasionally C. grotesca. These forms have been found as high as the Llandovery-Wenlock boundary in sub-surface material from Algeria and the Murzuk Basin of southern Libya. C. cf. tesselita is also present in Baltica in the upper Visby beds of the Gotland succession (Le Hérissé, 1989).
A new pulse of terrestrial palynomorphs, with chitinozoans species clearly attributable to the A. hemeri Zone, occurs from 13,175.26 ft to 13,126.0 ft in well Uthmaniyah-557. Only a few acritarchs are represented in this interval, probably due to the increase in terrestrial material. In Udaynan-1, the A. hemeri zone is tentatively identified in cuttings from 16,550.0 ft to 16,500.0 ft, and possibly to 16,000.0 ft. (Paris et al., 1995). Acritarchs are rare in this interval with the exception of an influx of netromorphs between 16,200.0 ft and 16,250.0 ft in Udaynan-1. Cuttings from Udaynan-1 yielded Anomaloplaisium johnsium, Dactylofusa maranhensis and Dactylofusa saudiarabiae that are interpreted as being reworked material. From the occurrence of A. johnsium, and by comparison with Biozone 7, this interval has been assigned to the latest Llandovery-?early Wenlock (as newly attributed palynomorph assemblages from 31 samples between 13,275.8 ft and 12,658.0 ft from well Hawiyah-151 identify the A. hemeri Zone (with an abundance of A. hemeri and P. dechei from 13,275.8 to 13,235.1 ft). From 13,275.8 ft upward there is a gradual increase in non-marine palynomorphs, such as cryptospores, trilete miospores and probable fresh-water algae. As a result, the diversity of acritarchs is extremely reduced from 12,858.6 ft to 12,733.0 ft. Amongst the species recovered, mixed with probable fresh-water algae, are Clypeolus tortugaides, Elektoriskos pogonius, Moyeria cabottii, some Evittia, Multiplicisphaeridium, Micrhystridium, Neoveryhachium carminae, Onondagaella, and simple veryhachids. The sphaeromorphs are abundant (for example, a Lophosphaeridium sp. A with rounded pylome), and many aggregates of simple cells of possible algal origin are present.
Clypeolus tortugaides illustrated here (Plate 9: e, f) was found in several samples. It is a lensoid species in a foveolate form as discussed by Miller et al. (1997). It may be a zygnematale zygospore as it shows morphological similarities with some zygospores (cf. Head, 1982) and other fossil morphons e.g. Maculatasporites that are considered to represent fossil zygnematacean spores (see Grenfell, 1995). As the zygnematales are mostly fresh-water or low-salinity algae, substantiating identification of Clypeolus as a zygnematale would make this taxon useful in palaeoenvironmental interpretations. Their abundance in the studied samples, together with abundant cryptospores suggests a fresh-water or brackish-water environment. These observations are also consistent with the sedimentological interpretation of a prodelta regime for the Qalibah Formation. The sporadic occurrences of terrestrial or fresh-water material in the Qusaiba Member, and more frequently in the Sharawra Member (for example, the coenobial algae), could correspond to the periodic influx of terrigenous organic matter in response to deltaic progradation across the shallow shelf.
Neoveryhachium carminae is relatively abundant. Several authors (see discussion in Tappan, 1980) have suggested that the presence of N. carminae indicates a highly turbid, near-coast environment (possibly tidal-flat sequences). From evidence in Hawiyah-151, Wellman et al. (1998) concluded that some units might have been exposed subaerially in the middle to late Telychian.
The assignment of Biozone 7 to the late Llandovery or Llandovery-Wenlock boundary is suggested from a re-examination of the chitinozoa, the presence of abundant A. macclurei, the persistence of P. pseudoagglutinans, and the characteristics of the acritarch assemblages. The age is also constrained by the reassignment of the basal Sharawra Member of the Qalibah Formation to the Sheinwoodian or earliest Homerian by Al-Hajri and Paris (1998). Some units in Kahf-1 equivalent to Biozone 8 defined by Le Hérissé et al. (1995), are reassigned now to the Wenlock.
