Skip to Main Content

ABSTRACT

The sub-surface Lower Palaeozoic succession in central Saudi Arabia includes a sandstone-dominated unit, up to 2,000 feet thick, which is overlain by either Silurian or Permian strata. Correlation of the sandstones with the exposed Lower Palaeozoic succession in northwest Saudi Arabia is problematical, partly because graptolite-bearing shales of the latter, notably the Hanadir Shale Member of the Qasim Formation, have not been identified in central Saudi Arabian wells. Based on Formation Microscanner (FMS) images, the succession in central Saudi Arabia was considered to correlate with the Quwarah Member (upper Caradoc to Ashgill) of the Qasim Formation, cutting down through the underlying members of the Qasim Formation (Ordovician) and the Saq Formation (Middle Cambrian to Lower Ordovician) to rest unconformably on Precambrian igneous basement. In contrast, palynological evidence presented here shows that the sandstone-dominated unit ranges in age from Early or Middle Cambrian, through Early, Middle and Late Ordovician, to Early Silurian. The palynological evidence suggests correlation with a substantial part of the Cambro-Ordovician succession at outcrop in northwest Saudi Arabia, rather than with just the Upper Ordovician. The palynological study also provides evidence for the depositional environments of the sandstone unit. Assemblages are generally of low diversity, and may be indicative of nearshore, marginal-marine conditions. Infrequent occurrences of more diverse assemblages suggest open-marine shelf sea environments for strata at some levels. Recognition of the latter has implications for stratigraphic modelling of the central Arabian succession, and might relate to episodes of coastal onlap recognised in the Llanvirn and lower Caradoc of northwest Saudi Arabia.

INTRODUCTION

The Lower Palaeozoic rocks of Saudi Arabia crop out adjacent to and east of the Arabian Shield, the main area of outcrop being located in northwest Saudi Arabia (Figure 1) where the succession comprises the Siq, Burj, Saq, Qasim, Zarqa, Sarah and Qalibah formations (Vaslet, 1989; Stump et al., 1995). At the base of this succession (Figure 2), the Lower Cambrian Siq Formation comprises red to white, variably sorted, fluvial sandstones and conglomerates, unconformably overlain by the well-sorted, non-marine to shallow-marine sandstones of the Middle Cambrian to Lower Ordovician Saq Formation. The Burj Formation is the marine equivalent of the basal, non-marine part of the Saq Formation, and consists of a thin basal shale overlain by sandstones with thin carbonate interbeds.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 2:

Correlation of sections in eight wells from central Saudi Arabia with lithostratigraphy in northwest Saudi Arabia and with Lower Palaeozoic chronostratigraphy. Ordovician ages were obtained from a further three wells, Haradh-57, Shiblah-1 and Tinat-2 (see text). For each well, the length of the bar between the indicated depths indicates the range of possible ages and correlations for the sampled interval.

Figure 2:

Correlation of sections in eight wells from central Saudi Arabia with lithostratigraphy in northwest Saudi Arabia and with Lower Palaeozoic chronostratigraphy. Ordovician ages were obtained from a further three wells, Haradh-57, Shiblah-1 and Tinat-2 (see text). For each well, the length of the bar between the indicated depths indicates the range of possible ages and correlations for the sampled interval.

The Qasim Formation rests on an erosion surface at the top of the Saq Formation. Above its basal, poorly-sorted sandstones, the Qasim Formation consists of the mid to outer shelf graptolitic mudstones of the Hanadir (Llanvirn) and Ra’an (mid-late Caradoc) members, representing periods of relatively high sea-level, interspersed with the shallow water sandstones and siltstones of the Kahfah (early-mid Caradoc) and Quwarah (latest Caradoc-mid Ashgill) members (the ages adopted here are after Al-Hajri, 1995). The upper part of the Quwarah Member is considered to correlate with the Zarqa and Sarah formations of mid Ashgill age. The Zarqa Formation (considered by the authors to be a lateral facies of the Sarah Formation) consists of variably sorted sandstones, conglomerates and siltstones, deposited mostly in non-marine to nearshore settings, often in deeply incised valleys formed during the Late Ordovician glaciation, while the Sarah Formation consists mainly of fluvial to shallow-marine sandstones.

A major hiatus separates the youngest Ordovician beds from earliest Silurian rocks of the Qalibah Formation. The latter includes the offshore, graptolite-bearing mudstones of the Qusaiba Member, overlain by the pro-deltaic siltstones and sandstones of the Sharawra Member.

Wells drilled to the basement in central Saudi Arabia, from south of Riyadh to the northern coast of the Arabian Gulf (Figure 1), have proved Palaeozoic sandstones that are overlain by either Silurian or Permian strata. Correlation of the sandstones with the Lower Palaeozoic succession at outcrop in northwest Saudi Arabia has proved difficult, partly because graptolite-bearing shale units of the latter, such as the Hanadir Shale Member of the Qasim Formation, have not been identified in the central Saudi Arabian wells (McGillivray and Husseini, 1992). Connally and Wiltse (1995) identified the Zarqa and Sarah formations in the central Saudi Arabian succession, and using Formation Microscanner (FMS) images, subdivided the underlying sandstones into five formations which they interpreted as having been deposited in non-marine, nearshore and shallow-marine settings. They correlated the sandstone succession, totalling some 2,000 feet (ft) in thickness, with the Quwarah Member of late Caradoc to Ashgill age at the top of the Qasim Formation, and depicted (Connally and Wiltse, 1995, Figure 1) their five formations as occupying a broad trough, cutting down through the rest of the Qasim Formation and the underlying Saq Formation to rest unconformably on Precambrian igneous basement.

In contrast to this scenario, palynological evidence presented here suggests a much greater range of ages for the sandstones, from Early or Middle Cambrian through Early and Middle Ordovician to early Silurian. In addition, the palynological assemblages provide supplementary evidence for depositional environments. They are generally indicative of nearshore, marginal-marine conditions, but infrequent occurrences of more diverse assemblages suggest open-marine shelf sea environments for strata at some levels. Recognition of the latter has implications for stratigraphic modelling of the central Arabian succession. They may relate to episodes of coastal onlap recognised in northwest Saudi Arabia (Stump et al., 1995), in which case they would constitute important criteria on which to build sequence stratigraphic models.

STRATIGRAPHIC PALYNOLOGY

The following discussion is based on an examination of slides from eleven wells: Abu Jifan-24 (ABJF-24), Abu Jifan-25 (ABJF-25), Farhah-1 (FRHH-1), Haradh-48 (HRDH-48), Haradh-52 (HRDH-52), Haradh-57 (HRDH-57), Hawtah-1 (HWTH-1), Khursaniyah-81 (KRSN-81), Mazalij-1 (MZLJ-1), Shiblah-1 (SHBL-1) and Tinat-2 (TINT-2) (see Figure 1 for their locations). Most of the slides are from core samples, but slides from cuttings samples at 14,320.0-14,330.0 ft in Khursaniyah-81 and 14,040.0-14,085.0 ft in Mazalij-1 were also examined. In addition, all the samples from Shiblah-1 are from sidewall cores. Samples were collected and prepared over a period of several years.

All the samples were prepared using conventional palynological techniques. They were subjected to 36% hydrochloric acid treatment, followed by a prolonged hydrofluoric acid treatment for two or three days, with occasional additions of hydrofluoric acid and frequent stirring. Samples were then treated with hydrochloric acid to dissolve the silica gel, and if necessary were oxidised using nitric acid. Slides representing three portions of the residue were prepared: a whole residue, a sieved residue and an oxidised portion.

Cambrian

Khursaniyah-81

Acritarch assemblages from two levels in Khursaniyah-81 suggest an Early to Middle Cambrian age (Figure 3). The lower microflora, from core at 15,504.0 ft, contains Annulum squamaceum (Volkova) Martin (Plate 1: b), a species that appears at about the base of the Cambrian and has its highest known occurrence in the lowest Middle Cambrian (e.g. in eastern Newfoundland and on the East European Platform; see Molyneux et al., 1996, text-figures 2 and 3). The acritarch assemblage from 15,504.0 ft also includes small sphaeromorph acritarchs and a distinctive species that bears some resemblance to Archaeodiscina (Plate 1: a). The specimens of Archaeodiscina? from Khursaniyah-81 occur singly or in clusters, have flattened vesicles that are circular in outline with narrow folds just inside and parallel to the circumference, and have a distinct cluster of granules at the centre of the flattened disc.

Figure 3:

Occurrences of acritarchs and cryptospores in Khursaniyah-81, with interpretations of ages and environments.

Figure 3:

Occurrences of acritarchs and cryptospores in Khursaniyah-81, with interpretations of ages and environments.

Plate 1:

Cambrian and Lower Ordovician acritarchs, cryptospores and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Archaeodiscina?, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,700). Specimen number FA118.

  • (b) Annulum squamaceum (Volkova) Martin, 1983. Lower-Middle Cambrian, Khursaniyah-81, 15,504.0 ft (x1,400). Specimen number FA119.

  • (c) Micrhystridium, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,400). Specimen number FA120.

  • (d) Vulcanisphaera cirritaRasul, 1976. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,296.0 ft (x850). Specimen number FA121.

  • (e) Acanthodiacrodium angustum (Downie) Combaz, 1967. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,320.0 to 14,330.0 ft (cuttings sample) (x1,700). Specimen number FA122.

  • (f) Virgatasporites rudiiCombaz, 1967. Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,256.8 to 14,257.8 ft (x1,700). Specimen number FA123.

  • (g) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,700). Specimen number FA124.

  • (h) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,900). Specimen number FA125.

  • (i) Laevolancis-like monad, Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,257.5 ft (x1,900). Specimen number FA126.

  • (j) Cymatiosphaera? Lower Ordovician (Arenig-Llanvirn), Haradh-52, 15,450.0 ft (x1,000). Specimen number FA127.

  • (k) sphaeromorph acritarch, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,600). Specimen number FA128.

  • (l) Eremochitina sp. 2 (Al-Hajri, 1991), Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x735). Specimen number FA129.

Plate 1:

Cambrian and Lower Ordovician acritarchs, cryptospores and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Archaeodiscina?, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,700). Specimen number FA118.

  • (b) Annulum squamaceum (Volkova) Martin, 1983. Lower-Middle Cambrian, Khursaniyah-81, 15,504.0 ft (x1,400). Specimen number FA119.

  • (c) Micrhystridium, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,400). Specimen number FA120.

  • (d) Vulcanisphaera cirritaRasul, 1976. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,296.0 ft (x850). Specimen number FA121.

  • (e) Acanthodiacrodium angustum (Downie) Combaz, 1967. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,320.0 to 14,330.0 ft (cuttings sample) (x1,700). Specimen number FA122.

  • (f) Virgatasporites rudiiCombaz, 1967. Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,256.8 to 14,257.8 ft (x1,700). Specimen number FA123.

  • (g) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,700). Specimen number FA124.

  • (h) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,900). Specimen number FA125.

  • (i) Laevolancis-like monad, Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,257.5 ft (x1,900). Specimen number FA126.

  • (j) Cymatiosphaera? Lower Ordovician (Arenig-Llanvirn), Haradh-52, 15,450.0 ft (x1,000). Specimen number FA127.

  • (k) sphaeromorph acritarch, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,600). Specimen number FA128.

  • (l) Eremochitina sp. 2 (Al-Hajri, 1991), Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x735). Specimen number FA129.

