Showing papers in "Journal of the Geological Society in 2016"

The Eurekan deformation in the Arctic: an outline

Abstract: The evolution of the Eurekan deformation zones in the Arctic is closely related to the development of the circum-Greenland plate boundaries in Early Cenozoic times (53 – 34 Ma). Mostly, the Eurekan Orogeny or deformation has been interpreted as a predominantly compressive tectonic event, but the Eurekan deformational history in the Arctic was not the result of a single tectonic episode. It rather represents a complex sequence of successive tectonic stages, which produced a number of intra-continental deformation zones with changing, sometimes opposing, lateral, oblique and convergent kinematics in the Canadian Arctic Archipelago, north and NE Greenland, and Svalbard. The interaction between the continental plates, especially in combination with the development of transform faults, resulted onshore in the formation of several complex deformation zones and areas of Eurekan deformation. The Eurekan deformation can be divided into two major tectonic stages: the first phase in the Early Eocene was dominated by orthogonal compression in the West Spitsbergen Fold-and-Thrust Belt along the west margin of the Barents Shelf and contemporaneous sinistral strike-slip tectonics along the Wegener Fault and on Ellesmere Island, whereas the second phase in the Late Eocene was characterized by dextral strike-slip and compression on Ellesmere Island and contemporaneous dextral transpression and transtension along the De Geer Fracture Zone or Hornsund Fault Complex between NE Greenland and Spitsbergen.

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

Abstract: The use of sedimentary structures as indicators of flow and sediment morphodynamics in ancient sediments lies at the very heart of sedimentology, and allows reconstruction of formative flow conditions generated in a wide range of grain sizes and sedimentary environments. However, the vast majority of past research has documented and detailed the range of bedforms generated in essentially cohesionless sediments that lack the presence of mud within the flow and within the sediment bed itself. Yet most sedimentary environments possess fine-grained sediments and recent work has shown how the presence of this fine sediment may substantially modify the fluid dynamics of such flows. It is increasingly evident that understanding the influence of mud, and the presence of cohesive forces, is essential to permit a fuller interpretation of many modern and ancient sedimentary successions. In this paper, the present state of knowledge on the stability of current- and wave-generated bedforms and their primary current stratification is reviewed, and a new extended bedform phase diagram is presented that summarizes the bedforms generated in mixtures of sand and mud under rapidly decelerated flows. This diagram provides a phase space using the variables of yield strength and grain mobility as the abscissa and ordinate axes, respectively, and defines the stability fields of a range of bedforms generated under flows that have modified fluid dynamics owing to the presence of suspended sediment within the flow. Our results also present unique data on a range of bedforms generated in such flows, whose recognition is essential to help interpret such deposits in the ancient sedimentary record, including the following: (1) heterolithic stratification, comprising alternating laminae or layers of sand and mud; (2) the preservation of low-amplitude bed-waves, large current ripples and bed scours with intrascour composite bedforms; (3) low-angle cross-lamination and long lenses and streaks of sand and mud formed by bed-waves; (4) complex stacking of reverse bedforms, mud layers and low-angle cross-lamination on the upstream face of bed scours; (5) planar bedding comprising stacked mud–sand couplets. Furthermore, the results shown herein demonstrate that flow variability is not required to produce deposits consisting of interbedded sand and muds, and that the nature of flaser, wavy and lenticular bedding ( sensu Reineck & Wunderlich 1968) may also need reconsideration in the deposits of such sediment-laden flows.

The Emu Bay Shale Konservat-Lagerstätte: a view of Cambrian life from East Gondwana

Abstract: Recent fossil discoveries from the lower Cambrian Emu Bay Shale (EBS) on Kangaroo Island, South Australia, have provided critical insights into the tempo of the Cambrian explosion of animals, such as the origin and seemingly rapid evolution of arthropod compound eyes, as well as extending the geographical ranges of several groups to the East Gondwanan margin, supporting close faunal affinities with South China. The EBS also holds great potential for broadening knowledge on taphonomic pathways involved in the exceptional preservation of fossils in Cambrian Konservat-Lagerstatten. EBS fossils display a range of taphonomic modes for a variety of soft tissues, especially phosphatization and pyritization, in some cases recording a level of anatomical detail that is absent from most Cambrian Konservat-Lagerstatten.

