Abstract Rock successions in Britain and Ireland, and more especially those in Wales, were instrumental in the founding and naming of the Ordovician System, and the Anglo-Welsh series established both initially and subsequently were used widely as a standard for Ordovician chronostratigraphy. Although now largely superseded in the global scheme of series and stages, they retain their local and regional importance. The Ordovician System in Britain and Ireland documents the history of a segment of the Earth's crust that incorporated opposing peri-Gondwanan and peri-Laurentian/Laurentian margins of the Iapetus Ocean during its closure, and is accordingly complex. The complexity arises from the volcanic and tectonic processes that accompanied oceanic closure coupled with the effects of eustatic sea-level changes, including the far-field effects of the Late Ordovician glaciation. For the past three decades, Ordovician successions in Britain and Ireland have been discussed in terms of terranes. Here we review Ordovician successions in each terrane, incorporating the results of recent research and correlating those successions via biostratigraphical schemes and radiometric dates to the global Ordovician series and stages.

Abstract Early to mid Palaeozoic marine phytoplankton are represented by acritarchs and associated forms, which had a global distribution from the early Cambrian to the early Carboniferous (Mississippian). Palaeozoic phytoplankton assemblages show varying degrees of cosmopolitanism and endemism through time. A high degree of cosmopolitanism was evidently characteristic of the Cambrian and much of the Late Ordovician, Silurian and Devonian, but provincialism was more marked in the Early Ordovician and Hirnantian (latest Ordovician), the latter at a time of major palaeoenvironmental perturbations. Distribution patterns of Palaeozoic phytoplankton are attributed to a number of interacting factors, including palaeolatitude, palaeotemperature, oceanic circulation patterns, the disposition of continents, differentiation between oceanic and cratonic (distal–proximal) assemblages, and sedimentary environments and facies. There are indications that biogeographical ranges of taxa shift over time. Moving our understanding of Palaeozoic phytoplankton biogeography forward requires (1) targeted investigation of regions and time periods for which no or little data exist, (2) quantitative analysis of data to investigate how similarity between regions varies through time and how this might correlate with other datasets such as carbon isotope stratigraphy or sea-level, and (3) rigorous application of well-defined time slices to compare coeval assemblages, at least within the limits of resolution.

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.