A similar assemblage occurs about 40 ft higher in the well, from core between 15,464.1 and 15,468.1 ft (Figure 3). The higher assemblage is characterised by small sphaeromorph acritarchs and the distinctive Archaeodiscina?, accompanied by Micrhystridium spp. (Plate 1: c) and an abundance of short, slender filaments. Although Annulum squamaceum is absent, the similarity between the higher assemblage and that from 15,504.0 ft suggests an Early to Middle Cambrian age.

Lower Ordovician (Tremadoc)

Khursaniyah-81

Diacromorph acritarchs are common in samples from 14,296.0 ft (core) and 14,320.0-14,330.0 ft (cuttings) in Khursaniyah-81, and include species of Acanthodiacrodium and Schizodiacrodium? (Figure 3). Acantho-diacrodium angustum (Downie) Combaz (Plate 1: e), present in both samples (Figure 3), ranges from the highest Cambrian to the top of the Tremadoc (Molyneux et al., 1996, text-figures 3 and 6). Vulcanisphaera cirrita Rasul (Plate 1: d) is also present in the core sample from 14,296.0 ft, and may range through the Tremadoc from the Upper Cambrian to the lower Arenig, especially if the synonymy with Vulcanisphaera africana suggested by Martin (in Martin and Dean, 1981) is accepted (Martin and Dean, 1988; Molyneux et al., 1996). These ranges do not discount a latest Cambrian age for the section in Khursaniyah-81, but the common occurrence of diacromorph acritarchs is a characteristic of Tremadoc assemblages and is indicative of a Tremadoc (Early Ordovician) age.

Lower-Middle Ordovician (Arenig-Llanvirn)

Khursaniyah-81 and Haradh-48

Assemblages from core at 14,256.8 to 14,259.3 ft in Khursaniyah-81 and core at 15,691.0 to 15,711.0 ft in Haradh-48 are similar (Figures 3 and 4). Acritarch diversity is low in both cases, with few acanthomorph (process-bearing) or other morphologically complex forms. Sphaeromorph acritarchs (Plate 1: k) are the most common forms in the microfloras from both wells, an observation of possible environmental significance as discussed below. Also present in both wells is Virgatasporites rudii Combaz (Plate 1: f), a species known from the Tremadoc and/or Arenig of Algeria (Combaz, 1967), England (Rasul and Downie, 1974), Bohemia (Vavrdová, 1990a, 1990b, 1992, 1993), and possibly Sardinia (Pittau, 1985; Albani, 1989).

Figure 4:

Occurrences of acritarchs and cryptospores in Haradh-48, with interpretations of age and environment.

Figure 4:

Occurrences of acritarchs and cryptospores in Haradh-48, with interpretations of age and environment.

The acritarch assemblages from Khursaniyah-81 and Haradh-48 recall those recorded by Jachowicz (1995) from the Saq Formation in Saudi Aramco well Qiba-1 (7,261.0 to 7,326.0 ft), situated towards the northwest (Figure 1). Like the assemblages from Khursaniyah-81 and Haradh-48, those from Qiba-1 are dominated by sphaeromorph acritarchs and contain Virgatasporites rudii. Jachowicz (1995) assigned her Qiba-1 assemblages to the Tremadoc-Arenig.

The main difference between the Khursaniyah-81 and Haradh-48 assemblages on the one hand, and the Qiba assemblages on the other, is the presence of cryptospores in the former. These include dyads (Plate 1: g, h) and possible tetrads as well as Laevolancis-like monads (Plate 1: i). The earliest records of such cryptospores are from the Llanvirn, including records from the Hanadir Member of the Qasim Formation in northwest Saudi Arabia (McLure, 1988; Strother et al., 1996; Wellman, 1996). Consequently, there is a discrepancy between the last known occurrence of Virgatasporites rudii in the Arenig, and the first appearance of cryptospores in the Llanvirn. Nevertheless, the evidence suggests that a level within the Arenig-Llanvirn is probable for the assemblages considered here, and points to correlation with the upper part of the Saq Formation or lower part of the Hanadir Member (Figure 2).

Haradh-52

The age of the section between 15,445.4 ft and 15,450.0 ft in Haradh-52 (Figure 5) is constrained in part by the Llanvirn age assigned to a higher section in the well (see below). The palynomorph assemblage obtained from core samples between 15,445.4 and 15,450.0 ft is distinctive, but contains little that can be used for definite correlation. Nevertheless, rare cryptospores including a tetrad and possible dyads are present, and as the earliest records of such cryptospores are from the early Middle Ordovician (Strother et al., 1996; Wellman, 1996), they indicate a possible maximum age for the section. Given the Llanvirn age suggested for the section between 14,874.9 ft and 14,945.2 ft in the same well, an undifferentiated Arenig-Llanvirn age is suggested here for the interval between 15,445.4 and 15,450.0 ft (Figure 2).

Figure 5:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-52, with interpretations of ages and environments.

Figure 5:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-52, with interpretations of ages and environments.

As with the assemblages from Khursaniyah-81 and Haradh-48, sphaeromorph acritarchs are a dominant component of the assemblage from Haradh-52. Also prominent is a form assigned to Cymatiosphaera? (Plate 1: j) which gives the assemblage its distinctive character. Determination of Cymatiosphaera? in this case is extremely tentative because the specimens’ preservation does not enable the delineation of polygonal fields. Nevertheless, the assemblage is sufficiently distinctive that it may be important for local correlation. Unlike the assemblages from Khursaniyah-81 and Haradh-48, that from Haradh-52 lacks Virgatasporites rudii (Figure 5).

Middle Ordovician (Llanvirn-Caradoc)

Middle Ordovician assemblages have been recorded from three wells, Abu Jifan-24, Farhah-1 and Haradh-52 (Figures 5, 6 and 7). Middle Ordovician (Llanvirn-Caradoc) palynostratigraphy presents a problem, in that relatively little has been published on the distribution of acritarch and chitinozoan taxa in the type areas of the Llanvirn and Caradoc series in England and Wales. Consequently, precise first and last appearance datums are not known for many species. Nevertheless, enough is known to be able to distinguish Llanvirn-Caradoc assemblages from pre-Llanvirn microfloras on the one hand, and from Ashgill assemblages on the other.

Figure 6:

Occurrences of acritarchs, chitinozoans and cryptospores in Abu Jifan-24, with interpretations of ages and environments.

Figure 6:

Occurrences of acritarchs, chitinozoans and cryptospores in Abu Jifan-24, with interpretations of ages and environments.

Figure 7:

Occurrences of acritarchs and cryptospores in Farhah-1, with interpretations of ages and environments.

Figure 7:

Occurrences of acritarchs and cryptospores in Farhah-1, with interpretations of ages and environments.

The Ordovician chronostratigraphy used in this paper follows Fortey et al.’s (1995) revision of British Ordovician series and stages. Fortey et al. redefined the base of the Caradoc Series in Britain, bringing it down to a level that correlated with the base of the Nemagraptus gracilis Biozone. The effect of this decision was to subsume a substantial part of the Llandeilo Series in the Caradoc. The pre-gracilis part of the Llandeilo Series was relegated to the rank of a stage (Llandeilian Stage) and included in the upper part of an extended Llanvirn Series. Thus the Caradoc Series now succeeds the Llanvirn Series with no intervening chronostratigraphical division of equivalent rank.

Abu Jifan-24

The diverse assemblage of acritarchs, chitinozoa and cryptospores recorded from core at 13,896.0 ft in Abu Jifan-24 (Figure 6) is Middle Ordovician, and a number of acritarch taxa, including Arkonia tenuata Burmann, Dicrodiacrodium ancoriforme Burmann (Plate 2: c), Stellechinatum celestum (Martin) Turner (Plate 2: d) and Striatotheca quieta (Martin) Rauscher, indicate a Llanvirn age or younger.

Plate 2:

Ordovician acritarchs. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Aureotesta clathrata clathrataVavrdová, 1972. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA130.

  • (b) Veryhachium subglobosumJardiné et al., 1974. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA131.

  • (c) Dicrodiacrodium ancoriformeBurmann, 1970. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA132.

  • (d) Stellechinatum celestum (Martin) Turner, 1984. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA133.

  • (e) Striatotheca quieta (Martin) Rauscher, 1973. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,926.0 to 13,927.5 ft (x1,400). Specimen number FA134.

  • (f) Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan, 1976. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA135.

Plate 2:

Ordovician acritarchs. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Aureotesta clathrata clathrataVavrdová, 1972. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA130.

  • (b) Veryhachium subglobosumJardiné et al., 1974. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA131.

  • (c) Dicrodiacrodium ancoriformeBurmann, 1970. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA132.

  • (d) Stellechinatum celestum (Martin) Turner, 1984. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA133.

  • (e) Striatotheca quieta (Martin) Rauscher, 1973. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,926.0 to 13,927.5 ft (x1,400). Specimen number FA134.

  • (f) Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan, 1976. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA135.

Determining the precise age of the sample from 13,896.0 ft is complicated by the association of the chitinozoan Eremochitina sp. 2 (of Al-Hajri, 1991; Plate 1: l) and the acritarch Aureotesta clathrata clathrata Vavrdová (Plate 2: a) on the one hand, with the acritarchs Veryhachium subglobosum Jardiné et al. (Plate 2: b) and Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan (Plate 2: f) on the other. The first two suggest a Llanvirn age; Eremochitina sp. 2 is a guide fossil for the early Llanvirn Siphonochitina formosa Biozone (Al-Hajri, 1995), while the highest confirmed records of A. clathrata clathrata are from the Llanvirn (Brocke et al., 1997). However, Veryhachium subglobosum and Villosacapsula setosapellicula are generally more indicative of upper Ordovician assemblages (Caradoc-Ashgill; Molyneux et al., 1996). This suggests either that ranges are poorly understood, or that some recycling has occurred.

Eremochitina sp. 2 is also present in a core sample from 14,124.8 ft in Abu Jifan-24. It is not associated with ‘younger’ Ordovician forms at that depth, and might therefore indicate a Llanvirn age (Figure 6), although Veryhachium subglobosum is questionably present in the basal core sample of the section from 14,166.8 ft. The presence of Stellechinatum celestum shows that the latter sample is probably not older than Llanvirn.

The problem presented by Abu Jifan-24 might be resolved by further investigation of acritarch assemblages from the Hanadir and Ra’an members. Jachowicz (1995), for example, gave no information on acritarch assemblages of Llanvirn age from northwest Saudi Arabia, and McLure (1988) provided only a preliminary list of the predominant genera in the Hanadir Member, and a brief list of the more characteristic species in the Ra’an. In the meantime, the entire interval between 13,896.0 ft and 14,166.8 ft in Abu Jifan-24 is assigned a Llanvirn age, based on the presence of Eremochitina sp. 2.

Haradh-52

Eremochitina sp. 2 also occurs in core between 14,874.9 ft and 14,945.2 ft in Haradh-52 (Figure 5), but the palynomorph assemblage is dominated by sphaeromorph acritarchs with few other forms that are diagnostic of age. The presence of Stellechinatum celestum (Martin) Turner in the core sample from 14,943.4 ft suggests that the section at that depth is not older than Llanvirn, and based on the occurrence of these two taxa, a Llanvirn age is suggested for the entire cored section between 14,874.9 ft and 14,945.2 ft.

Farhah-1

The acritarchs Arkonia tenuata Burmann, Stellechinatum celestum (Martin) Turner and Striatotheca quieta (Martin) Rauscher (Plate 2: e) indicate that the 3 ft cored section between 13,925.0 ft and 13,928.1 ft in Farhah-1 is Llanvirn or younger (Figure 7), but there are no forms that might be indicative of the Ashgill Series such as specimens of Diexallophasis. Comparison with the acritarch assemblages recorded from Saudi Arabia by McLure (1988) and Jachowicz (1995) also suggests a pre-Ashgill age, while Stellechinatum helosum? Turner (Plate 3: a) is similar to forms recorded from the Caradoc Series in Britain (Turner, 1984). Based on this evidence, an age within the Llanvirn-Caradoc is suggested.