Age constraints of the glaciation in the Paraná Basin: evidence from new U–Pb dates

Abstract: The absence of absolute dating in glacial deposits of the Parana Basin prevents better understanding of the complete history of the Late Palaeozoic Ice Age in Western Gondwana and the climatic change that took place in the late Carboniferous. In this study, new U–Pb radiometric ages date the topmost glacial deposits of the Itarare Group at 307.7 ± 3.1 Ma (Kasimovian−Moscovian) and the base of post-glacial deposits of the Rio Bonito Formation at 298.8 ± 1.9 Ma (Ghzelian−Asselian). Correlation of glacial and post-glacial deposits of the Parana Basin with other southwestern Gondwana basins shows that the end of the Late Palaeozoic Ice Age is asynchronous. Supplementary material: Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon data and methods for the ash sample of the Itarare Group and the tonstein sample of the Rio Bonito Formation are available at https://doi.org/10.6084/m9.figshare.c.3278273.

Retro-wedge foreland basin evolution along the ECORS line, eastern Pyrenees, France

Abstract: The eastern Aquitaine basin and North Pyrenean Zone show many characteristics of retro-wedge models. However, they differ significantly in that slow subsidence and low deformation continued throughout orogenesis so that growth and steady-state phases cannot be distinguished. We show that the eastern Pyrenees record two clear phases of convergence and probably never attained steady state. Analysis of the Aquitaine retro-foreland basin along the Ariege ECORS deep seismic line, eastern French Pyrenees, integrates a new litho- and chronostratigraphy, subsidence analysis, low-temperature thermochronology data, new interpretations of seismic lines and a balanced cross-section. Within an overall regression, two shallowing-up cycles (Latest Santonian–Danian, Thanetian–Oligocene) record slow tectonic subsidence of the eastern Aquitaine basin separated by a quiet period. Continuing thick-skinned shortening was low to moderate. The early marine basin, generated by loading of the weak, extended margin, was supplied axially from an unknown eastern edifice while the young Pyrenean orogeny to the south remained submerged. During the quiet period of ultra-slow subsidence, no basin migration and negligible sediment supply, continental conditions characterized the eastern orogen. The second marine transgression was quickly followed by continental conditions. The basin was supplied by the now emerging Pyrenean orogen and continued to subside until Miocene time.

Accommodation succession (δA/δS) sequence stratigraphy: observational method, utility and insights into sequence boundary formation

Abstract: The future of sequence stratigraphy depends on stratigraphers making observations with a common method so that physical frameworks can be clearly separated from interpretations of driving mechanisms. Depositional sequence boundary selection is a well-known controversy that could be resolved with objective recognition criteria. Accommodation succession sequence stratigraphy refines traditional methods, using sedimentary facies, facies associations, vertical stacking, stratal geometries and stratal terminations as the objective record of competing rates of accommodation change and sediment fill through time. Observations are placed in context of lateral (transgression and regression) and vertical (aggradation and degradation) movement of shoreline through time, across multiple timescales in hierarchal stacks. The repeating motif consists of a subaerial unconformity and its correlative subaqueous surface overlain in coastal settings by a basinward shift in coastal onlap and strata with progradational to aggradation stacking, then retrogradation and aggradation–progradation–degradation stacking. These stacking patterns are bounded by key surfaces, recognized by stratal terminations and characteristic vertical successions of facies. This pattern is independent of time duration or position on a sea-level curve, but incorporates data resolution, regional extent and hierarchal stacking. Examples from multiple datasets show the utility and objectivity of the method and provide insights into sequence boundary formation.

