P. Stone

Are we now living in the Anthropocene

The term Anthropocene, proposed and increasingly employed to denote the current interval of anthropogenic global environmental change, may be discussed on stratigraphic grounds. A case can be made for its consideration as a formal epoch in that, since the start of the Industrial Revolution, Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable. The boundary may be defined either via Global Stratigraphic Section and Point (“golden spike”) locations or by adopting a numerical date. Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions. This datum, from the perspective of the far future, will most probably approximate a distinctive stratigraphic boundary.

A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland

Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

Stratigraphy of the Anthropocene

The Anthropocene, an informal term used to signal the impact of collective human activity on biological, physical and chemical processes on the Earth system, is assessed using stratigraphic criteria. It is complex in time, space and process, and may be considered in terms of the scale, relative timing, duration and novelty of its various phenomena. The lithostratigraphic signal includes both direct components, such as urban constructions and man-made deposits, and indirect ones, such as sediment flux changes. Already widespread, these are producing a significant ‘event layer’, locally with considerable long-term preservation potential. Chemostratigraphic signals include new organic compounds, but are likely to be dominated by the effects of CO2 release, particularly via acidification in the marine realm, and man-made radionuclides. The sequence stratigraphic signal is negligible to date, but may become geologically significant over centennial/millennial time scales. The rapidly growing biostratigraphic signal includes geologically novel aspects (the scale of globally transferred species) and geologically will have permanent effects.

Short Paper: Timing and regional implications of deformation in the Southern Uplands of Scotland

The timing of deformation in the Southern Uplands is constrained by tectonostratigraphical and intrusive relationships. Deformation migrated diachronously southwards, affecting the southern part of the Southern Uplands in the mid-Silurian. In contrast deformation in the Lake District cacan be shown by the same criteria to be Early Devonian. Thus, since the possibility of post-Emsian strike-slip along the Iapetus suture is remote, the regional tectonism cannot be related to a climactic episode due simply to closure of the Iapetus Ocean. Instead a southeast migrating deformation front is envisaged linking Silurian closure of Iapetus and the Southern Uplands backarc basin with Early Devonian collision farther south. The timing and geometry of the main cleavage-producing deformation within the paratectonic Caledonides of northern Britain is generally debated in terms of closure of the Iapetus Ocean. Structures developed are related to accretion of a collage of terranes onto the southern margin of Laurentia, within a sinistral transpressive stress regime, during the collision of the three plates Laurentia, Baltica and Eastern Avalonia (Soper & Hutton 1984). However, within this overall model there is a considerable divergence of opinion as to the chronology and correlation of the main episode of deformation across the Iapetus suture collision zone. In north-west England and Wales, recent regional syntheses propose that the main structures were formed during an end-Silurian/Early Devonian collision and subsequent NW-directed underthrusting. This climaxed with the main cleavage-producing episode in the Emsian (Soper et al. 1987; McKerrow 1988) which has been correlated with the Acadian Orogeny of the

Biostratigraphical control of thrust models for the Southern Uplands of Scotland

Graptolite biostratigraphy affords a robust and relatively accurate means of correlating Ordovician and Silurian hemipelagite and turbidite sequences and has been used to establish the structural development of the regional thrust belt in the Southern Uplands of Scotland. The overall structural pattern has long been recognised: greywackes within individual thrust slices, deposited within a relatively short time-interval, become sequentially younger southwards; each overlies the basal Moffat Shale Group which was deposited over a longer time. However, recent refinement of the graptolite biozonal scheme has allowed the better assessment of along-strike variations within the thrust belt which are here illustrated by two transects; one, based on work in the Rhins of Galloway and the Kirkcudbright areas (SW Southern Uplands), and the other in the Peebles-Hawick area (NE Southern Uplands). The SW transect most closely approximates to the regular pattern wherein a southward-propagating thrust-front incorporated sequentially younger greywacke units. The uniform geometry is interrupted only locally, towards the southern margin of the thrust belt, by a system of back-thrusts producing structural pop-ups. The NE transect departs from this regular model: a northern sector shows the orderly initiation of the thrust belt, but towards the SE a more irregular distribution of the thrust-slice agescan be best explained by outof- sequence movement. This transect also shows more repetitive imbrication of the same biostratigraphic interval than is apparent farther SW. In both transects the fundamental changes in thrustbelt geometry took place from mid-Llandovery times onwards, with a reversion to forward-breaking, in-sequence thrusting at the beginning of the Wenlock. The cause is a matter for speculation, but may be linked withthe accommodation of an obstacle to forward-thrust propagation. However it is recognised that such variationsin thrust geometry are a fundamental feature of most thrust belts and do not require a single regionally significant cause.

Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Palaeozoic terranes of southern Scotland: an assessment

The Notre Dame and Exploits subzones of Newfoundlands Dunnage Zone are correlated with the Midland Valley and Southern Uplands of Scotland, using detailed comparisons of two key Lower Palaeozoic successions which record similar histories of extension and compression. It follows that the Baie Verte Line, Red Indian Line and Dover Fault are equivalent to the Highland Boundary Fault, Southern Upland Fault and Solway Line, respectively. The Betts Cove Complex and overlying Snooks Arm Group of the Notre Dame Subzone are analogous to the Ballantrae Complex of the Midland Valley, both recording the Arenig evolution and subsequent obduction of an arc and back-arc system

Patterns of ostracod migration for the ‘North Atlantic’ region during the Ordovician