Plate 3:

Ordovician and Silurian acritarchs and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Stellechinatum helosum? Turner, 1984. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,925.0 ft (x1,200). Specimen number FA136.

  • (b) Stelliferidium?, Ordovician?, Hawtah-1, 8,549.0 ft (x1,425). Specimen number FA137.

  • (c) Diexallophasis denticulata (Stockmans and Willière) Loeblich, 1970. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA138.

  • (d) Jenkinochitina lepta (Jenkins, 1969). Upper Ordovician, Mazalij-1, 14,040.0-14,085.0 ft (cuttings sample) (x800). Specimen number FA139.

  • (e) Tylotopalla cf. aniae (Jardiné et al.) Eisenack et al., 1979. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA140.

  • (f) Neoveryhachium carminae constrictaLe Hérissé et al., 1995. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x1,700). Specimen number FA141.

Plate 3:

Ordovician and Silurian acritarchs and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Stellechinatum helosum? Turner, 1984. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,925.0 ft (x1,200). Specimen number FA136.

  • (b) Stelliferidium?, Ordovician?, Hawtah-1, 8,549.0 ft (x1,425). Specimen number FA137.

  • (c) Diexallophasis denticulata (Stockmans and Willière) Loeblich, 1970. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA138.

  • (d) Jenkinochitina lepta (Jenkins, 1969). Upper Ordovician, Mazalij-1, 14,040.0-14,085.0 ft (cuttings sample) (x800). Specimen number FA139.

  • (e) Tylotopalla cf. aniae (Jardiné et al.) Eisenack et al., 1979. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA140.

  • (f) Neoveryhachium carminae constrictaLe Hérissé et al., 1995. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x1,700). Specimen number FA141.

A core sample from 13,932.2 ft in Farhah-1 has yielded forms that are mainly undiagnostic of age, although the occurrence of cryptospores suggests that it is probably not older than Llanvirn (Figure 7) (but see discussion of the Arenig-Llanvirn assemblages from Khursaniyah-81 and Haradh-48).

Upper Ordovician (Ashgill)

Abu Jifan-25

Old slides from core at 13,849.8 ft in Abu Jifan-25 yielded a low diversity assemblage of poorly preserved chitinozoans. However, broken specimens of Armoricochitina nigerica (Bouché) and Belonechitina spp. indicate an Ashgill age. Armoricochitina nigerica is particularly significant, as it is restricted in northwest Saudi Arabia to the A. nigerica and Ancyrochitina merga biozones of the Ashgill (Al-Hajri, 1995).

Ordovician (undifferentiated)

Ordovician rocks are inferred to be present in five wells, Mazalij-1, Haradh-57, Hawtah-1, Shiblah-1 and Tinat-2, but assemblages are insufficient to suggest more precise ages.

Mazalij-1

Ordovician rocks, possibly of Ashgill age, are inferred to underlie lower Silurian strata at 13,998.2 ft in Mazalij-1 (Figure 8). There is a sharp drop in diversity between core samples from 13,997.7 ft and 13,998.2 ft in the well, and Silurian acritarchs present at 13,997.7 ft (see below) are missing from the lower depth. A single specimen of Stelliferidium? from core at 13,998.2 ft suggests an Ordovician age, and there is nothing in the assemblage to refute that suggestion. Evidence from a cuttings sample taken at 14,040.0-14,085.0 ft also suggests that Upper Ordovician rocks occur at or above those depths in the well. The cuttings sample contains obviously caved specimens in the form of bisaccate pollen grains, and probable Silurian forms are present, including for example the acritarch Visbysphaera aff. oligofurcata (Eisenack) Lister. However, the chitinozoan Jenkinochitina lepta (Jenkins) (Plate 3: d), which occurs in Ashgill faunas in Saudi Arabia (Al-Hajri, 1995), is also present, and one specimen is tentatively assigned to the Late Ordovician acritarch species Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan.

Figure 8:

Occurrences of acritarchs and cryptospores in Mazalij-1, with interpretations of ages and environments.

Figure 8:

Occurrences of acritarchs and cryptospores in Mazalij-1, with interpretations of ages and environments.

Haradh-57

The Ordovician age of the cored section at 16,311.4 ft in Haradh-57 (Figure 9) is based on a specimen of Frankea?, specimens of Stelliferidium, and cryptospores. The cryptospores suggest that the sample is probably not older than Llanvirn (Strother et al., 1996; Wellman, 1996) (but see discussion of the Arenig-Llanvirn assemblages from Khursaniyah-81 and Haradh-48). Frankea and Stelliferidium both range from the Lower Ordovician into the Caradoc, but neither have well documented Ashgill occurrences.

Figure 9:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-57, with interpretations of ages and environments.

Figure 9:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-57, with interpretations of ages and environments.

Hawtah-1

Evidence for the presence of Ordovician rocks in Hawtah-1 is weak, and is based on the presence of a single specimen, assigned tentatively to Stelliferidium (Plate 3: b), in a core sample from 8,549.0 ft (Figure 10). Nothing else in the sample from that depth can be used to confirm or refute the suggestion of an Ordovician age. The suggested Ordovician age contrasts with the conclusion of Le Hérissé et al. (1995), who considered the entire interval between 8,171.0 ft and 9,468.9 ft in Hawtah-1 to be of early Rhuddanian (earliest Silurian) age (see below).

Figure 10:

Occurrences of acritarchs, chitinozoans and cryptospores in Hawtah-1, with interpretations of ages and environments.

Figure 10:

Occurrences of acritarchs, chitinozoans and cryptospores in Hawtah-1, with interpretations of ages and environments.

Shiblah-1

Moyeria cabottii suggests a Middle Ordovician to Middle Silurian age for a sidewall core from 6,974 ft in Shiblah-1 (Figure 11). Combined with the presence of a possible diacromorph acritarch from a sidewall core at 6,807 ft, a Middle to Late Ordovician age is indicated for the section between these depths.

Figure 11:

Occurrences of acritarchs and cryptospores in Shiblah-1, with interpretations of ages and environments.

Figure 11:

Occurrences of acritarchs and cryptospores in Shiblah-1, with interpretations of ages and environments.

Tinat-2

The Ordovician age of the 37.6 ft section between 17,953.0 ft and 17,990.6 ft in Tinat-2 is based on individual specimens from each of these depths (Figure 12). The occurrence of a single specimen of Veryhachium lairdii Deflandre ex Loeblich s.l. in a core sample from 17,990.6 ft suggests that this level, and therefore the entire sampled section, is latest Tremadoc or younger. A single specimen of Acanthodiacrodium cf. costatum Burmann from a core sample at 17,953.0 ft is compared with rather than assigned to that species, but diacromorph acritarchs of this type are known only from Ordovician strata. A. costatum was described from upper Llanvirn strata of Germany (Burmann, 1968), and Downie (1984, p. 13) depicted A. costatum and the similar form A. rectinerve Burmann as both having ranges restricted to the Llanvirn. Whether this represents the full range of these Acanthodiacrodium species has yet to be established but, for the time being, and combined with the specimen of V. lairdii, it forms the basis of the Ordovician age suggested here.

Figure 12:

Occurrences of acritarchs in Tinat-2, with interpretations of ages and environments.

Figure 12:

Occurrences of acritarchs in Tinat-2, with interpretations of ages and environments.

Lower Silurian (Llandovery)

Hawtah-1

The section between 7,816 ft and 7,839 ft in Hawtah-1 is early Silurian (Figure 2). Forms recovered from this interval include Lagenochitina nuayymensis Paris and Al-Hajri, Spinachitina sp., Neoveryhachium carminae (Cramer), cryptospores and sphaeromorph acritarchs. Lagenochitina nuayymensis is restricted to the middle Rhuddanian in central Saudi Arabia (Paris et al., 1995).

Le Hérissé et al. (1995) believed the interval between 8,171.0 ft and 9,468.9 ft in Hawtah-1 to be of early Rhuddanian (earliest Silurian) age, but the evidence is weak, many acritarch species are long-ranging, and there is no independent biostratigraphical control (Le Hérissé et al., 1995, p. 52). The assemblages recorded during this study contain no specimens that would confirm a Silurian age (Figure 10), but a single specimen of Stelliferidium? from 8,549.0 ft (Plate 3: b) raises the possibility of an Ordovician age for strata at that depth (see above).

Mazalij-1

The section at 13,997.7 ft in Mazalij-1 is considered to be of Llandovery age (Figure 8), based on an acritarch assemblage from a core sample at that depth which includes relatively common Diexallophasis denticulata (Stockmans and Willière) Loeblich (Plate 3: c), accompanied by single specimens of Neoveryhachium carminae constricta Le Hérissé et al. (Plate 3: f) and Tylotopalla cf. aniae (Jardiné et al.) Eisenack et al. (Plate 3: e). Neoveryhachium carminae constricta was reported by Le Hérissé et al. (1995) to range through their zones 2-4 (Qusaiba Member, Qalibah Formation), considered to be of Rhuddanian to early Aeronian age, and Tylotopalla aniae was described by Jardiné et al. (1974) from rocks of Llandovery age in Algeria.

Reworking

Jachowicz (1995) reported reworked acritarchs, mainly Tremadoc forms, in her lower Caradoc and lower Ashgill assemblages from Saudi Arabia. In contrast, there is no definite evidence for reworking in any of the assemblages recorded here. Reworking might account for the apparent mixing of Llanvirn and younger Ordovician palynomorphs in Abu Jifan-24 (13,896.0ft-14,166.8 ft), and for the possible occurrence of the Ordovician acritarch Stelliferidium at 8,549.0 ft in Hawtah-1, at a level previously considered to be Rhuddanian, but the evidence is not conclusive.

PALYNOFACIES

There is a body of evidence to suggest that acritarch assemblages vary systematically in relation to environment of deposition, and that abundance and diversity both increase with distance from shore. In consequence, acritarch abundance is at its maximum in open-marine facies whereas marginal-marine facies generally contain no or very few acritarchs (Smith and Saunders, 1970). Diversity is also at its maximum in offshore marine shelf facies (Dorning, 1981; Al-Ameri, 1983), for example in palynofacies type 4 of Al-Ameri (1983), considered to represent outer neritic conditions. A number of authors (Gray and Boucot, 1972; Jacobson, 1979; Dorning, 1981; Al-Ameri, 1983) have noted that sphaeromorph-dominated assemblages are characteristic of nearshore shallow-water conditions, but Dorning (1981) also reported sphaeromorph acritarchs to be dominant in deep-water basinal assemblages, and Al-Ameri (1983) described sphaeromorph acritarchs as being abundant in his most distal palynofacies (type 5), considered to represent a continental break environment. Sphaeromorph-dominated assemblages therefore represent the two poles of an inshore-offshore gradient, more diverse assemblages occurring in the intervening environments.