Towards a sequence stratigraphic solution set for autogenic processes and allogenic controls: Upper Cretaceous strata, Book Cliffs, Utah, USA

Abstract: Upper Cretaceous strata exposed in the Book Cliffs of east–central Utah are widely used as an archetype for the sequence stratigraphy of marginal-marine and shallow-marine deposits. Their stratal architectures are classically interpreted in terms of accommodation controls that were external to the sediment routing system (allogenic), and that forced the formation of flooding surfaces, sequence boundaries, and parasequence and parasequence-set stacking patterns. Processes internal to the sediment routing system (autogenic) and allogenic sediment supply controls provide alternatives that can plausibly explain aspects of the stratal architecture, including the following: (1) switching of wave-dominated delta lobes, expressed by the internal architecture of parasequences; (2) river avulsion, expressed by the internal architecture of multistorey fluvial sandbodies and related deposits; (3) avulsion-generated clustering of fluvial sandbodies in delta plain strata; (4) ‘autoretreat’ owing to increasing sediment storage on the delta plain as it lengthened during progradation, expressed by progradational-to-aggradational stacking of parasequences; (5) sediment supply control on the stacking of, and sediment grain-size fractionation within, parasequence sets. The various potential allogenic controls and autogenic processes are combined to form a sequence stratigraphic solution set. This approach avoids anchoring of sequence stratigraphic interpretations on a specific control and acknowledges the non-unique origin of stratal architectures.

Should Sibumasu be renamed Sibuma? The case for a discrete Gondwana-derived block embracing western Myanmar, upper Peninsular Thailand and NE Sumatra

Abstract: The Luxi–Nujiang suture, which extends from Yunnan into Myanmar, is shown to continue into Thailand and the Malacca Strait. It forms the eastern boundary of a discrete block, here called the Irrawaddy Block, which was formerly considered to be part of Gondwana-derived Sibumasu. It has a distinct Palaeozoic succession, with thick Lower Permian, glacially influenced, diamictite-bearing, marine mass-flow intervals deposited at the Gondwana margin. The rest of Sibumasu, east of the suture, also has local, thin, Lower Permian diamictite units; however, they have been interpreted as ice-rafted deposits, laid down at a greater distance from the Gondwana margin. In NE Sumatra a diamictite-bearing (?)Carboniferous–Lower Permian succession has also been described as of mass-flow origin and it is suggested that it is also part of the Irrawaddy Block. It requires that the bounding suture is aligned SE, beneath the Malacca Strait. In the Late Cretaceous–Palaeogene, the dextral India–Australia oceanic transform propagated onshore as a strike-slip fault, bounding on the east the north-going India Plate. It disrupted the Irrawaddy Block and for much of its length coincided with the line of the earlier suture. Insofar as the acronym Sibumasu was coined to include Sumatra, consideration should be given to renaming this block Sibuma.

Integrated stratigraphy and palaeoclimate history of the Carnian Pluvial Event in the Boreal realm : new data from the Upper Triassic Kapp Toscana Group in central Spitsbergen (Norway).

Abstract: The Late Triassic climate is characterized by arid conditions interrupted by a humid phase known as the Carnian Pluvial Event (CPE). This wet phase is well documented in the Tethyan realm, but evidence from the Boreal realm is scarce. Here we present evidence from quantitative palynology for the CPE from the Kapp Toscana Group on central Spitsbergen integrated with organic carbon isotope data linked to the geomagnetic polarity time scale. Our data reveal an early to mid-Julian-1 age for the Tschermakfjellet Formation. The transition to the Julian-2 is located in the De Geerdalen Formation and the Isfjorden Member is confirmed as mostly Tuvalian-3 in age. The Aulisporites astigmosus pollen assemblage zone that marks the base of the CPE in the Tethys realm at the base of the Julian-2 is assigned to the Julian-1 in the Boreal region. Palaeoclimate proxy data inferred from principal component analysis indicate wetter conditions from the Julian-2 onwards, which is in agreement with the establishment of local swamp vegetation on top of a delta plain. The palaeotemperature curve indicates a period of cooler climate during the early Julian-1 followed by warming during the late Julian-1. Supplementary materials: A list of all identified morphotaxa of pollen, spores and aquatic palynomorphs is available at http://www.geolsoc.org.uk/SUP18879.

