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 Although now largely superseded in the global scheme of series and stages, they retain their local and regional importance.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 This chapter describes and presents a newly compiled map illustrating the paleogeography of Laurentia during the earliest Ordovician, a time when the great American carbonate bank was at one of its greatest extents and a period for which the most is understood. The map depicts the known or postulated extent of the inner detrital belt, the great American carbonate bank and the more problematic (commonly structurally relocated) outer detrital belt. The period on which the map is based and discussed in the accompanying text is based on the Early Ordovician (early Ibexian) (early Tremadocian) Stonehenge transgression.

Abstract The Cambrian–Ordovician succession of northwestern Scotland represents a fragment of a once-continuous southeastern Laurentian margin stretching between western Newfoundland and eastern Greenland. The subdivisions of the Sauk megasequence are recognized within the Scottish succession for the first time. The Sauk I supersequence corresponds to deposition of the Ardvreck Group, which is composed of a succession of arkoses, quartz arenites, and siltstones. The overlying Sauk II and III supersequences are represented by carbonates of the Durness Group. Within the Durness Group, several smaller sequences can be identified (interpreted as third order). The Sauk II supersequence can be divided into two sequences of 83- and 75-m (272- and 246-ft) thickness. These may correlate with Cambrian grand cycles recognized in other parts of Laurentia. The Sauk III supersequence contains four smaller sequences, some of which correlate with sequences observed in western Newfoundland. The top of the Sauk megasequence is not observed because of truncation of the available section by a thrust fault.

The East Greenland Caledonides, which make up an ∼1300-km-long stretch of North-East Greenland, were formed by the collision of Laurentia and Baltica in mid-Silurian time. Geological mapping and research in this remote and poorly accessible segment of the circum-Atlantic Caledonide orogen began in connection with geographical exploration voyages in the early part of the nineteenth century. The first regional geological mapping took place during the long series of “The Danish Expeditions to North-East Greenland” between 1926 and 1958. Modern geological research and regional mapping by the Geological Survey of Denmark and Greenland between 1968 and 1998 have resulted in the publication of a series of 1:500,000 geological maps of the orogen, and an overview geological map at 1:1,000,000 scale, which accompanies this volume. This article reviews the history of geological research and the evolution of interpretations of the orogen. The recent systematic studies by the Geological Survey of Denmark and Greenland supplement and build on the considerable existing published literature and demonstrate that the North-East Greenland segment of the Caledonide orogen consists of a westward-propagating thrust sheet pile, with displacements estimated at 300–500 km. The thrust sheets incorporate major segments of reworked Laurentian gneiss basement, and a thick succession of Neoproterozoic to Ordovician sediments that accumulated in a major basin originally located outboard of the present coastline.

The East Greenland Caledonian orogen can be divided into distinct structurally bound geological domains composed of Archean to Lower Paleozoic lithostrati graphic and lithodemic components derived from the eastern margin of Laurentia. These domains originally evolved as major westward-displaced thrust units in the overriding plate during the collision with Baltica. The western border of the 1300-km-long and up to 300-km-wide segment of the orogen preserved onshore in East Greenland is thrust against the rocks of the Laurentian craton and is largely concealed beneath the Inland Ice. A foreland-propagating thrust pile is well-preserved in the extreme north of the orogen (79°N–82°N), and in the southern half (70°N–76°N), with less-well-preserved remnants in the western nunataks of the intervening region. Between 76°N and 81°N, the outer coastal region is dominated by high-grade Paleoproterozoic orthogneisses that were reworked during the Caledonian orogeny; most of this region is characterized by the presence of eclogitic mafic enclaves, which testify to exhumation from depths in excess of 50 km in late Caledonian time. Caledonian granites are confined to the southern orogen (70°N–76°N), where they intrude rock units now contained within the upper thrust sheet. Devonian continental basins are conspicuous in the southern part of the orogen and occur offshore farther north; their deposition can be linked to syn- to late-orogenic extension. Carboniferous and younger rocks are exposed onshore in the extreme north of the orogen (80°N–81°N) and are widespread in the south between 71°N and 75°N.