Samples from the wells studied here have yielded sphaeromorph-dominated assemblages. In the context of the Ordovician-lower Silurian of Saudi Arabia, deposited in fluvio-deltaic to mid-shelf settings on a stable continental margin (Stump et al., 1995), the sphaeromorph-dominated assemblages are considered to indicate nearshore, marginal-marine rather than deep basinal environments. Based on this understanding, an inshore index has been calculated for samples from nine wells (Table 1). The index shows the proportion of those marine palynomorphs considered to indicate proximity to shore, in this case sphaeromorph acritarchs, to the total number of marine microfossils, the result being expressed as a percentage:

 

Inshore Index=Sphaeromorph AcritarchsTotal marine palynomorphsx100

Table 1

Inshore indices and absolute abundances of sphaeromorph acritarchs and total marine palynomorphs for samples from nine wells. Abundance data are based on complete counts of one slide per sample; see the Palynofacies section in the text for discussion. Fully marine intervals are indicated in blue. Normal font indicates probable nearshore, marginal-marine assemblages. No data are available for Shiblah-1 and Abu Jifan-25.

WellDepth (ft)Sphaeromorph AcritarchsTotal Marine PalynomorphsInshore Index
Abu Jifan-2413,853.822100.00
 13,860.0010.00
 13,896.05214934.90
 13,897.5223661.11
 14,123.0141593.33
 14,124.863020.00
 14,166.8090.00
Farhah-113,925.0132552.00
 13,927.0131776.47
 13,926.0-13,927.56639.52
 13,925.3-13,928.1183158.07
 13,932.2263086.67
Haradh-4815,685.06785.71
 15,691.0929695.83
 15,691.44242100.00
 15,691.5757994.94
 15,711.0506181.97
Haradh-5214,874.9708087.50
 14,878.644100.00
 14,880.066100.00
 14,881.022100.00
 14,886.866100.00
 14,890.833100.00
 14,899.022100.00
 14,903.299100.00
 14,912.755100.00
 14,919.56785.71
 14,920.0545794.74
 14,922.5597380.82
 14,921.1-14,923.5486080.00
 14,925.510711791.45
 14,927.0151883.33
 14,932.4192382.61
 14,935.510111190.99
 14,936.0121866.67
 14,943.0-14,945.01333.33
 14,943.491464.29
 14,945.2606690.91
 15,445.4408050.00
 15,447.8213953.85
 15,450.0357050.00
 15,464.522100.00
Haradh-5716,304.233100.00
 16,311.0141593.33
 16,311.4184341.86
Hawtah-18,506.0303488.24
 8,509.5505198.04
 8,509.6485390.57
 8,518.6252986.21
 8,526.0363992.31
 8,526.2889097.78
 8,549.0566981.16
Khursaniyah-8114,256.8-14257.8808297.56
 14,257.0919298.91
 14,296.085315.09
 15,464.1-15468.14610643.39
 15,504.0152268.18
Mazalij-113,997.7105518.18
 13,998.26785.71
Tinat-217,953.0111478.57
 17,963.5010.00
 17,976.011100.00
 17,981.622100.00
 17,990.6121392.31
WellDepth (ft)Sphaeromorph AcritarchsTotal Marine PalynomorphsInshore Index
Abu Jifan-2413,853.822100.00
 13,860.0010.00
 13,896.05214934.90
 13,897.5223661.11
 14,123.0141593.33
 14,124.863020.00
 14,166.8090.00
Farhah-113,925.0132552.00
 13,927.0131776.47
 13,926.0-13,927.56639.52
 13,925.3-13,928.1183158.07
 13,932.2263086.67
Haradh-4815,685.06785.71
 15,691.0929695.83
 15,691.44242100.00
 15,691.5757994.94
 15,711.0506181.97
Haradh-5214,874.9708087.50
 14,878.644100.00
 14,880.066100.00
 14,881.022100.00
 14,886.866100.00
 14,890.833100.00
 14,899.022100.00
 14,903.299100.00
 14,912.755100.00
 14,919.56785.71
 14,920.0545794.74
 14,922.5597380.82
 14,921.1-14,923.5486080.00
 14,925.510711791.45
 14,927.0151883.33
 14,932.4192382.61
 14,935.510111190.99
 14,936.0121866.67
 14,943.0-14,945.01333.33
 14,943.491464.29
 14,945.2606690.91
 15,445.4408050.00
 15,447.8213953.85
 15,450.0357050.00
 15,464.522100.00
Haradh-5716,304.233100.00
 16,311.0141593.33
 16,311.4184341.86
Hawtah-18,506.0303488.24
 8,509.5505198.04
 8,509.6485390.57
 8,518.6252986.21
 8,526.0363992.31
 8,526.2889097.78
 8,549.0566981.16
Khursaniyah-8114,256.8-14257.8808297.56
 14,257.0919298.91
 14,296.085315.09
 15,464.1-15468.14610643.39
 15,504.0152268.18
Mazalij-113,997.7105518.18
 13,998.26785.71
Tinat-217,953.0111478.57
 17,963.5010.00
 17,976.011100.00
 17,981.622100.00
 17,990.6121392.31

The figures for sphaeromorph acritarchs and total marine palynomorphs shown in Table 1, and used to calculate the index, are based on complete counts of one slide per sample. The index is analogous to the inshore index of Richardson and Rasul (1990), although the calculation is slightly different (different groups were used in Richardson and Rasul’s calculation).

Two issues should be borne in mind when considering the results of this exercise. In the first place, there is no indication whether the palynological slides were prepared using quantitative techniques, and so the numbers of specimens cannot be related to a predetermined quantity of rock. Consequently, it is not clear whether the figures presented in Table 1 are truly comparable. Furthermore, the palaeoenvironmental interpretations presented below are based solely on the palynological evidence, and ought to be tested against evidence from other sources, including lithologies, lithofacies and sedimentological data. Until both issues are resolved, the interpretations presented here should be regarded as preliminary.

Abu Jifan-24

Samples from the upper part of the section studied in Abu Jifan-24, between 13,826.0 ft and 13,868.0 ft, were either barren or yielded very sparse marine palynomorphs, including two sphaeromorph acritarchs from 13,853.8 ft and a single chitinozoan, possibly Conochitina, from 13,860.0 ft (Figure 6). A more diverse assemblage, with an inshore index of 34.90, occurs in the Llanvirn section at 13,896.0 ft, but below that depth, the index increases to 61.11 at 13,897.5 ft and 93.33 at 14,123.0 ft (Table 1). The inshore index decreases in the lower part of the section, to 20.00 at 14,124.8 ft and 0.00 at 14,166.8 ft, but abundance in both cases is low, and the value for the lower sample reflects the fact that no sphaeromorph acritarchs were included among the nine acritarchs recorded (Table 1).

The figures outlined above suggest a fully marine interval in the Llanvirn section at 13,896.0 ft, with possible non-marine or nearshore, marginal-marine beds in higher parts of the section (13,826.0-13,868.0 ft). The low abundance, low diversity, sphaeromorph-dominated assemblages from 13,897.5 ft and 14,123.0 ft suggest marginal-marine beds below the fully marine interval, with increased marine influence in the lower part of the section (14,124.8 ft and 14,166.8 ft).

Farhah-1

The diversity of acritarch assemblages from the upper part of the section studied in Farhah-1 varies considerably. The composite sample shown in Figure 7 is based on three samples between 13,925.3 and 13,928.1 ft, i.e. 13,925.3 to 13,928.1 ft, 13,926.0 to 13,927.5 ft and 13,927.0 ft. The highest abundance and greatest diversity occurs in the sample from 13,926.0 to 13,927.5 ft, with an inshore index of 9.52 (Table 1). Other indices are higher, although the assemblages from 13,925.0 ft and 13,925.3-13,928.1 ft are reasonably diverse, reflected in their inshore indices being less than 60.00 (Figure 7, Table 1). The highest inshore index is that from 13,927.0 ft (76.47), the relatively sparse assemblage from that sample being dominated by sphaeromorph acritarchs.

Interpretation of the Llanvirn-Caradoc section in Farhah-1 is difficult because the range of sample depths means that superpositional relationships cannot be established. Nevertheless, the assemblage from 13,926.0 to 13,927.5 ft is fully marine, and may indicate the advance of shallow shelf seas into an otherwise nearshore, marginal-marine environment.

The sample from 13,932.2 ft in Farhah-1 yielded a sphaeromorph-dominated assemblage with an inshore index of 86.67, and is considered to indicate nearshore, marginal-marine conditions.

Haradh-48

All the samples from Haradh-48 yielded sphaeromorph-dominated assemblages with high inshore indices (Table 1). The highest sample, from 15,685.0 ft, yielded a sparse microflora, but abundance in the other samples is reasonably high. The inshore indices for the Arenig-Llanvirn section in Haradh-48 (Table 1) are considered to indicate marginal-marine conditions throughout (compare with the Arenig-Llanvirn section in Khursaniyah-81).

Haradh -52

Inshore indices throughout the Llanvirn section in Haradh-52 (14,874.9 to 14,945.2 ft) are generally greater than 80.00, reflecting the high sphaeromorph content of the samples. Lower values for samples from 14,936.0 ft, 14,943.0 to 14,945.0 ft and 14,943.4 ft (Table 1) are probably a consequence of their low yields rather than any change in environmental conditions.

The marine component of the samples from 14,878.6 ft to 14,919.5 ft consists almost entirely of rare sphaeromorph acritarchs, and may indicate extreme marginal-marine conditions. Greater marine influence is evident for the highest sample of the Llanvirn section (14,874.9 ft), and for the Llanvirn section below 14,920.0 ft (i.e. 14,921.1 to 14,945.2 ft). In both cases, there is an increase in acritarch diversity and chitinozoa are present (Figure 5). Nevertheless, the high proportion of sphaeromorph acritarchs suggests a nearshore, marginal-marine environment throughout.

Inshore indices for the Arenig-Llanvirn section in Haradh-52 (15,445.4 to 15,450.0 ft) are moderate (Table 1), but diversity is low (Figure 5). The inshore index values of about 50.00 reflect the fact that assemblages from the three samples in this interval are dominated by sphaeromorph acritarchs and Cymatiosphaera? in almost equal measure, with few other marine forms; 38, 13 and 32 specimens of Cymatiosphaera? were recorded from 15,445.4 ft, 15,447.8 ft and 15,450.0 ft respectively (compare with numbers of sphaeromorph acritarchs listed in Table 1). Given the low diversity of the assemblages and the high sphaeromorph content of the samples, nearshore, marginal-marine conditions are suggested. The lowest sample from Haradh-52, from 15,464.5 ft, yielded a very sparse assemblage of only two sphaeromorph acritarchs, possibly indicating extreme marginal-marine conditions.

Haradh-57

The upper two samples from Haradh-57 (16,304.2 ft and 16,311.0 ft) yielded relatively sparse, sphaeromorph-dominated assemblages (Table 1) which may indicate marginal-marine conditions. Greater marine influence is evident for the sample from 16,311.4 ft, although sphaeromorph acritarchs remain an important component of the assemblage (Table 1), and combined with the low diversity of the assemblage (Figure 9) suggest marginal-marine rather than fully marine conditions.

Hawtah-1

Microfloras throughout the section in Hawtah-1 (8,506.0 to 8,549.0 ft) are dominated by sphaeromorph acritarchs, resulting in high inshore indices (Table 1). The assemblage from the lowest sample is reasonably diverse (Figure 10), but a marginal-marine environment is nevertheless suggested for the entire section.

Khursaniyah-81

Samples from the Arenig-Llanvirn section in Khursaniyah-81 (Table 1) yielded sphaeromorph-dominated acritarch assemblages with high inshore indices, similar to those from the Arenig-Llanvirn section in Haradh-48. As in the case of the Haradh-48 assemblages, those from the Arenig-Llanvirn section in Khursaniyah-81 are thought to indicate nearshore, marginal-marine conditions. In contrast, sphaeromorph acritarchs are rare in the Tremadoc core sample from 14,296.0 ft, and the low inshore index of 15.09 (Table 1) is considered to indicate fully marine conditions. Moderate to high inshore indices for the Lower-Middle Cambrian section in Khursaniyah-81, combined with the extremely low diversity of the Cambrian assemblages (Figure 3), may indicate marginal-marine conditions.