Baltisphaeridium diabolicum (Le Hérissé et al., 1995) present between 4,921.2 ft and 4,900.0 ft in Kahf-1, is a significant biostratigraphic marker. Acritarch species that persist from underlying levels are Crassiangulina cf. tesselita, Dactylofusa maranhensis, Multiplicisphaeridium breviculum and Veryhachium? owensii. A large Pterospermopsis and a granulate veryhachid were recovered from a sample at 4,921.0 ft. These specimens have been found near to the Llandovery-Wenlock boundary in Libya. Acritarch diversity declined at 4,900.0 ft and is differentiated from the 4,921.2 ft level by the abundance of eurypterid fragments that could indicate a marginal environment. Pro-deltaic conditions prevailed at this level and persist in the overlying parts of the Sharawra Member. Also identified at 4,900.0 ft are the first occurrences of Ovnia sp. and Pardaminella crassicosta (also known in the succeeding Zones 8 and 9 of the Middle and Upper Silurian) that are now suspected to indicate a fresh-water environment.
CONCLUSIONS: RADIATION AND SURVIVORSHIP PATTERNS OF EARLY SILURIAN ACRITARCHS
Acritarchs progressively established their diversity following climatic amelioration and a global marine transgression in the latest Ordovician and earliest Silurian (Brenchley et al., 1997). From the persculptus Zone (latest Ordovician) to the acuminatus Zone (earliest Silurian), is a prolonged period of impoverishment, with only simple forms present. The early Rhuddanian equivalent of the acuminatus Zone in the black shales stratotype of Dob’s Linn, Scotland, is no more favourable to the diversification of acritarchs (Whelan, 1988). A Leiosphaeridia sp. 1 is present in the studied interval but does not allow correlation with the stratotype area. A more striking microfloral change takes place in the middle Rhuddanian (interval equivalent to the atavus-acinaces Zones) where the acritarch associations show a rapid speciation. The same tendency toward high speciation rates is also reported for graptolites in the middle Rhuddanian (Melchin et al., 1998).
By comparison, the first acritarch assemblages 1a and 1b, described by Hill and Dorning (1984) from the Llandovery type area, in the atavus-acinaces Zones are no more diversified than equivalent strata in Saudi Arabia. Major important morphological and evolutionary developments are also documented here from late Rhuddanian to earliest Aeronian assemblages. A gradual increase in the diversity has been also recorded in the Telychian, with several diversification peaks in the early, middle and late Telychian. Intermittent incursions of prograding sandy material are marked by more frequent trilete miospores. Acritarch associations dominated by cosmopolitan species reflect palaeoenvironmental changes. The fresh-water and terrestrial elements increase in abundance as the deltaic progradation culminates near the Llandovery-Wenlock boundary.
Good analogies exist with associations described from the Lower Silurian of Libya by Hill and Molyneux (1988) and from Jordan (Keegan et al., 1990), demonstrating common sedimentation environments on the margin of Gondwana. Trends in acritarch diversity in the Aeronian and Telychian are particularly consistent with similar tendencies reported elsewhere, for example, in the Arisaig Group of Nova Scotia, Canada (Beck and Strother, 1997).
Some large gaps in the stratigraphic ranges of several taxa (illustrating the Lazarus phenomenon) coincide with major palaeoenvironmental changes, such as lowstand deposition. It is important to identify those species that are responding to environmental fluctuations, in order to avoid confusion in the biostratigraphical conclusions. For example, P. crassicosta known in the late Llandovery disappears temporarily and reappears in the early Ludlow.
