Abstract A critique of palynostratigraphic zonation schemes associated with the Langsettian-Duckmantian Stage boundary shows that, at high levels of stratigraphic resolution, most criteria used as palynological proxies for the position of the boundary are unsuitable. Most importantly, in the Duckmantian stratotype section the highest stratigraphic occurrence of the index species Schulzospora rara is above (rather than below or within) the Vanderbeckei Marine Band, the base of which defines the Duckmantian Stage. A cored section through the boundary in southern North Sea well 44/22-1 displays comparable microfloral distributions. While data are as yet too sparse to provide detailed answers, the findings have implications for understanding how marine flooding events affected Carboniferous microfloral evolution and extinction. Ultimately this influences how microfloral biozone boundaries relate to the regional European Upper Carboniferous stage boundaries, which are largely defined in relation to strata deposited during marine flooding events similar to that associated with the Langsettian-Duckmantian boundary. The miospore genus Sinuspores Artüz is emended, and two new taxa are described: Gondisporites bulboides sp.nov. and Hymenospora murdochensis sp.nov.

ABSTRACT A Kingdom-wide palynological analysis was carried out on the Palaeozoic section within the framework of the Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) Project. Thirty-three biostratigraphic assemblages and zones have been documented as a result of this study. A palynostratigraphic and lithostratigraphic review of the Palaeozoic of Saudi Arabia is provided herein.

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.

ABSTRACT Biostratigraphic investigations have been carried out on Ordovician chitinozoans mostly from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation in central Saudi Arabia. Among the 96 core samples processed from seven wells, about half of them (from wells Berri-84, Shedgum-239, Ain Dar-196 and Ain Dar-277) provided workable specimens, whereas the other wells, namely Ain Dar-281, Haradh-51 and Abu Jifan-25 were barren. Some chitinozoan species of Baltic affinities (e.g. Laufeldochitina striata ) or of Laurentian affinities (e.g. Lagenochitina cf. pirum ) are present, but most of the recovered chitinozoan species are of northern Gondwana affinities. These chitinozoans allow accurate correlation with the local chitinozoan biozones already established for northern Saudi Arabia. Precise correlation with some of the Ordovician chitinozoan biozones for the northern Gondwana regions can also be proposed (e.g. lower part of the pissotensis biozone). The available samples are not, however, sufficiently closely spaced for characterizing all the Ordovician chitinozoan biozones. It is not yet possible, therefore, to document eventual hiatuses in the Ordovician sedimentary succession of central Saudi Arabia. The investigated samples from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation are respectively dated as Ashgill, late Caradoc/earliest Ashgill, Caradoc and Llanvirn. Strata referred to the Sarah Formation are probably of topmost Ashgill age but may range into earliest Llandovery. Several new species have been observed. They are presently kept in open nomenclature until better preserved material is available for proposing well-documented diagnosis.

ABSTRACT 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.

ABSTRACT Diverse and well-preserved acritarchs have been recovered from Qalibah Formation of Llandovery-Ludlow age, from well Kahf-1 drilled in northwestern Saudi Arabia. One new genus and 8 new species are described and illustrated: Baltisphaeridium qusaibaense sp. nov., Dictyotidium callum sp. nov., Duvernaysphaera siderea sp. nov., Eupoikilofusa tenua sp. nov., Geron ludlovensis sp. nov., Membranites minutus gen. et sp. nov., Muraticavea irregularis sp. nov., Proteolobus cylindrus sp. nov., and Veryhachium ? owensii sp. nov. Two new combinations are proposed: Eupoikilofusa punctalirella comb. nov. and Eupoikilofusa rhikne comb. nov. Ludlow age acritarchs were encountered from the uppermost Sharawra Member. Unlike in the reference sections, the palynofacies and lithofacies of the Sharawra Member indicate an offshore environment. The presence of fresh-water algae with marine palynomorphs in the upper Sharawra Member indicates a fresh-water influx into the marine system.

ABSTRACT A diversified assemblage of cryptospores and trilete miospores are reported from the Nuayyim-2 borehole, Saudi Arabia. Four new genera and six new species are described: Chelinohilates maculatus sp. nov., Complectitetras alhajrii gen. and sp. nov., Imperfectotriletes patinatus gen. and sp. nov., Laevolancis chibrikovae sp. nov., Sphaerasaccus glabellus gen. and sp. nov., and Vestitusdyadus qalibahinus gen. and sp. nov. Independent biostratigraphical control using chitinozoans and graptolites indicates that all the studied samples are Llandovery (Rhuddanian) in age. A new biostratigraphic scheme for strata near the Ashgill-Llandovery boundary is proposed: one assemblage biozone is divided into two interval biozones that are both further subdivided into two sub-biozones. The criteria used to create these biozones are the first occurrences of taxa (including the earliest trilete spores) and the relative abundance of envelope-enclosed cryptospores. The palaeogeographically widespread distribution of similar spore assemblages during the Late Ordovician-Early Silurian suggested that the parent plants were cosmopolitan and thrived in a variety of climatic conditions. Therefore, they were little affected by climatic changes associated with the Hirnantian glaciation. This might provide an explanation for the apparent lack of significant impoverishment in spore biodiversity during this glaciation.

ABSTRACT Spore assemblages are described from three cored intervals in the Silurian sequence in borehole Hawiyah-151 from central Saudi Arabia. Those from the oldest cored interval are correlated with the Imperfectotriletes spp.- Laevolancis divellomedia spore Assemblage Biozone [ divellomedia Interval Biozone (α Subzone)], suggesting a Rhuddanian (possibly early Aeronian) age. The older samples from the middle cored interval are correlated with the Imperfectotriletes spp.- Laevolancis divellomedia spore Assemblage Biozone [ divellomedia Interval Biozone (β Subzone)], suggesting a late Aeronian-early Telychian age, but the younger samples from this interval are correlated with the chulus-nanus spore Assemblage Biozone, suggesting a late Telychian-early Homerian age. Samples from the youngest cored interval are correlated with the brevicosta-verrucatus spore Assemblage Biozone, suggesting an early (but not earliest) Homerian-?earliest Gorstian age. Palynofacies analysis indicates that the deposits of the oldest and middle cored interval accumulated in a nearshore marine environment, but those of the youngest cored interval were deposited during a nearshore marine to non-marine regressive sequence. In the middle cored interval, the abundance of chitinozoans coupled with the paucity of acritarchs is intriguing. The sequence of Silurian spore assemblages in Saudi Arabia is clearly different to that reported in Laurentia, Avalonia and probably also Baltica. In Saudi Arabia, trilete spores and ornamented hilate cryptospores appear to make their inception earlier, and envelope-enclosed cryptospores persist in relative abundance for longer.