Mazalij-1

There is a marked contrast between the assemblages from core at 13,997.7 ft and 13,998.2 ft in Mazalij-1. The Llandovery assemblage from 13,997.7 ft is diverse (Figure 8), with a low inshore index of 18.18 reflecting the relatively low sphaeromorph content (Table 1), whereas the sample from 13,998.2 ft yielded a sparse assemblage in which sphaeromorph acritarchs are the dominant form. These features suggest fully marine conditions for the Llandovery section at 13,997.7 ft, but possible marginal-marine conditions for the Ordovician section at 13,998.2 ft.

Tinat-2

The section between 17,953.0 ft and 17,990.6 ft in Tinat-2 yielded sparse assemblages in which the dominant forms are sphaeromorph acritarchs (Figure 12, Table 1). Inshore indices are consequently high throughout the section, suggesting possible nearshore, marginal-marine conditions for the entire interval.

The analysis presented above identifies fully marine conditions at restricted intervals in four wells: the Llanvirn section in Abu Jifan-24, especially 13,896.0 ft; the Llanvirn-Caradoc section at 13,926.0-13,927.5 ft in Farhah-1; the Tremadoc section in Khursaniyah-81; and the Llandovery section in Mazalij-1 (Table 1). The remainder of the sections studied in these wells, and the entire sections studied in the other five wells, yielded sphaeromorph-dominated assemblages of variable but often relatively high abundance that are considered to indicate nearshore, marginal-marine conditions. It is emphasised that the interpretations presented here are preliminary, for reasons outlined in the introduction to this section. Nevertheless, the results of the palynological study are consistent with Connally and Wiltse’s (1995) FMS-based recognition of non-marine, nearshore and shallow-marine depositional environments in the Lower Palaeozoic sandstones of central Saudi Arabia.

CORRELATION

Dating of sections in the central Saudi Arabian wells means that correlations can be suggested with the lithostratigraphy in northwest Saudi Arabia (Figure 2). Thus, the Cambrian, Tremadoc and Arenig-Llanvirn sections in Khursaniyah-81, Haradh-48 and Haradh-52 probably correlate with the Saq Formation and/or (in the case of the Llanvirn sections) the basal part of the Hanadir Member; the similarity between an assemblage from the Saq Formation in Qiba-1 and the Arenig-Llanvirn assemblages from Khursaniyah-81 and Haradh-48 has already been noted. The Llanvirn section between 14,874.9 ft and 14,945.2 ft in Haradh-52 may correlate with the Hanadir Member of the Qasim Formation, and would certainly correlate with the Hanadir if the assemblage represents the Siphonochitina formosa Biozone. The same may be true for the postulated Llanvirn section in Abu Jifan-24 (13,896.0 to 14,166.8 ft). Correlation of the Llanvirn-Caradoc section between 13,925.0 ft and 13,928.1 ft in Farhah-1 is more problematical because of the range of possible ages, but nevertheless correlation with some part of the Qasim Formation is indicated. The sample from 13,849.0 ft in Abu Jifan-25 is definitely from the Sarah Formation, and the marine Llandovery strata at 13,997.7 ft in Mazalij-1 probably correlate with the Qusaiba Member of the Qalibah Formation or a younger than usual Sarah Formation.

The occurrence of fully marine assemblages in Khursaniyah-81 (Tremadoc), Abu Jifan-24 (Llanvirn), Farhah-1 (Llanvirn-Caradoc) and Mazalij-1 (Llandovery) may also be important for correlation, because they might represent episodes of coastal onlap in otherwise marginal-marine (or non-marine) successions. If so, they may provide a means of precise correlation based on event stratigraphy, assisting in the construction of a sequence stratigraphic framework for the successions in these wells and for problematic Ordovician-Silurian successions in other wells. Stump et al. (1995), for example, identified major episodes of coastal onlap in the Middle Cambrian, Late Cambrian, early Llanvirn, mid Llandeilo (i.e. earliest Caradoc in the revised chronostratigraphy) and early Caradoc. The Llanvirn and Caradoc episodes resulted in the development of mid to outer shelf graptolitic mudstone facies in northwest Saudi Arabia, represented by the Hanadir and Ra’an members, the corresponding episodes of coastal offlap being represented by the shallow water sandstones and siltstones of the Kahfah and Quwarah members. It is conceivable that the fully marine palynomorph assemblages from Abu Jifan-24 and Farhah-1 record the spread of one or other of these episodes of coastal onlap into central Saudi Arabia.

ACKNOWLEDGEMENT

The authors publish with permission of the Saudi Arabian Ministry of Petroleum and Minerals, the Saudi Arabian Oil Company and the Director, British Geological Survey (N.E.R.C.).

APPENDIX

Authors of species cited

Acanthodiacrodium angustum (Downie) Combaz, 1967

Acanthodiacrodium cf. costatumBurmann, 1968

Acanthodiacrodium tuberatum? (Downie) Martin, 1972

Actinotodissus cf. A. longitaleosusLoeblich and Tappan, 1978

Annulum squamaceum (Volkova) Martin in Martin and Dean, 1983

Arkonia tenuataBurmann, 1970

Aureotesta clathrata clathrataVavrdová, 1972

Baiomeniscus cf. camurusLoeblich, 1970

Desmochitina minorEisenack, 1931

Dicrodiacrodium ancoriformeBurmann, 1970

Diexallophasis denticulata (Stockmans and Willière) Loeblich, 1970

Eremochitina sp. 2 of Al-Hajri, 1991

Frankea breviusculaBurmann, 1970

Frankea longiusculaBurmann, 1970

Frankea sartbernardensis (Martin) Colbath, 1986

Jenkinochitina lepta (Jenkins, 1969)

Moyeria cabottii (Cramer) Miller and Eames, 1982

Neoveryhachium carminae constrictaLe Hérissé, Al-Tayyar and van der Eem, 1995

Stellechinatum celestum (Martin) Turner, 1984

Stellechinatum helosumTurner, 1984

Striatotheca quieta (Martin) Rauscher, 1973

Trichosphaeridium annolovaenseTimofeev, 1966

Tylotopalla aff. digitiferaLoeblich, 1970

Tylotopalla cf. aniae (Jardiné et al.) Eisenack, Cramer and Díez, 1979

Veryhachium cf. longispinosum Jardiné, Combaz, Magloire, Peniguel and Vachey, 1974

Veryhachium lairdii Deflandre exLoeblich, 1970

Veryhachium subglobosum Jardiné, Combaz, Magloire, Peniguel and Vachey, 1974

Veryhachium trispinosum (Eisenack) Stockmans and Willière, 1962

Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan, 1976

Virgatasporites rudiiCombaz, 1967

Visbysphaera aff. oligofurcata (Eisenack) Lister, 1970

Vulcanisphaera cirritaRasul, 1976

REFERENCES

Al-Ameri
,
T.K.
1983
.
Acid-resistant microfossils used in the determination of Palaeozoic environments in Libya
.
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
44
, p.
103
-
116
.
Al-Hajri
,
S.
1991
.
Systematics and Biostratigraphy of Middle Ordovician to Lower Silurian chitinozoans of the Arabian Peninsula
. Master thesis of Geosciences,
University of Pennsylvania
.
311
p. (Unpublished).
Al-Hajri
,
S.
1995
.
Biostratigraphy of the Ordovician chitinozoa of northwestern Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
27
-
48
.
Albani
,
R.
1989
.
Ordovician (Arenigian) Acritarchs from the Solanas Sandstone Formation
,
Central Sardinia, Italy. Bollettino della Società Paleontologica Italiana
 , v.
28
, p.
3
-
37
.
Brocke
,
R.
,
O.
Fatka
and
T.
Servais
1997
.
A review of the Ordovician acritarchs Aureotesta and Marrocanium
.
Annales de la Société Géologique de Belgique
 , v.
120
, p.
1
-
21
.
Burmann
,
G.
1968
.
Diacrodien aus dem unteren Ordovizium
.
Paläontologische Abhandlungen, Abteilung B
 , v.
2
, p.
635
-
652
.
Burmann
,
G.
1970
.
Weitere organische Mikrofossilien aus dem unteren Ordovizium
.
Paläontologische Abhandlungen, Abteilung B
 , v.
3
, p.
289
-
332
.
Colbath
,
G.K.
1986
.
The Lower Palaeozoic organic-walled phytoplankton (‘acritarch’) genus Frankea Burmann 1970
.
Micropaleontology
 , v.
32
, p.
72
-
73
.
Combaz
,
A.
1967
.
Un microbios du Trémadocien dans un sondage d’Hassi-Messaoud
.
Actes de la Société linnéenne de Bordeaux
 , v.
104
, série B, no.
29
,
26
p.
Connally
,
T.C.
and
E.W.
Wiltse
1995
.
Correlation of Ordovician Sandstones in central Saudi Arabia
.
In M.I. Al-Husseini (Ed.), GEO’94: The Middle East Petroleum Geosciences. Gulf PetroLink, Bahrain
 , v.
1
, p.
321
-
333
.
Dorning
,
K.J.
1981
.
Silurian acritarch distribution in the Ludlovian shelf sea of South Wales and the Welsh Borderland
. In
J.W.
Neale
and
M.D.
Brasier
(Eds.),
Microfossils from recent and fossil shelf seas. Ellis Horwood Ltd
.,
Chichester, England
, p.
31
-
36
.
Downie
,
C.
1984
.
Acritarchs in British stratigraphy
.
Geological Society of London, Special Report
 
17
, p.
1
-
26
.
Eisenack
,
A.
1931
.
Neue Mikrofossilien des baltischen Silurs I
.
Paläontologische Zeitschrift
 , v.
13
, p.
74
-
118
.
Eisenack
,
A.
,
F.H.
Cramer
and
M.
del
C.R.
Díez
1979
.
Katalog der fossilen Dinoflagellaten, Hystrichosphären und verwandten Mikrofossilien
.
Band VI Acritarcha 4. Teil. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart
 .
531
p.
Fortey
,
R.A.
,
D.A.T.
Harper
,
J.K.
Ingham
,
A.W.
Owen
and
A.W.A.
Rushton
1995
.
A revision of Ordovician series and stages from the historical type area
.
Geological Magazine
 , v.
132
, p.
15
-
30
.
Gray
,
J.
and
A.J.
Boucot
1972
.
Palynological evidence bearing on the Ordovician-Silurian paraconformity in Ohio
.
Bulletin of the Geological Society of America
 , v.
83
, p.
1299
-
1313
.
Jachowicz
,
M.
1995
.
Ordovician acritarch assemblages from central and northwestern Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
19
-
25
.
Jacobson
,
S.R.
1979
.
Acritarchs as paleoenvironmental indicators in Middle and Upper Ordovician rocks from Kentucky
,
Ohio and New York. Journal of Paleontology
 , v.
53
, p.
1197
-
1212
.
Jardiné
,
S.
,
A.
Combaz
,
L.
Magloire
,
G.
Peniguel
and
G.
Vachey
1974
.
Distribution stratigraphique des Acritarches dans le Paléozoïque du Sahara Algérien
.
Review of Palaeobotany and Palynology
 , v.
18
, p.
99
-
129
.
Jenkins
,
W.A.M
1969
.
Chitinozoa from the Ordovician Sylvan Shale of the Arbuckle Mountains
,
Oklahoma. Palaeontology
 , v.
13
, p.
261
-
288
.
Le Hérissé
,
A.
,
H.
Al-Tayyar
and
H.
Van der Eem
1995
.
Stratigraphic and paleogeographical significance of Silurian acritarchs from Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
49
-
74
.
Lister
,
T.R.
1970
.
The acritarchs and chitinozoa from the Wenlock and Ludlow Series of the Ludlow and Millichope areas, Shropshire
.
Part 1. Palaeontographical Society Monograph, London, p. 1-100, Plates 1-13 (publication number 528, part of Volume 124 for 1970)
 .
Loeblich
,
A.R.
1970
.
Morphology, ultrastructure and distribution of Paleozoic acritarchs
.
Proceedings of the North American Paleontological Convention
,
1969
, Part G, p.
705
-
788
.
Loeblich
,
A.R.
and
H.
Tappan
1976
.
Some new and revised organic-walled phytoplankton microfossil genera
.
Journal of Paleontology
 , v.
50
, p.
301
-
308
.
Loeblich
,
A.R.
and
H.
Tappan
1978
.
Some Middle and Late Ordovician microphytoplankton from central North America
.
Journal of Paleontology
 , v.
52
, p.
1233
-
1287
.
Martin
,
F.
1972
.
Les Acritarches de l’Ordovicien inférieur de la Montagne Noire (Hérault
,
France). Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre
 , v.
48
, no.
10
, p.
1
-
61
.
Martin
,
F.
and
W.T.
Dean
1981
.
Middle and Upper Cambrian and lower Ordovician acritarchs from Random Island, eastern Newfoundland
.
Geological Survey of Canada, Bulletin
 