The important implications of this study are in the discussion of the biological significance and origin of several taxa or morphological groups. It is clear that a better stratigraphic resolution can be achieved by continuing the improvement of zonal schemes, by basic research on acritarch succession, and by identifying ecological signals delivered by the dominance of some taxa or morphological groups in relation to their environmental requirements.
APPENDIX: SYSTEMATIC DESCRIPTIONS
Type species: Eupoikilofusa striatifera (Cramer, 1964) Cramer, 1970
Eupoikilofusa saetosa sp. nov. (Plate 3: i)
Derivation of name: from the Latin saeta, bristle, for the ornamentation on the vesicle.
Holotype: Hawtah-1, 7,760.0 ft, slide FA67 (P57).
Material: 12 specimens.
Description: fusiform vesicle with commonly a crescent shape, terminated at each pole by a fine process tapering to a sharp point. The central body has a fine striation parallel to the longitudinal axis of the vesicle, and the crests are ornamented by microspines.
Dimensions: Total length 78-95 μm; width 19-22 μm; length of the processes 20-22 μm; spinous ornaments 1 μm high.
Remarks: The species shows some analogies with E. ampulliformis of Martin (1974), as well as E. aff. E. ampulliformis in Martin (1988), but the vesicle here is not rounded and the ornamentation seems to be more dense and composed of fine filose spines, not of short conical spines.
Occurrence: In several boreholes, from the upper part of the Rhuddanian up to the Telychian.
Type species: Evittia sommeri Brito, 1967
Material: 4 specimens
Description: vesicle spherical, finely granulate surface, with 5 to 6 processes, short, robust, characterised by prominent denticulate ornaments and protuberances grouped in the basal part, and longitudinal ridges along the trunk. The distal ends of the processes are heteromorphic but ramifications are short and quite simple.
Dimensions: vesicle 26-28 μm in diameter; process length 10-11 μm.
Remarks: the form of the processes in this species, and their limited number is an interesting characteristic compared to other morphotypes.
Occurrence: Central Saudi Arabia, Nuayyim-2 borehole, 12,431.0 ft; late acuminatus-early atavus Zone.
Type species: Geron guerillerus Cramer, 1967
Geron sp. 1 (Plate 3: h)
Remarks: this species is described in open nomenclature because only a few specimens have been observed. It is included here in order to avoid confusion with Tunisphaeridium caudatum. Its characteristics, compared to other published species, are a relatively short cylindrical skirt surrounding the central body and closely appressed to it. This genus in Saudi Arabia has a precocious appearance compared to Silurian sequences elsewhere.
Occurrence: Central Saudi Arabia, Nuayyim-2 borehole, 11,908.0 ft; acinaces Zone, late Rhuddanian.
Type species: Lophosphaeridium rarum Timofeev, 1959, designated by Downie, 1963.
Lophosphaeridium minimum sp. nov. (Plate 4: e)
Derivation of name: from the Latin minima, little.
Holotype: Nuayyim-2, 11,150.0 ft, slide FA43 (F36).
Material: 8 specimens
Description: central body spherical, of small size, densely covered by small baculate sculptural elements. The ornaments are distributed all over the vesicle.
Dimensions: central body spherical, of small size (13-14.5 μm), densely covered by small (1 μm) baculate ornaments.
Remarks: This species is characterised by its small size. It is of interest because of its restricted distribution.
Occurrence: L. minimum sp. nov. is apparently limited to the late Rhuddanian cyphus Zone.
Type species: Multiplicisphaeridium ramispinosum Staplin, 1961
Multiplicisphaeridium breviculum sp. nov. (Plate 6: a, b).
Holotype: Kahf-1, 4,900.2 ft, slide FA52 (G50.4)
Derivation of name: from the Latin breviculus, dumpy, for the short and stout processes.
Material: numerous specimens (more than 20) examined in several boreholes
Description: species of Multiplicisphaeridium with subspherical central body, psilate and thin walled. The processes communicate freely with the central cavity; they are very short and large at the base, with a rosette of branches divided up to the third-order.