The Caledonian orogen of East Greenland contains remnants of Archean, Paleoproterozoic, late Mesoproterozoic, and early Neoproterozoic rocks that occur within far-traveled thrust sheets, and bear witness to a complex polyorogenic history of the region prior to Caledonian orogenesis. Archean and Paleoproterozoic complexes consist mainly of granitoid orthogneisses. A succession of Paleoproterozoic tholeiitic metabasalts is present in some of the foreland windows. A major unit of late Meso-proterozoic metasedimentary rocks (Krummedal supracrustal sequence) contains early Neoproterozoic (ca. 950 Ma) as well as Caledonian granites. There is evidence for Archean (ca. 2800–2600 Ma), Paleoproterozoic (2000–1750 Ma), and late Grenvillian (ca. 950 Ma) deformation and metamorphism, but Caledonian overprinting complicates the study of these events. This paper presents a broad overview of the various rock units with structural, geochemical, and geochronologic data. The Paleoproterozoic metabasaltic rocks from the foreland windows are described in more detail.

The crystalline basement within the northern parts of the Caledonian orogen, and in the adjacent foreland, is overlain by a several-kilometer-thick succession of sedimentary and volcanic rocks, the Paleoproterozoic–Mesoproterozoic Independence Fjord Group and the Mesoproterozoic Zig-Zag Dal Basalt Formation. The lowermost strata of the Independence Fjord Group, composed of quartzitic and feldspathic sandstones and conglomerates with interbedded volcanic rocks, occur within the Caledonian orogen and are strongly deformed. These strata were deposited around 1740 Ma ago, and they were associated with a period of rifting that succeeded a long sequence of Paleoproterozoic orogenic events. Similar sandstones, interbedded with siltstone units but without volcanic rocks, are widespread in the Caledonian foreland, where they are virtually undeformed. These foreland deposits were laid down in a continental sag basin under semiarid conditions. Sedimentary structures indicate a largely fluvial origin, with intermittent eolian transport. The siltstones were deposited in extensive shallow lakes. Desiccated bedding surfaces show that these periodically dried out. The sandstones of the Independence Fjord Group are cut by a multitude of doleritic sheets and dikes, the ca. 1380 Ma Midsommersø Dolerites, and more silicic intrusions, most of which show evidence of hydrothermal alteration and variable contamination with components derived from the crystalline basement and the sandstones. Some intrusions consist almost entirely of crustally derived material. The Zig-Zag Dal Basalt Formation conformably overlies the Independence Fjord Group. Compositional similarities suggest a genetic relationship with the Midsommersø Dolerites, but the basalts appear to be less crustally contaminated. The basalts were deposited within a basin that underwent subsidence during and after volcanic activity. The Zig-Zag Dal Basalt Formation is unconformably overlain by Neoproterozoic sedimentary successions. The unconformity represents a stratigraphic hiatus of some 500 m.y., for which no information is available from North Greenland.

Two major Neoproterozoic sedimentary basins that probably formed in response to an early pulse of Iapetan rifting along the Laurentian margin are well exposed in the East Greenland Caledonides. The Hekla Sund Basin is exposed at the northern termination of the East Greenland Caledonides, and it is represented by the Rivieradal and Hagen Fjord Groups, which attain a cumulative thickness of 8–11 km. The evolution of this basin reflects deposition during active rifting and a postrift thermal equilibration stage. The Eleonore Bay Basin of East Greenland includes the deposits of the Eleonore Bay Supergroup of early Neoproterozoic age overlain by Cryogenian (mid-Neoproterozoic) glacial deposits of the Tillite Group, which have a combined thickness in excess of 14 km. Four stages of basin evolution may be distinguished based on paleogeographic reorganizations of the shelf and a change from siliciclastic to carbonate deposition, and the final stage was dominated by glacigenic deposition. Major regional stratigraphic breaks seem to be absent, as is other evidence of rift-related sedimentation, suggesting deposition in one or a series of connected ensialic basins. A comparison with other Neoproterozoic basins along the Laurentian margin of the Iapetus Ocean shows similarities between the Eleonore Bay Basin and coeval deposits on Svalbard and the Central Highlands of Scotland. The development of an extensive carbonate platform during the later stages of both the Eleonore Bay and Hekla Sund Basins testifies to a period of tectonic stability prior to onset of Iapetus rifting. The extent of this carbonate platform may have been even larger, since similar successions are present in the Caledonides of Scotland and Ireland.