343
,
43
p.
Martin
,
F.
and
W.T.
Dean
1983
.
Late Early Cambrian and early Middle Cambrian Ordovician acritarchs from Manuels River, eastern Newfoundland
.
Geological Survey of Canada
, Paper 83-1B, p.
353
-
363
.
Martin
,
F.
and
W.T.
Dean
1988
.
Middle and Upper Cambrian acritarch and trilobite zonation at Manuels River and Random Island, eastern Newfoundland
.
Geological Survey of Canada, Bulletin
 
381
,
91
p.
McGillivray
,
J.G.
and
M.I.
Husseini
1992
.
The Paleozoic petroleum geology of central Arabia
.
American Association of Petroleum Geologists Bulletin
 , v.
76
, p.
1473
-
1490
.
McLure
,
H.A.
1988
.
Chitinozoan and acritarch assemblages, stratigraphy and biogeography of the Early Palaeozoic of Northwest Arabia
.
Review of Palaeobotany and Palynology
 , v.
56
, p.
41
-
60
.
Miller
.
M.A.
and
L.E.
Eames
1982
.
Palynomorphs from the Silurian Medina Group (lower Llandovery) of the Niagara Gorge
,
Lewiston, New York, U.S.A. Palynology
, v.
6
, p.
221
-
254
.
Molyneux
,
S.G.
,
A.
Le Hériss
é and
R.
Wicander
1996
.
Paleozoic phytoplankton
. In
J.
Jansonius
and
D.C.
McGregor
(Eds.),
Palynology: principles and applications. American Association of Stratigraphic Palynologists Foundation
 , v.
2
, chapter 16, p.
493
-
529
.
Paris
,
F.
,
J.
Verniers
,
S.
Al-Hajri
and
H.
Al-Tayyar
1995
.
Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from Central Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
86
, p.
75
-
90
.
Pittau
,
P.
1985
.
Tremadocian (Early Ordovician) Acritarchs of the Arburese Unit
,
Southwest Sardinia (Italy). Bollettino della Società Paleontologica Italiana
 , v.
23
, p.
161
-
204
.
Rasul
,
S.M.
1976
.
New species of the genus Vulcanisphaera (Acritarcha) from the Tremadocian of England
.
Micropaleontology
 , v.
22
, p.
479
-
484
.
Rasul
,
S.M.
and
C.
Downie
1974
.
The stratigraphic distribution of Tremadoc acritarchs in the Shineton Shales succession
,
Shropshire, England. Review of Palaeobotany and Palynology
 , v.
18
, p.
1
-
9
.
Rauscher
,
R.
1973
.
Recherches micropaléontologiques et stratigraphiques dans l’Ordovicien et le Silurien en France. Étude des Acritarches, des Chitinozoaires et des Spores
.
Sciences Géologiques, Université Louis Pasteur de Strasbourg, Institut de Géologie, Mémoire
 , v.
38
, p.
1
-
224
.
Richardson
,
J.B.
and
S.M.
Rasul
1990
.
Palynofacies in a Late Silurian regressive sequence in the Welsh Borderland and Wales
.
Journal of the Geological Society, London
 , v.
147
, p.
675
-
686
.
Smith
,
N.D.
and
R.S.
Saunders
1970
.
Paleoenvironments and their control of acritarch distribution: Silurian of east-central Pennsylvania
.
Journal of Sedimentary Petrology
 , v.
40
, p.
324
-
333
.
Stockmans
,
F.L.
and
Y.
Willière
1962
.
Hystrichosphères du Dévonien belge (sondage de l’Asile d’aliénés à Tournai)
.
Bulletin de la Société belge de Géologie, de Paléontologie et d’Hydrologie
 , v.
71
, p.
41
-
77
.
Strother
,
P.K.
,
S.
Al-Hajri
and
A.
Traverse
1996
.
New evidence for land plants from the lower Middle Ordovician of Saudi Arabia
.
Geology
 , v.
24
, p.
55
-
58
.
Stump
,
T.E.
,
S.
Al-Hajri
and
J.G.L.A.
Van der Eem
1995
.
Geology and biostratigraphy of the Late Precambrian through Palaeozoic sediments of Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
5
-
17
.
Timofeev
,
B.V.
1966
.
Mikropaleofitologicheskoe issledovanie drevnikh svit. Akademiya Nauk SSSR
 ,
Isdatelskvo Nauka
,
Moscow
, p.
1
-
147
.
Turner
,
R.E.
1984
.
Acritarchs from the type area of the Ordovician Caradoc Series
,
Shropshire, England. Palaeontographica, Abteilung B
 , v.
190
, p.
87
-
157
.
Vaslet
,
D.
1989
.
Late Ordovician glacial deposits in Saudi Arabia: a lithostratigraphic revision of the Early Paleozoic succession
.
Kingdom of Saudi Arabia, Deputy Ministry for Mineral Resources
 , Professional Paper 3.
Vavrdová
,
M.
1972
.
Acritarchs from Klabava Shales (Arenig)
.
Vestník Ústredního ústavu geologického
 , v.
47
, p.
79
-
86
, plates I and II.
Vavrdová
,
M.
1990a
.
Early Ordovician acritarchs from the locality M to near Rokycany (late Arenig
,
Czechoslovakia). Casopis pro mineralogii a geologii
 , v.
35
, p.
239
-
250
, plates I-IV.
Vavrdová
,
M.
1990b
.
Coenobial acritarchs and other palynomorphs from the Arenig/Llanvirn boundary
,
Prague basin. Vestník Ústredního ústavu geologického
 , v.
65
, p.
237
-
242
, plates I-IV.
Vavrdová
,
M.
1992
.
Excystment opening and the affinities of acritarchs
.
Acta Universitatis Carolinae, Geologica
 , nos.
3 and 4
, p.
361
-
383
.
Vavrdová
,
M.
1993
.
Acritarch assemblages in the Arenig Series of the Prague Basin, Czech Republic
. In
S.G.
Molyneux
and
K.J.
Dorning
(Eds.),
Contributions to acritarch and chitinozoan research. Special Papers in Palaeontology
 , no.
48
, p.
125
-
139
.
Wellman
,
C.H.
1996
.
Cryptospores from the type area of the Caradoc Series in southern Britain
. In
C.J.
Cleal
(Ed.),
Studies on early land plant spores from Britain. Special Papers in Palaeontology, no
 .
55
, p.
103
-
136
.

ABOUT THE AUTHORS

Stewart G. Molyneux is a Palynologist who has worked for the British Geological Survey, Nottingham, UK for the past 19 years. He specializes in Lower Palaeozoic palynology, and has published extensively on the stratigraphic application of palynology to Lower Palaeozoic successions in the UK. Stewart has also worked on Lower Palaeozoic successions and microfloras in North Africa and and Middle East.

Sa’id Al-Hajri (see page 17)

Figures & Tables

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 1:

Palaeozoic outcrops on the Arabian Peninsula, and the location of wells discussed in this study.

Figure 2:

Correlation of sections in eight wells from central Saudi Arabia with lithostratigraphy in northwest Saudi Arabia and with Lower Palaeozoic chronostratigraphy. Ordovician ages were obtained from a further three wells, Haradh-57, Shiblah-1 and Tinat-2 (see text). For each well, the length of the bar between the indicated depths indicates the range of possible ages and correlations for the sampled interval.

Figure 2:

Correlation of sections in eight wells from central Saudi Arabia with lithostratigraphy in northwest Saudi Arabia and with Lower Palaeozoic chronostratigraphy. Ordovician ages were obtained from a further three wells, Haradh-57, Shiblah-1 and Tinat-2 (see text). For each well, the length of the bar between the indicated depths indicates the range of possible ages and correlations for the sampled interval.

Figure 3:

Occurrences of acritarchs and cryptospores in Khursaniyah-81, with interpretations of ages and environments.

Figure 3:

Occurrences of acritarchs and cryptospores in Khursaniyah-81, with interpretations of ages and environments.

Plate 1:

Cambrian and Lower Ordovician acritarchs, cryptospores and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Archaeodiscina?, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,700). Specimen number FA118.

  • (b) Annulum squamaceum (Volkova) Martin, 1983. Lower-Middle Cambrian, Khursaniyah-81, 15,504.0 ft (x1,400). Specimen number FA119.

  • (c) Micrhystridium, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,400). Specimen number FA120.

  • (d) Vulcanisphaera cirritaRasul, 1976. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,296.0 ft (x850). Specimen number FA121.

  • (e) Acanthodiacrodium angustum (Downie) Combaz, 1967. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,320.0 to 14,330.0 ft (cuttings sample) (x1,700). Specimen number FA122.

  • (f) Virgatasporites rudiiCombaz, 1967. Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,256.8 to 14,257.8 ft (x1,700). Specimen number FA123.

  • (g) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,700). Specimen number FA124.

  • (h) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,900). Specimen number FA125.

  • (i) Laevolancis-like monad, Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,257.5 ft (x1,900). Specimen number FA126.

  • (j) Cymatiosphaera? Lower Ordovician (Arenig-Llanvirn), Haradh-52, 15,450.0 ft (x1,000). Specimen number FA127.

  • (k) sphaeromorph acritarch, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,600). Specimen number FA128.

  • (l) Eremochitina sp. 2 (Al-Hajri, 1991), Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x735). Specimen number FA129.

Plate 1:

Cambrian and Lower Ordovician acritarchs, cryptospores and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Archaeodiscina?, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,700). Specimen number FA118.

  • (b) Annulum squamaceum (Volkova) Martin, 1983. Lower-Middle Cambrian, Khursaniyah-81, 15,504.0 ft (x1,400). Specimen number FA119.

  • (c) Micrhystridium, Lower-Middle Cambrian, Khursaniyah-81, 15,464.1 ft (x1,400). Specimen number FA120.

  • (d) Vulcanisphaera cirritaRasul, 1976. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,296.0 ft (x850). Specimen number FA121.

  • (e) Acanthodiacrodium angustum (Downie) Combaz, 1967. Lower Ordovician (Tremadoc), Khursaniyah-81, 14,320.0 to 14,330.0 ft (cuttings sample) (x1,700). Specimen number FA122.