The use of chemostratigraphy to refine ambiguous sequence stratigraphic correlations in marine mudrocks. An example from the Woodford Shale, Oklahoma, USA

Abstract: Application of sequence stratigraphy to fine-grained lithologies has previously been hindered by the difficulty of identifying distinct facies shifts within mudrocks. Three cores from Lincoln, Pottawatomie and Pontotoc Counties in Oklahoma and two outcrops at the Hunton Anticline Quarry in Murray County, OK record both proximal and distal regions of the Arkoma Basin. Handheld X-ray fluorescence data and gamma-ray profiles together with lithological descriptions can be used to develop sequence-stratigraphic interpretations. Stratigraphic successions that are challenging to correlate based on gamma-ray (GR) profiles alone can be correlated accurately by utilizing surfaces that are recognized within chemostratigraphic profiles. Within the Arkoma Basin, Oklahoma, the chemostratigraphic profile of the Woodford Shale is interpreted within a sequence-stratigraphic framework using the following general criteria. The Lowstand Systems Tract (LST) is defined by increasing concentrations of Ti, Zr, Al and K as well as high levels of Mo and V. The Transgressive Systems Tract (TST) is defined by declining concentrations of Ti and Zr, although Al and K may remain elevated; Mo and V also show general declines. The Highstand Systems Tract (HST) is defined by increasing concentrations of Ti, Zr, Al and K, but is distinguished from the LST by low levels of Mo and V. Supplementary material: Chemofacies profiles are available at http://doi.org/10.6084/m9.figshare.c.2849599.

Decoupling seasonal fluctuations in fluvial discharge from the tidal signature in ancient deltaic deposits: an example from the Neuquén Basin, Argentina

Abstract: Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-energy successions. Large-scale outcrops from the Middle Jurassic Lajas Formation (Argentina) expose a well-constrained stratigraphic succession of marginal-marine deposits with a strong fluvial influence and well-known tidal indicators. The studied deposits show decimetre-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities. The alternation of these two types of beds forms non-cyclic successions that are interpreted to be the result of seasonal variation in river discharge, rather than regular and predictable changes in current velocity caused by tides. Seasonal bedding is present in bar deposits that form within or at the mouth of minor and major channels. Seasonal bedding is not preserved in channel thalweg deposits, where river flood processes were too powerful, or in floodplain, muddy interdistributary-bay, prodelta and transgressive deposits, where the river signal was weak and sporadic. The identification of sedimentary facies characteristic of seasonal river discharge variations is important for accurate interpretation of ancient deltaic process regime.

40Ar/39Ar ages of alkaline and tholeiitic rocks from the northern Deccan Traps: implications for magmatic processes and the K–Pg boundary

Abstract: The Deccan large igneous province in India was emplaced temporally close to the Cretaceous–Palaeogene (K–Pg) boundary and is formed by tholeiitic flood basalts and less abundant alkaline rocks. Definition of the origin of Deccan magmatism and of its environmental impact relies on precise and accurate geochronological analyses. We present new 40 Ar/ 39 Ar ages from the northern sector of the province. In this area, tholeiitic and alkaline rocks were contemporaneously emplaced at 66.60 ± 0.35 to 65.25 ± 0.29 Ma in the Phenai Mata area, whereas rocks from Rajpipla and Mount Pavagadh yielded ages ranging from 66.40 ± 2.80 to 64.90 ± 0.80 Ma. The indistinguishable ages for alkaline and tholeiitic magmatism suggest that distinct mantle sources were synchronously active. The new ages are compared with previous ages, which were carefully screened and filtered and then recalculated to be comparable. The entire dataset of geochronological data does not support a time-related migration of the magmatism related to the northward Indian plate movement relative to the Reunion mantle plume. The main phase of magmatism, including the newly dated rocks from the northern Deccan, occurred at the K–Pg boundary. This suggests a causal link between the emplacement of the province and the K–Pg mass extinction. Supplementary material: Whole-rock and mineral compositions and the complete 40 Ar/ 39 Ar dataset are available at https://doi.org/10.6084/m9.figshare.c.2674441.