The Iapetus margin of Laurentia is preserved, with varying degrees of deformation, along a belt that extends for 1300 km along the eastern coast of Greenland, from Scoresby Sund in the south to Kronprins Christian Land at the northernmost extent of the Caledonian–Appalachian orogen. Along the length of the Greenland Caledonides, deformation is restricted to a single orogenic phase, the Scandian, at around 425 Ma, which represents the continent-continent collision of Laurentia and Baltica. The Lower Paleozoic stratigraphy can be closely correlated with the palinspastically contiguous terranes of NE Spitsbergen, Bjørnøya, and NW Scotland, and, farther to the south, that of western Newfoundland. In Greenland itself, Lower Paleozoic sediments are present in the foreland, parautochthon, and the highest allochthonous sheet of the orogen, the Franz Joseph allochthon. In the Franklinian Basin of eastern North Greenland, unconformity-bounded Lower Cambrian sediments can be correlated with the Sauk I sequence of cratonic North America. These Cambrian sediments are separated from younger units by a significant hiatus, the sub–Wandel Valley unconformity, but above that surface, the succession extends without major breaks from the major flooding event at the base of Sauk IV (Early Ordovician) through to the early Wenlock. The carbonate platform in this region foundered from late Llandovery time onward due to loading by thrust sheets, and turbidite deposition replaced platform carbonate deposition. Caledonian thrusts truncate the youngest preserved sediments, which are of early Wenlock age. The punctuated, attenuated stratigraphy seen in Kronprins Christian Land continues southward along the length of the parautochthon, through Lambert Land, Nørreland, and Dronning Louise Land, to a series of tectonic windows in the southern part of the Greenland Caledonides. In contrast to the stratigraphy seen in the parautochthon, the Franz Joseph allochthon contains one of the thickest Cambrian–Middle Ordovician successions in Laurentia, including a complete succession from Sauk I to Tippecanoe II.

The 1300-km-long, up to 300-km-wide onshore segment of the East Greenland Caledonian orogen is divided into distinct structurally bound geological domains that originally evolved as major westward-displaced thrust units during collision with Baltica. The thrust systems accommodated contraction of an already complex Laurentian assembly of Archean to Neoproterozoic and Cambrian to Silurian lithostratigraphic units and are a consequence of the convergence, and final collision, of Baltica with Laurentia in the mid- to late Silurian Scandian orogeny. The transition from undisturbed foreland to orogen is perfectly preserved in the extreme north of the East Greenland Caledonides, where a younger lower (Vandredalen) thrust sheet carrying older thrust sheets (Western thrust belt) is displaced westward across a thin-skinned fold-and-thrust belt. In the southern half of the orogen, a pile of far-traveled thrust sheets (from youngest to oldest, Gemmedal, Niggli Spids, Hagar Bjerg thrust sheets) is displaced WNW across parautochthonous foreland windows, and the intact foreland is only intermittently exposed at the margin of the Inland Ice in the far west. These westward- and foreland-propagating systems are distinct from the Nørreland thrust sheet, the coastal region between 76°N and 79°N, in which Paleoproterozoic basement gneiss lithologies host enclaves of Devonian and Carboniferous eclogite-facies rocks. These rocks must have been exhumed from the roots of the collisional orogen, and their age suggests that the Nørreland thrust may be out of sequence relative to the main WNW foreland-propagating systems.