  • (f) Virgatasporites rudiiCombaz, 1967. Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,256.8 to 14,257.8 ft (x1,700). Specimen number FA123.

  • (g) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,700). Specimen number FA124.

  • (h) dyad, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,900). Specimen number FA125.

  • (i) Laevolancis-like monad, Lower Ordovician (Arenig-Llanvirn), Khursaniyah-81, 14,257.5 ft (x1,900). Specimen number FA126.

  • (j) Cymatiosphaera? Lower Ordovician (Arenig-Llanvirn), Haradh-52, 15,450.0 ft (x1,000). Specimen number FA127.

  • (k) sphaeromorph acritarch, Lower Ordovician (Arenig-Llanvirn), Haradh-48, 15,691.0 ft (x1,600). Specimen number FA128.

  • (l) Eremochitina sp. 2 (Al-Hajri, 1991), Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x735). Specimen number FA129.

Figure 4:

Occurrences of acritarchs and cryptospores in Haradh-48, with interpretations of age and environment.

Figure 4:

Occurrences of acritarchs and cryptospores in Haradh-48, with interpretations of age and environment.

Figure 5:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-52, with interpretations of ages and environments.

Figure 5:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-52, with interpretations of ages and environments.

Figure 6:

Occurrences of acritarchs, chitinozoans and cryptospores in Abu Jifan-24, with interpretations of ages and environments.

Figure 6:

Occurrences of acritarchs, chitinozoans and cryptospores in Abu Jifan-24, with interpretations of ages and environments.

Figure 7:

Occurrences of acritarchs and cryptospores in Farhah-1, with interpretations of ages and environments.

Figure 7:

Occurrences of acritarchs and cryptospores in Farhah-1, with interpretations of ages and environments.

Plate 2:

Ordovician acritarchs. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Aureotesta clathrata clathrataVavrdová, 1972. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA130.

  • (b) Veryhachium subglobosumJardiné et al., 1974. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA131.

  • (c) Dicrodiacrodium ancoriformeBurmann, 1970. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA132.

  • (d) Stellechinatum celestum (Martin) Turner, 1984. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA133.

  • (e) Striatotheca quieta (Martin) Rauscher, 1973. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,926.0 to 13,927.5 ft (x1,400). Specimen number FA134.

  • (f) Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan, 1976. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA135.

Plate 2:

Ordovician acritarchs. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Aureotesta clathrata clathrataVavrdová, 1972. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA130.

  • (b) Veryhachium subglobosumJardiné et al., 1974. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,300). Specimen number FA131.

  • (c) Dicrodiacrodium ancoriformeBurmann, 1970. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA132.

  • (d) Stellechinatum celestum (Martin) Turner, 1984. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA133.

  • (e) Striatotheca quieta (Martin) Rauscher, 1973. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,926.0 to 13,927.5 ft (x1,400). Specimen number FA134.

  • (f) Villosacapsula setosapellicula (Loeblich) Loeblich and Tappan, 1976. Middle Ordovician (Llanvirn), Abu Jifan-24, 13,896.0 ft (x1,400). Specimen number FA135.

Plate 3:

Ordovician and Silurian acritarchs and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Stellechinatum helosum? Turner, 1984. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,925.0 ft (x1,200). Specimen number FA136.

  • (b) Stelliferidium?, Ordovician?, Hawtah-1, 8,549.0 ft (x1,425). Specimen number FA137.

  • (c) Diexallophasis denticulata (Stockmans and Willière) Loeblich, 1970. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA138.

  • (d) Jenkinochitina lepta (Jenkins, 1969). Upper Ordovician, Mazalij-1, 14,040.0-14,085.0 ft (cuttings sample) (x800). Specimen number FA139.

  • (e) Tylotopalla cf. aniae (Jardiné et al.) Eisenack et al., 1979. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA140.

  • (f) Neoveryhachium carminae constrictaLe Hérissé et al., 1995. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x1,700). Specimen number FA141.

Plate 3:

Ordovician and Silurian acritarchs and chitinozoan. Figured material is housed in the Palaeontological Collections of the Natural History Museum, London.

  • (a) Stellechinatum helosum? Turner, 1984. Middle Ordovician (Llanvirn-Caradoc), Farhah-1, 13,925.0 ft (x1,200). Specimen number FA136.

  • (b) Stelliferidium?, Ordovician?, Hawtah-1, 8,549.0 ft (x1,425). Specimen number FA137.

  • (c) Diexallophasis denticulata (Stockmans and Willière) Loeblich, 1970. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA138.

  • (d) Jenkinochitina lepta (Jenkins, 1969). Upper Ordovician, Mazalij-1, 14,040.0-14,085.0 ft (cuttings sample) (x800). Specimen number FA139.

  • (e) Tylotopalla cf. aniae (Jardiné et al.) Eisenack et al., 1979. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x950). Specimen number FA140.

  • (f) Neoveryhachium carminae constrictaLe Hérissé et al., 1995. Lower Silurian (Llandovery), Mazalij-1, 13,997.7 ft (x1,700). Specimen number FA141.

Figure 8:

Occurrences of acritarchs and cryptospores in Mazalij-1, with interpretations of ages and environments.

Figure 8:

Occurrences of acritarchs and cryptospores in Mazalij-1, with interpretations of ages and environments.

Figure 9:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-57, with interpretations of ages and environments.

Figure 9:

Occurrences of acritarchs, chitinozoans and cryptospores in Haradh-57, with interpretations of ages and environments.

Figure 10:

Occurrences of acritarchs, chitinozoans and cryptospores in Hawtah-1, with interpretations of ages and environments.

Figure 10:

Occurrences of acritarchs, chitinozoans and cryptospores in Hawtah-1, with interpretations of ages and environments.

Figure 11:

Occurrences of acritarchs and cryptospores in Shiblah-1, with interpretations of ages and environments.

Figure 11:

Occurrences of acritarchs and cryptospores in Shiblah-1, with interpretations of ages and environments.

Figure 12:

Occurrences of acritarchs in Tinat-2, with interpretations of ages and environments.

Figure 12:

Occurrences of acritarchs in Tinat-2, with interpretations of ages and environments.

Table 1

Inshore indices and absolute abundances of sphaeromorph acritarchs and total marine palynomorphs for samples from nine wells. Abundance data are based on complete counts of one slide per sample; see the Palynofacies section in the text for discussion. Fully marine intervals are indicated in blue. Normal font indicates probable nearshore, marginal-marine assemblages. No data are available for Shiblah-1 and Abu Jifan-25.

WellDepth (ft)Sphaeromorph AcritarchsTotal Marine PalynomorphsInshore Index
Abu Jifan-2413,853.822100.00
 13,860.0010.00
 13,896.05214934.90
 13,897.5223661.11
 14,123.0141593.33
 14,124.863020.00
 14,166.8090.00
Farhah-113,925.0132552.00
 13,927.0131776.47
 13,926.0-13,927.56639.52
 13,925.3-13,928.1183158.07
 13,932.2263086.67
Haradh-4815,685.06785.71
 15,691.0929695.83
 15,691.44242100.00
 15,691.5757994.94
 15,711.0506181.97
Haradh-5214,874.9708087.50
 14,878.644100.00
 14,880.066100.00
 14,881.022100.00
 14,886.866100.00
 14,890.833100.00
 14,899.022100.00
 14,903.299100.00
 14,912.755100.00
 14,919.56785.71
 14,920.0545794.74
 14,922.5597380.82
 14,921.1-14,923.5486080.00
 14,925.510711791.45
 14,927.0151883.33
 14,932.4192382.61
 14,935.510111190.99
 14,936.0121866.67
 14,943.0-14,945.01333.33
 14,943.491464.29
 14,945.2606690.91
 15,445.4408050.00
 15,447.8213953.85
 15,450.0357050.00
 15,464.522100.00
Haradh-5716,304.233100.00
 16,311.0141593.33
 16,311.4184341.86
Hawtah-18,506.0303488.24
 8,509.5505198.04
 8,509.6485390.57
 8,518.6252986.21
 8,526.0363992.31
 8,526.2889097.78
 8,549.0566981.16
Khursaniyah-8114,256.8-14257.8808297.56
 14,257.0919298.91
 14,296.085315.09
 15,464.1-15468.14610643.39
 15,504.0152268.18
Mazalij-113,997.7105518.18
 13,998.26785.71
Tinat-217,953.0111478.57
 17,963.5010.00
 17,976.011100.00
 17,981.622100.00
 17,990.6121392.31
WellDepth (ft)Sphaeromorph AcritarchsTotal Marine PalynomorphsInshore Index
Abu Jifan-2413,853.822100.00
 13,860.0010.00
 13,896.05214934.90
 13,897.5223661.11
 14,123.0141593.33
 14,124.863020.00
 14,166.8090.00
Farhah-113,925.0132552.00
 13,927.0131776.47
 13,926.0-13,927.56639.52
 13,925.3-13,928.1183158.07
 13,932.2263086.67
Haradh-4815,685.06785.71
 15,691.0929695.83
 15,691.44242100.00
 15,691.5757994.94
 15,711.0506181.97
Haradh-5214,874.9708087.50
 14,878.644100.00
 14,880.066100.00
 14,881.022100.00
 14,886.866100.00
 14,890.833100.00
 14,899.022100.00
 14,903.299100.00
 14,912.755100.00
 14,919.56785.71
 14,920.0545794.74
 14,922.5597380.82
 14,921.1-14,923.5486080.00
 14,925.510711791.45
 14,927.0151883.33
 14,932.4192382.61
 14,935.510111190.99
 14,936.0121866.67
 14,943.0-14,945.01333.33
 14,943.491464.29
 14,945.2606690.91
 15,445.4408050.00
 15,447.8213953.85
 15,450.0357050.00
 15,464.522100.00
Haradh-5716,304.233100.00
 16,311.0141593.33
 16,311.4184341.86
Hawtah-18,506.0303488.24
 8,509.5505198.04
 8,509.6485390.57
 8,518.6252986.21
 8,526.0363992.31
 8,526.2889097.78
 8,549.0566981.16
Khursaniyah-8114,256.8-14257.8808297.56
 14,257.0919298.91
 14,296.085315.09
 15,464.1-15468.14610643.39
 15,504.0152268.18
Mazalij-113,997.7105518.18
 13,998.26785.71
Tinat-217,953.0111478.57
 17,963.5010.00
 17,976.011100.00
 17,981.622100.00
 17,990.6121392.31