Early Cretaceous exhumation of the Sulu orogenic belt as a consequence of the eastern Eurasian tectonic extension: insights from the newly discovered Wulian metamorphic core complex, eastern China

Abstract: The mechanisms of thinning of the lithosphere and destruction of the North China Craton have been debated in recent decades. Their causal link with regional tectonic extension is well exemplified by the exhumation of the Wulian metamorphic core complex (mcc), along the Sulu orogenic belt to the SE of the North China Craton. The Wulian mcc is a Cordilleran type mcc with a northwestward dipping low-angle master detachment fault zone (the Wulian detachment fault zone), an Early Cretaceous supradetachment basin (the Zhucheng Basin) and a lower plate of ultrahigh-pressure metamorphic rocks in the Sulu orogenic belt. Synthetic structural, fabric and thermochronological data suggest that tectonic extension is responsible for the exhumation of the lower plate and triggered syn- and post-tectonic magmatisms. A new cooling path for the lower plate rocks indicates that the lower plate was exhumed at a high rate of 2.0 km Ma −1 from before c . 128 Ma to 123 Ma and at a moderate rate of 0.35 km Ma −1 from 81 to 61 Ma. The rapid exhumation is consistent temporally with the sedimentation of terrestrial deposits of the Laiyang Group in the Zhucheng supradetachment basin. The post-kinematic granitic intrusions are dated as c. 122 Ma, which marks the cessation of rapid exhumation of the lower plate. Vast amount of andesitic volcanic rocks from 120 to 105 Ma in the Qingshan Group in the Zhucheng Basin may be related to the peak of lithospheric thinning in the Wulian area. These data highlight the importance of regional tectonic extension in the formation of the detachment fault zone and the exhumation of the Wulian mcc, and in triggering syn- and post-kinematic magmatisms. The scenario is consistent with the parallel extension tectonics model in which tectonic extension of the lithosphere led to detachment faulting in both the crust and mantle, resulted in the loss of some of the subcontinental roots, gave rise to the exhumation of the mccs, and triggered plutonic emplacement and volcanic eruptions of hybrid magmas.

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

Abstract: The Lagavulin exploration well 217/15-1Z penetrated a c . 2.6 km thick volcanic sequence dominated by extrusive basaltic rocks spanning the Paleocene–Eocene boundary in the NE Faroe–Shetland Basin. The well comprises one of the thickest drilled sequences through the North Atlantic Igneous Province. Integrated analysis of drill cuttings and wireline-log data reveals key volcanic lithofacies: (1) tabular lava flows; (2) compound lava flows; (3) hyaloclastite; (4) volcaniclastic rocks. The volcanic facies reveal two major sub-aqueous to subaerial sequences consistent with lava delta progradation. These sequences are separated by a volcanic hiatus represented by extensive reddened soils, which preceded the re-submergence of the area. Emergence followed by submergence of the first lava delta is interpreted to record an intra-T40 transient uplift event near the Paleocene–Eocene boundary. Basalts from the lower c . 1.3 km have low TiO 2 ( 85–91 ). The hiatus correlates precisely with a change to high-TiO 2 (2.5–3.2 wt%), high-Zr/Y (>4) compositions, which dominate the upper sequence. The associated change in lava geochemistry, transient uplift and volcanic hiatus appears consistent with a transient pulse of hot buoyant plume material passing beneath the area. Supplementary material: All raw geochemical data and supplementary analyses are available at http://www.geolsoc.org.uk/SUP18888.

Devonian magmatism in the accretionary complex of southern Chile

Abstract: Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U–Pb dating of zircon, with Hf- and O-isotope analyses of selected grains. Ages of 384 ± 3 and 382 ± 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 ± 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c . 330 – 385 Ma. Devonian ages of 391 ± 10 and 374 ± 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaiten (361 ± 7 and 364 ± 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian–Carboniferous times. Supplementary material: Description of analytical methods and tables of isotope analytical data are available at http://doi.org/10.6084/m9.figshare.c.2336728.

U–Pb and Lu–Hf zircon data in young sediments reflect sedimentary recycling in eastern South Africa