Abstract A review of Ordovician neritic ostracods from the ‘North Atlantic’ region including Europe and North America identifies over 100 genera (including 44 palaeocopes and 31 binodicopes) which show a complex pattern of migration between two or more of the palaeocontinents Gondwana, Ibero-Armorica, Perunica, Avalonia, Baltica and Laurentia. Many dispersals were relatively slow, and the migration of a genus between palaeocontinents often took the duration of one or more graptolite biozones. Over 70 migrations appear to have occurred more rapidly, including those of Pseudulrichia, a genus which dispersed to five palaeocontinents within the duration of three graptolite biozones. Longevity clearly facilitated the chances of migration, as the most widespread genera such as Vannieria, Platybolbina, Medianella and Euprimites, are often the most long-ranging. Low migration rates prior to the Llanvirn are, at least in part, related to low ostracod taxonomic diversity. Greatly increased diversity from the late Llanvirn coincided with a much higher rate of migration. Coupled with the spread of carbonate–mudstone shelf marine facies in Laurentia during the early and mid Caradoc, this resulted in the migration of up to 18 Baltic-origin genera to Laurentia. Relative to overall ostracod diversity, migration rates were generally higher during periods of lower global sea level, suggesting that ostracod dispersal may have been aided by mid-ocean islands or outer-shelf carbonate platforms, which provided more extensive island-hopping routes during periods of low sea level. The palaeogeographical convergence of Avalonia, Perunica and Baltica, and subsequently of Avalonia and Baltica with Laurentia, in low latitudes and warm surface waters, is suggested by increasing ostracod migration between these palaeocontinents from the late Llanvirn onwards. This culminated, during the Ashgill, in numerous species–level links. Baltica may have been the source area for more than 40 migrant genera, reflecting its high-diversity faunas and its intermediate palaeogeographical position between Laurentia and Avalonia. Several ostracod genera used Baltica as a staging-post in migrations between Avalonia and Laurentia. Migrations continued during the late Ashgill Hirnantian Stage (24 migrations), especially between Laurentia, Baltica and Avalonia (up to 19 migrations of genera), suggesting close geographical proximity for these palaeocontinents. Some ostracods, particularly the binodicopes Pseudulrichia, Klimphores, Kinnekullea, Aechmina and Spinigerites, could occupy outer-shelf and cooler-water benthic palaeoenvironments. They were part of a widespread deep-shelf fauna from the mid Caradoc onwards, for which distances or climatic barriers were less of an obstacle for trans-oceanic migration. None of these ostracods were bathyal.

Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland

The Silurian–Ordovician Southern Uplands terrane occupies a key position in the Caledonian orogen, yet its genesis is controversial. Marginal-basin, backarc, and forearc tectonic regimes have all been invoked as operative at the Laurentian margin of the Iapetus Ocean. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician suprasubduction volcanic arc, a central feature of most models. However, high-precision thermal-ionization mass spectrometer U-Pb and laser-ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557 ± 6 Ma (2σ) and probably also at 613 ± 12 Ma (2σ). The complex crystallization history recorded by the zircons shows assimilation of 1043 ± 7 Ma (2σ) Grenvillian basement into the andesite magma. The fact that no zircons have been found having ages that overlap the Caradocian depositional age of the host sedimentary rocks undermines all extant terrane models. The age profile of the detrital zircons is typical of Gondwana and Avalonia. This finding has important implications for the paleogeography of the Iapetus Ocean during the Ordovician, because the zircon data require the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.

Short Paper: Arc detritus in the Southern Uplands: mineralogical characterization of a ‘missing’ terrane

Within the late Ordovician-early Silurian Southern Uplands thrust belt, quartzo-feldspathic greywackes derived from the north are interbedded with volcanic-rich greywackes of southerly provenance. Abundant fresh clinopyroxenes which occur in the latter, both as single crystals and phenocrysts in lithic fragments, have compositions that indicate a calc-alkaline island arc origin.

Petrogenesis of boninites in the Ordovician Ballantrae Complex ophiolite, southwestern Scotland

Primitive lava and hyaloclastite with unusual, highly refractory compositions, form part of the Early Ordovician Balcreuchan Group within the ophiolitic Ballantrae Complex, southwestern Scotland. They are identified as likely high-Ca boninites on the basis of new XRF and INAA results and are the first unambiguous boninites to be discovered in the British Isles. The boninites are interbedded with low-Ti tholeiitic lavas with which they share some distinctive geochemical characteristics suggestive of a close petrogenetic relationship. The low-Ti tholeiite lavas have been interpreted as island-arc tholeiites but they also resemble back-arc basin basalts. The newly discovered boninites confirm an intra-oceanic environment of eruption; their distinctive features include relatively high SiO2, MgO, Cr and Ni but low Al2O3 and HFSE abundances, U-shaped REE patterns, low TiZr and high ZrHf ratios. Bulk geochemical trends are indicative of low-temperature, seawater-dominated alteration of the lavas but these alteration conditions apparently had little effect on the distribution of critical diagnostic elements such as Zr, Ti, Sc, Ta and the mid-heavy rare earths. We suggest that the Ballantrae boninites and low-Ti tholeiites represent different batch melts derived from a common, depleted mantle source region variably modified compositionally (i.e., made “streaky”) by fluids and/or melts during slab interaction (subduction metasomatism). A contribution from slab-derived pelagic sediments and/or a carbonatite melt is necessary to account for the fractionated, non-chondritic ZrHf ratios in the boninites. In view of the close compositional similarity of the Ballantrae lavas to Cenozoic boninite suites, we believe that these interpretations may have wider application to the petrogenesis of boninites in general.