Dimensions: central body diameter 21-28 μm; process length 3.5-4.5 μm; process width 2-3 μm; number of processes 12-15.
Remarks: There are no published species with morphologic characters similar to M. breviculum.
Occurrence: Found throughout most of the Telychian; occurs in several boreholes, e.g. Kahf-1, Turabah-1, Uthmaniyah-557.
Type species: Circinatisphaera aenigmaMiller, 1987
Dimensions (taken from many specimens): central body diameter 24-34 μm, process length 17-24.5 μm, process width 2-3.5 μm.
Remarks: A lot of material corresponding to the two morphons Circinatisphaera aenigma and C. cf. C. aenigma distinguished in Miller (1987) has been examined. It is considered that the more verrucate specimens (such as the specimen Plate 7: i) illustrate an intraspecific variability. Probably, the two specimens illustrated by Hill et al. (1985, Plate 10, Figures 3 and 5), are conspecific to C. aenigma, as suggested by Miller (1987), but not the specimen illustrated as Plate 10, Figure 1, which more probably corresponds to an Oppilatala sp. Even badly preserved specimens are easy to identify by the style of the processes (sometimes of darker appearance due to infilling material), the verrucate ornamentation, and the circular opening visible on broken specimens.
Occurrence: In Saudi Arabia, the species has been found in several boreholes. Locally the species is interesting because it is limited to the early to middle Telychian. It is present in the late Llandovery in Libya (Hill et al., 1985).
Type species: Muraticavea enteichia Wicander, 1974
Muraticavea sp. 1 (Plate 1: a)
Material: only one specimen.
Description: relatively large form, unornamented, with vesicle subpolygonal in outline, divided by low costae into polygonal fields of variable size.
Dimensions: vesicle 67 μm in diameter; diameter of polygonal areas 8-11 μm.
Occurrence: Central Saudi Arabia, Mukassir-1 borehole, 16,783.0 ft, acuminatus graptolite Zone.
Type species: Pteroverricatus pequantus Al-Ameri, 1984
Pteroverricatus occuliformis sp. nov. (Plate 6: f, g).
Derivation of name: from the Latin occulus, eye.
Material: more than 10 specimens in Uthmaniyah-557.
Holotype: Uthmaniyah-557, 13,258.9 ft, slide FA61 (K29.1)
Description: central body circular, surrounded by a thin flange at the equator. The boundary between the central body and flange is commonly dark-coloured. The central part shows a large macula or blot, linked to the vesicle border by thin radial ridges.
Dimensions: central body diameter 22-26 μm; dark blot 9 μm; flange 6 μm.
Comparison: the two species P. pequantus and P. zonocylindrus are differentiated from P. occuliformis sp. nov. by the ornamentation of their verrucae.
Occurrence: early Telychian, turriculatus-early crispus Zone.
Reviews and improvements to the text by Bernard Owens, Mansour Al-Ruwaili, and an anonymous referee are gratefully acknowledged. This paper is a contribution to the French Crisevole Program on biodiversity. The author is grateful to the Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company for permission to publish this study.
ABOUT THE AUTHOR
Alain Le Hérissé is a Palynologist and Researcher at the French National Center of Scientific Research. After studying geology at the University of Rennes, and graduating in 1981 with a thesis on Lower Devonian miospores, he received the title of Doctor of Sciences from the University of Brest in 1988. His doctoral thesis concerned the study of Upper Ordovician and Silurian acritarchs from Gotland, Sweden. His current research interests include the taxonomy, biodiversity and distribution in space and time of Palaeozoic acritarchs, especially of Europe and Gondwanan and peri-Gondwanan domains. Alain is now working on European collaborative projects in PICG 410 and 453, inside the CIMP-Saudi Aramco project on Lower Palaeozoic acritarchs and on acritarch biostratigraphy of Upper Devonian Brazilian basins with Petrobras.