Abstract: Detrital zircon from unconsolidated, Cenozoic sediments from eastern South Africa has been analysed for U–Pb and Lu–Hf isotopes by laser ablation inductively coupled plasma mass spectrometry. Identifiable bedrock sources have made local contributions to the detrital zircon populations, but the dominant zircon components are of regional distribution: late Mesoproterozoic (e Hf = –5 to +10), Neoproterozoic to early Palaeozoic (e Hf = –10 to +10), and minor late Palaeozoic (e Hf ≈ 0). Archaean zircons are scarce even in sediments deposited on exposed Archaean basement or by rivers eroding it. The dominant components cannot be tied to specific first-generation sources in southern Africa or its former Gondwana neighbours. Instead, we see the effect of mixing and remobilization of debris from large parts of the supercontinent in the early Phanerozoic, which was stored in the Karoo basin and other continental cover sequences and shed from there to the present site of deposition. Therefore, data from detrital zircon in these deposits tell us less about the path of detritus from source to sink in a recent sedimentary system than about processes in much earlier erosion–transport–deposition cycles. To facilitate comparison of detrital zircon age distribution patterns, a simple and intuitive method that takes sampling uncertainty explicitly into account is proposed. Supplementary materials: U–Pb and Lu–Hf data, and concordia diagrams and discussion of effects of discordance are available at http://www.geolsoc.org.uk/SUP18884.

The dynamic influence of microbial mats on sediments: fluid escape and pseudofossil formation in the Ediacaran Longmyndian Supergroup, UK

Abstract: Microbial mats are thought to have been widespread in marine settings before the advent of bioturbation, and the range of their influence on sediments is gradually becoming recognized. We propose that mat sealing can dynamically affect porewater conditions, and allow the build-up of overpressure that can drive dewatering and degassing to produce a suite of atypical fluid-escape features. Finely bedded silty and sandy laminae from the c . 560 Ma Burway Formation of the Longmyndian Supergroup, Shropshire, England, reveal evidence for sediment injection, including disrupted bedding, clastic injections, sill-like features and sediment volcanoes at sub-millimetre scale. These features are associated with crinkly laminae diagnostic of microbial matgrounds. Matground-associated sediment injection can explain the formation of several types of enigmatic discoidal impressions, common in rocks of this age, which have previously been attributed to the Ediacaran macrobiota. Serial grinding of Longmyndian forms previously described as Medusinites aff. asteroides and Beltanelliformis demonstrates that such discoidal features can be fully explained by fluid escape and associated load structures. Our observations emphasize the non-actualistic nature of shallow-marine Ediacaran sediments. Matground-associated sediment injection features provide a new insight into the interpretation of Proterozoic rocks and the biogenicity of their enigmatic discoidal markings. Supplementary materials: A document containing further images of fluid escape and loading features observed in the upper Burway Formation at Ashes Hollow, together with an annotated diagram of features appearing in one typical vertical cross-section, is available at www.geolsoc.org.uk/SUP18870.

Detrital zircon in a supercontinental setting: locally derived and far-transported components in the Ordovician Natal Group, South Africa

Abstract: U–Pb and Lu–Hf signatures of detrital zircon from conglomerates and sandstones of the Ordovician Natal Group, South Africa were determined using laser ablation inductively coupled plasma mass spectrometry. The basal conglomerates are dominated by Palaeo- to Mesoarchaean detrital zircon with e Hf values from +3 to −4 with minor Mesoproterozoic input, indicating a proximal source in the Kaapvaal Craton and minor input from rocks of the Natal Sector of the Mesoproterozoic Namaqua–Natal Province. The sandstones are all dominated by a combination of juvenile Mesoproterozoic zircon and Neoproterozoic zircon derived from Mesoproterozoic rocks that were reworked during the Pan-African Orogeny. Several sedimentary sequences from former Gondwana with Neoproterozoic to Permian depositional ages show similar detrital zircon signatures. Sedimentary sequences of such vast temporal and geographical distribution are unlikely to have been fed by a single source, making it more likely that these sequences were fed by several different (Pan-Gondwana) source terranes with closely similar U–Pb and Lu–Hf zircon signatures. The results show that source terrane non-uniqueness can make ascertaining sedimentary provenance from detrital zircon impossible, and should be taken as a reminder when using detrital zircon as evidence for far-reaching conclusions in basin evolution studies and palaeogeography. Supplementary materials: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18880.

An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain)

Abstract: Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprian massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U > 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U Supplementary materials: Code and data to generate Figures 1 and 5 are available at http://www.geolsoc.org.uk/SUP18885.