Contents

GeoRef

References

REFERENCES

Al-Ameri
,
T.K.
1983
.
Acid-resistant microfossils used in the determination of Palaeozoic environments in Libya
.
Palaeogeography, Palaeoclimatology, Palaeoecology
 , v.
44
, p.
103
-
116
.
Al-Hajri
,
S.
1991
.
Systematics and Biostratigraphy of Middle Ordovician to Lower Silurian chitinozoans of the Arabian Peninsula
. Master thesis of Geosciences,
University of Pennsylvania
.
311
p. (Unpublished).
Al-Hajri
,
S.
1995
.
Biostratigraphy of the Ordovician chitinozoa of northwestern Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
27
-
48
.
Albani
,
R.
1989
.
Ordovician (Arenigian) Acritarchs from the Solanas Sandstone Formation
,
Central Sardinia, Italy. Bollettino della Società Paleontologica Italiana
 , v.
28
, p.
3
-
37
.
Brocke
,
R.
,
O.
Fatka
and
T.
Servais
1997
.
A review of the Ordovician acritarchs Aureotesta and Marrocanium
.
Annales de la Société Géologique de Belgique
 , v.
120
, p.
1
-
21
.
Burmann
,
G.
1968
.
Diacrodien aus dem unteren Ordovizium
.
Paläontologische Abhandlungen, Abteilung B
 , v.
2
, p.
635
-
652
.
Burmann
,
G.
1970
.
Weitere organische Mikrofossilien aus dem unteren Ordovizium
.
Paläontologische Abhandlungen, Abteilung B
 , v.
3
, p.
289
-
332
.
Colbath
,
G.K.
1986
.
The Lower Palaeozoic organic-walled phytoplankton (‘acritarch’) genus Frankea Burmann 1970
.
Micropaleontology
 , v.
32
, p.
72
-
73
.
Combaz
,
A.
1967
.
Un microbios du Trémadocien dans un sondage d’Hassi-Messaoud
.
Actes de la Société linnéenne de Bordeaux
 , v.
104
, série B, no.
29
,
26
p.
Connally
,
T.C.
and
E.W.
Wiltse
1995
.
Correlation of Ordovician Sandstones in central Saudi Arabia
.
In M.I. Al-Husseini (Ed.), GEO’94: The Middle East Petroleum Geosciences. Gulf PetroLink, Bahrain
 , v.
1
, p.
321
-
333
.
Dorning
,
K.J.
1981
.
Silurian acritarch distribution in the Ludlovian shelf sea of South Wales and the Welsh Borderland
. In
J.W.
Neale
and
M.D.
Brasier
(Eds.),
Microfossils from recent and fossil shelf seas. Ellis Horwood Ltd
.,
Chichester, England
, p.
31
-
36
.
Downie
,
C.
1984
.
Acritarchs in British stratigraphy
.
Geological Society of London, Special Report
 
17
, p.
1
-
26
.
Eisenack
,
A.
1931
.
Neue Mikrofossilien des baltischen Silurs I
.
Paläontologische Zeitschrift
 , v.
13
, p.
74
-
118
.
Eisenack
,
A.
,
F.H.
Cramer
and
M.
del
C.R.
Díez
1979
.
Katalog der fossilen Dinoflagellaten, Hystrichosphären und verwandten Mikrofossilien
.
Band VI Acritarcha 4. Teil. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart
 .
531
p.
Fortey
,
R.A.
,
D.A.T.
Harper
,
J.K.
Ingham
,
A.W.
Owen
and
A.W.A.
Rushton
1995
.
A revision of Ordovician series and stages from the historical type area
.
Geological Magazine
 , v.
132
, p.
15
-
30
.
Gray
,
J.
and
A.J.
Boucot
1972
.
Palynological evidence bearing on the Ordovician-Silurian paraconformity in Ohio
.
Bulletin of the Geological Society of America
 , v.
83
, p.
1299
-
1313
.
Jachowicz
,
M.
1995
.
Ordovician acritarch assemblages from central and northwestern Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
19
-
25
.
Jacobson
,
S.R.
1979
.
Acritarchs as paleoenvironmental indicators in Middle and Upper Ordovician rocks from Kentucky
,
Ohio and New York. Journal of Paleontology
 , v.
53
, p.
1197
-
1212
.
Jardiné
,
S.
,
A.
Combaz
,
L.
Magloire
,
G.
Peniguel
and
G.
Vachey
1974
.
Distribution stratigraphique des Acritarches dans le Paléozoïque du Sahara Algérien
.
Review of Palaeobotany and Palynology
 , v.
18
, p.
99
-
129
.
Jenkins
,
W.A.M
1969
.
Chitinozoa from the Ordovician Sylvan Shale of the Arbuckle Mountains
,
Oklahoma. Palaeontology
 , v.
13
, p.
261
-
288
.
Le Hérissé
,
A.
,
H.
Al-Tayyar
and
H.
Van der Eem
1995
.
Stratigraphic and paleogeographical significance of Silurian acritarchs from Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
49
-
74
.
Lister
,
T.R.
1970
.
The acritarchs and chitinozoa from the Wenlock and Ludlow Series of the Ludlow and Millichope areas, Shropshire
.
Part 1. Palaeontographical Society Monograph, London, p. 1-100, Plates 1-13 (publication number 528, part of Volume 124 for 1970)
 .
Loeblich
,
A.R.
1970
.
Morphology, ultrastructure and distribution of Paleozoic acritarchs
.
Proceedings of the North American Paleontological Convention
,
1969
, Part G, p.
705
-
788
.
Loeblich
,
A.R.
and
H.
Tappan
1976
.
Some new and revised organic-walled phytoplankton microfossil genera
.
Journal of Paleontology
 , v.
50
, p.
301
-
308
.
Loeblich
,
A.R.
and
H.
Tappan
1978
.
Some Middle and Late Ordovician microphytoplankton from central North America
.
Journal of Paleontology
 , v.
52
, p.
1233
-
1287
.
Martin
,
F.
1972
.
Les Acritarches de l’Ordovicien inférieur de la Montagne Noire (Hérault
,
France). Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre
 , v.
48
, no.
10
, p.
1
-
61
.
Martin
,
F.
and
W.T.
Dean
1981
.
Middle and Upper Cambrian and lower Ordovician acritarchs from Random Island, eastern Newfoundland
.
Geological Survey of Canada, Bulletin
 
343
,
43
p.
Martin
,
F.
and
W.T.
Dean
1983
.
Late Early Cambrian and early Middle Cambrian Ordovician acritarchs from Manuels River, eastern Newfoundland
.
Geological Survey of Canada
, Paper 83-1B, p.
353
-
363
.
Martin
,
F.
and
W.T.
Dean
1988
.
Middle and Upper Cambrian acritarch and trilobite zonation at Manuels River and Random Island, eastern Newfoundland
.
Geological Survey of Canada, Bulletin
 
381
,
91
p.
McGillivray
,
J.G.
and
M.I.
Husseini
1992
.
The Paleozoic petroleum geology of central Arabia
.
American Association of Petroleum Geologists Bulletin
 , v.
76
, p.
1473
-
1490
.
McLure
,
H.A.
1988
.
Chitinozoan and acritarch assemblages, stratigraphy and biogeography of the Early Palaeozoic of Northwest Arabia
.
Review of Palaeobotany and Palynology
 , v.
56
, p.
41
-
60
.
Miller
.
M.A.
and
L.E.
Eames
1982
.
Palynomorphs from the Silurian Medina Group (lower Llandovery) of the Niagara Gorge
,
Lewiston, New York, U.S.A. Palynology
, v.
6
, p.
221
-
254
.
Molyneux
,
S.G.
,
A.
Le Hériss
é and
R.
Wicander
1996
.
Paleozoic phytoplankton
. In
J.
Jansonius
and
D.C.
McGregor
(Eds.),
Palynology: principles and applications. American Association of Stratigraphic Palynologists Foundation
 , v.
2
, chapter 16, p.
493
-
529
.
Paris
,
F.
,
J.
Verniers
,
S.
Al-Hajri
and
H.
Al-Tayyar
1995
.
Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from Central Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
86
, p.
75
-
90
.
Pittau
,
P.
1985
.
Tremadocian (Early Ordovician) Acritarchs of the Arburese Unit
,
Southwest Sardinia (Italy). Bollettino della Società Paleontologica Italiana
 , v.
23
, p.
161
-
204
.
Rasul
,
S.M.
1976
.
New species of the genus Vulcanisphaera (Acritarcha) from the Tremadocian of England
.
Micropaleontology
 , v.
22
, p.
479
-
484
.
Rasul
,
S.M.
and
C.
Downie
1974
.
The stratigraphic distribution of Tremadoc acritarchs in the Shineton Shales succession
,
Shropshire, England. Review of Palaeobotany and Palynology
 , v.
18
, p.
1
-
9
.
Rauscher
,
R.
1973
.
Recherches micropaléontologiques et stratigraphiques dans l’Ordovicien et le Silurien en France. Étude des Acritarches, des Chitinozoaires et des Spores
.
Sciences Géologiques, Université Louis Pasteur de Strasbourg, Institut de Géologie, Mémoire
 , v.
38
, p.
1
-
224
.
Richardson
,
J.B.
and
S.M.
Rasul
1990
.
Palynofacies in a Late Silurian regressive sequence in the Welsh Borderland and Wales
.
Journal of the Geological Society, London
 , v.
147
, p.
675
-
686
.
Smith
,
N.D.
and
R.S.
Saunders
1970
.
Paleoenvironments and their control of acritarch distribution: Silurian of east-central Pennsylvania
.
Journal of Sedimentary Petrology
 , v.
40
, p.
324
-
333
.
Stockmans
,
F.L.
and
Y.
Willière
1962
.
Hystrichosphères du Dévonien belge (sondage de l’Asile d’aliénés à Tournai)
.
Bulletin de la Société belge de Géologie, de Paléontologie et d’Hydrologie
 , v.
71
, p.
41
-
77
.
Strother
,
P.K.
,
S.
Al-Hajri
and
A.
Traverse
1996
.
New evidence for land plants from the lower Middle Ordovician of Saudi Arabia
.
Geology
 , v.
24
, p.
55
-
58
.
Stump
,
T.E.
,
S.
Al-Hajri
and
J.G.L.A.
Van der Eem
1995
.
Geology and biostratigraphy of the Late Precambrian through Palaeozoic sediments of Saudi Arabia
.
Review of Palaeobotany and Palynology
 , v.
89
, p.
5
-
17
.
Timofeev
,
B.V.
1966
.
Mikropaleofitologicheskoe issledovanie drevnikh svit. Akademiya Nauk SSSR
 ,
Isdatelskvo Nauka
,
Moscow
, p.
1
-
147
.
Turner
,
R.E.
1984
.
Acritarchs from the type area of the Ordovician Caradoc Series
,
Shropshire, England. Palaeontographica, Abteilung B
 , v.
190
, p.
87
-
157
.
Vaslet
,
D.
1989
.
Late Ordovician glacial deposits in Saudi Arabia: a lithostratigraphic revision of the Early Paleozoic succession
.
Kingdom of Saudi Arabia, Deputy Ministry for Mineral Resources
 , Professional Paper 3.
Vavrdová
,
M.
1972
.
Acritarchs from Klabava Shales (Arenig)
.
Vestník Ústredního ústavu geologického
 , v.
47
, p.
79
-
86
, plates I and II.
Vavrdová
,
M.
1990a
.
Early Ordovician acritarchs from the locality M to near Rokycany (late Arenig
,
Czechoslovakia). Casopis pro mineralogii a geologii
 , v.
35
, p.
239
-
250
, plates I-IV.
Vavrdová
,
M.
1990b
.
Coenobial acritarchs and other palynomorphs from the Arenig/Llanvirn boundary
,
Prague basin. Vestník Ústredního ústavu geologického
 , v.
65
, p.
237
-
242
, plates I-IV.
Vavrdová
,
M.
1992
.
Excystment opening and the affinities of acritarchs
.
Acta Universitatis Carolinae, Geologica
 , nos.
3 and 4
, p.
361
-
383
.
Vavrdová
,
M.
1993
.
Acritarch assemblages in the Arenig Series of the Prague Basin, Czech Republic
. In
S.G.
Molyneux
and
K.J.
Dorning
(Eds.),
Contributions to acritarch and chitinozoan research. Special Papers in Palaeontology
 , no.
48
, p.
125
-
139
.
Wellman
,
C.H.
1996
.
Cryptospores from the type area of the Caradoc Series in southern Britain
. In
C.J.
Cleal
(Ed.),
Studies on early land plant spores from Britain. Special Papers in Palaeontology, no
 .
55
, p.
103
-
136
.

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal