Abstract The dating of terrestrial impact craters and impact glasses that exhibit high degrees of mineralogical complexity can be problematic. However, if the maximum potential of the terrestrial impact crater record is to be realized, accurate and precise ages for crater-forming events are critical. Here we report a high-precision 40 Ar/ 39 Ar age for the Dellen impact structure, Sweden. Previous radio-isotopic constraints show a wide variation in age as a result of poor sample characterization and analytical approach. A detailed petrographical and mineralogical study provides a solid foundation for interpretation of step-heating 40 Ar/ 39 Ar data, culminating in a statistically robust age of 140.82±0.51 Ma (2σ; full external precision) for the Dellen impact event, for which data disfavour an inherited argon component. Primary hydration of the impact melt during cooling–quenching and entrapment of molecular water promoted rapid loss of inherited 40 Ar from the impact melt of rhyolitic composition. Duplicate analyses of the water content and ∂D of the glass give similar values for the former (1.9±0.1 μmol mg −1 ) but unexpectedly low values for the latter (−159±8‰), with scatter beyond the expected analytical reproducibility due to isotopic heterogeneity. This study highlights that the 40 Ar/ 39 Ar technique is unrivalled in its ability to precisely and accurately date the products of hypervelocity collisional events. Supplementary material: Raw 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18633 .

Abstract The Witvlei Group is preserved in two regional synclinoria in the Gobabis-Witvlei area of east-central Namibia and as isolated outcrops 90 km SW of Rehoboth, itself some 200 km south of that area. It consists of mixed, coarse- to fine-grained siliciclastic and carbonate strata deposited in deep- to shallow-marine, and locally non-marine, settings along the post-rift continental margin of the Kalahari Craton prior to the onset of foreland basin sedimentation recorded by the overlying terminal Neoproterozoic–Cambrian Nama Group. No direct age constrains exist for the Witvlei Group, but it post-dates c. 800 Ma rift-related rocks and pre-dates the c . 548 Ma base of the Nama Group, thereby placing it as Cryogenian to Ediacaran in age. The Witvlei Group consists of three main units, from oldest to youngest, the Blaubeker, Court and Buschmannsklippe Formations. The Blaubeker Formation is highly variable in thickness and can be as much as 1000 m thick. It consists mostly of massive, polymict diamictite and, in the area of the type locality, contains conglomerate and pebbly sandstone beds. The diamictic strata combined with the presence of numerous faceted and striated clasts provide the evidence for glaciogenic influences on sedimentation. The highly variable thickness pattern likely reflects the infill of palaeo-valleys formed by the deep erosion and scouring of bedrock by ice, and the conglomerates and pebbly sandstones record glacial outwash processes. The Tahiti Formation is a locally developed, fine-grained sandstone above the Blaubeker Formation. It is poorly exposed and its exact stratigraphic relationship to the Blaubeker rocks and overlying Court Formation remains to be determined. The Blaubeker rocks are overlain sharply by the basal unit of the Court Formation, the Gobabis Member. This Member is from 20 to 60 m thick and consists mostly of dark and light grey laminated dolostones that display a δ 13 C carbonate profile that rises from values of −4‰ in the lowermost beds to values of 5‰ in the topmost. The Gobabis Member is conformably overlain by the shales, marls and thin limestones of the Constance Member followed by quartzites of the uppermost unit of the Court Formation, the Simmenau Member. The basal unit of the Buschmannsklippe Formation is the light to tan and pink grey dolostone of the Bildah Member. Its basal contact is sharp everywhere, and it is gradationally overlain by a coarsening (shoaling) upward succession from shales, thin limestones (some exhibiting formerly aragonitic fans) and fine sandstones of the La Fraque Member, to interbedded quartzites and stromatolitic and cherty dolostones of the Okambara Member. The δ 13 C carbonate profile for the Buschmannsklippe rocks shows that the basal beds of the Bildah Member begin at –4‰, followed by a decline to –6‰ in the lower La Fraque limestones and then a rise to –3‰ in the dolostones of the Okambara Member before being truncated by the base of the regionally unconformably overlying basal Weissberg Quartzite Member of the lower Nama Group. Although no glacial sediments have been recognized below the Bildah Member, its lithofacies character, stratigraphic position and C-isotopic profile are compatible with and strikingly similar to younger Cryogenian cap carbonates. Thus, the Witvlei Group arguably contains both the older and younger cap carbonates of Neoproterozoic time, but only the older Cryogenian glacial deposit.

Abstract Of the three major Neoproterozoic supracrustal units in the Scottish and Irish Highlands (the Torridonian, Moine and Dalradian Supergroups), only the latter contains evidence of Neoproterozoic glaciations. The Dalradian is siliciclastic-dominated and constitutes much of the Scottish–Irish Highlands between the Great Glen and Highland Boundary Fault Zones, and their correlatives in Ireland. At the time of writing, three stratigraphically distinct glacial intervals in the Dalradian have been documented in the literature. The oldest is the Port Askaig Formation (Fm.) at the base of the Argyll Group (see Arnaud & Fairchild 2011 ). It ranges from several tens to many hundreds of metres in thickness and occurs in numerous localities in Scotland and the north of Ireland. A second glacial is recorded in the middle part of the Argyll Group (Easdale Subgroup) and consists of localised sedimentary breccias as well as pelites and schists containing dropstone/lonestone units inferred to be ice-rafted debris; these rocks are patchily preserved and typically a few metres or less in thickness. It is sharply overlain by a variably developed carbonate unit that is marked by a 1–7-m-thick, light-coloured, basal dolostone or dolomitic limestone interpreted as a cap carbonate. This succession is best preserved in Donegal, Ireland, as the Stralinchy–Reelan glacial and Cranford cap-carbonate sequence. A correlative cap carbonate, the Whiteness Limestone, has been identified in the Shetland Islands. The third and youngest glacial is represented by locally preserved dropstone and polymict diamictite beds ranging in thickness from several to a few tens of metres in thickness in the lower Southern Highland Group. These include the MacDuff and Loch na Cille Boulder Beds in, respectively, NE and SW Scotland, and the Inishowen Beds in Donegal, Ireland.

Abstract The Strathclyde and Clackmannan Groups comprise the Lower Carboniferous successions exposed in the eastern Midland Valley of Scotland (MVS) and adjacent areas of northern England. They form a thick succession (2–3 km in thickness) of similar-appearing shallow-marine to shoreline and fluvial–deltaic sandstones and shales punctuated by thin (typically < 1–2 m thick) shallow-marine carbonate rocks. This study obtained stable-isotope data on the carbonate units in order to assess the utility of C isotopes as an independent means of testing and refining stratigraphic correlations. The δ 13 C carbonate data: (1) corroborate most of the lithostratigraphic correlations determined previously using other stratigraphic methodologies; (2) diagnose miscorrelations based on markedly different C-isotope profiles exhibited by carbonate units originally thought to be correlative; and (3) help discern patterns of varying rates of sediment flux and accommodation-space genesis across the eastern MVS basin. These results prove the utility of C-isotope profiles in helping construct and evaluate the stratigraphic framework of a sedimentary basin and highlight their usefulness as a tool that could be applied relatively quickly and inexpensively in areas of lesser-known geology and when time and financial investment are at a premium. The data also show that the Strathclyde Group has mostly negative C-isotope values (ca. -1 to -5‰) that shift abruptly to consistently positive values (0 to 2‰) at the contact with and into the overlying Clackmannan Group. Radiometric ages on volcanic rocks in the MVS constrain the negative interval to between ca. 343–335 Ma and that the shift to positive values occurred close to ca. 335 Ma. This trend coincides with a decline and recovery in δ 13 C trends documented in Visean rocks elsewhere and likely records a widespread (global?) shift in the isotopic composition of Early Carboniferous oceans. Application of Modern Stratigraphic Techniques: Theory and Case Histories SEPM Special Publication No. 94, Copyright © 2010 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-199-5, p. 143–151.

Abstract Field and diagenetic studies of injected sandstones occurring within the Dinantian oil-shale group of Scotland show that they provided subvertical pathways for petroleum fluid flow from host oil shales. Despite rapid cementation by pre- compactional ankerite, the injected sandstones became reactivated as fluid conduits during subsequent deformation that caused pervasive fracturing. An early phase of Fe dolomite and Fe ankerite veins shows no evidence of petroleum migration; however, abundant primary petroleum inclusions in a second phase of calcite veins, probably formed during Upper Carboniferous to Lower Permian Variscan deformation, suggests that the oil shales had reached maturity. Igneous intrusions may have been a heat source that caused oil maturation. This study shows that injected sandstones may form important fluid conduits for prolonged periods. Even when cemented, they form competent, subvertical structures that are ideally suited for localized faulting, fracturing, and fluid flow during deformation.

Abstract This study of four well characterized and adjacent terranes in Northern Britain outlines the sulphur isotope variations, assesses the overall importance of crustal and mantle sulphur, and presents a model that can be applied to terrane distinction throughout the North Atlantic Caledonides. The characteristics of metal components within the mineralization provide additional information that can be related to the nature of underlying basement and events from the onset of sedimentation to the cessation of mineralization within stratigraphically linked packages of rock. The δ 34 S data show that the dominant crustal units in each terrane, whether upper crustal sediments or cratonic basement, provide the main alternative sulphur source to the mantle and act also as the main contaminant of subcrustal melts. The δ 34 S values of granitoid-related mineralization are either within the subcrustal melt-range of −3‰ to +3‰ or deviate toward the values of major crustal units in the terrane, i.e. toward 34 S depletion in the Southern Uplands and toward 34 S enrichment in the Lakesman and Grampian terranes. More complex mineralization in the Northern Highland terrane is linked to the presence of thick North Atlantic craton beneath upper crustal metasediments. Across the region the vein systems beyond the influence of magmatic components represent homogenized sulphur, metals and fluids from local upper crustal units. The sulphur isotope data and style of mineralization for the British terranes are compared with terranes of similar age along strike in Eastern Canada revealing notable correlations.