Mesozoic orogens of the Arctic from Novaya Zemlya to Alaska

Abstract: Mesozoic orogenic belts fringe the Alaska and eastern Russia portion of the Arctic Basin. From west to east, these include the fold belts of Novaya Zemlya, Taimyr Peninsula, northern Verkhoyansk–Kolyma, Chukotka and the Brooks Range, as well as their continuations onto the continental shelves. The Taimyr and Novaya Zemlya structures were traditionally interpreted as the continuation of the late Palaeozoic Uralian orogenic belt. This is probably correct for Taimyr, but not for Novaya Zemlya, where shortening post-dates Uralian deformation. The Mesozoic tectonic evolution of the Verkhoyansk–Kolyma, Chukotka and Brooks Range orogens relates to the accretion of numerous continental and arc terranes to the Siberian and North American margins starting in the Late Jurassic and driven by palaeo-Pacific dynamics. This history is complicated by the opening of the Amerasia Basin of the Arctic, which displaced the Arctic Alaska–Chukotka microplate from a position adjacent to Arctic Canada towards the palaeo-Pacific. Although the Chukotka fold belt and the Brooks Range both formed along the southern edge of Arctic Alaska–Chukotka, most shortening took place prior to Amerasia Basin opening. The remoteness of this region and the complexity of its geology has left numerous questions regarding its tectonic evolution unresolved, providing rich avenues for future research.

Provenance shifts in an evolving Eurekan foreland basin: the Tertiary Central Basin, Spitsbergen

Abstract: Detrital zircon U–Pb laser ablation inductively coupled plasma mass spectrometry age data for Paleocene and Eocene sandstones from the Central Tertiary Basin on Spitsbergen, Arctic Norway are used to investigate provenance and to test the filling history of the basin in response to the evolving Eurekan orogeny. The zircon age data from 16 analysed samples allow us to distinguish five sources. The zircon age populations of the Paleocene Firkanten and Basilika formations have a distinct Uralide signature ( c . 275 Ma) and are interpreted to represent four mixtures of reworked Late Triassic, Jurassic and older sandstones affected by Early Cretaceous volcanism. The zircon age population of the Eocene Battfjellet Formation lacks Uralide- and Early Cretaceous-aged zircons and is dominated by Proterozoic and Archaean zircons indicating sourcing from Middle Triassic and older rocks. The data support models implying a marked shift from easterly to westerly source areas during the evolution of the basin in response to the evolving Eurekan orogeny. The population of Cretaceous-aged zircons indicates that volcanism in the High Arctic Large Igneous Province took place during two separate events at 80–100 and 118–150 Ma, thus supporting previous models for Cretaceous volcanic activity in the High Arctic. Supplementary materials: Operating conditions, data acquisition parameters and zircon discordances are available at https://doi.org/10.6084/m9.figshare.c.2187247.

Interpreting syndepositional sediment remobilization and deformation beneath submarine gravity flows; a kinematic boundary layer approach

Abstract: Turbidite sandstones and related deposits commonly contain deformation structures and remobilized sediment that might have resulted from post-depositional modification such as downslope creep (e.g. slumping) or density-driven loading by overlying deposits. However, we consider that deformation can occur during the passage of turbidity currents that exerted shear stress on their substrates (whether entirely pre-existing strata, sediment deposited by earlier parts of the flow itself or some combination of these). Criteria are outlined here, to avoid confusion with products of other mechanisms (e.g. slumping or later tectonics), which establish the synchronicity between the passage of overriding flows and deformation of their substrates. This underpins a new analytical framework for tracking the relationship between deformation, deposition and the transit of the causal turbidity current, through the concept of kinematic boundary layers. Case study examples are drawn from outcrop (Miocene of New Zealand, and Apennines of Italy) and subsurface examples (Britannia Sandstone, Cretaceous, UK Continental Shelf). Example structures include asymmetric flame structures, convolute lamination, some debritic units and injection complexes, together with slurry and mixed slurry facies. These structures may provide insight into the rheology and dynamics of submarine flows and their substrates, and have implications for the development of subsurface turbidite reservoirs.

The geology and tectonics of central Bhutan

Abstract: Lithotectonic mapping, metamorphic observations and U–Pb zircon ages underpin a substantial revision of central Bhutan geology, notably a more extensive and continuous outcrop of the Tethyan Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene ( c . 17–20 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8 ± 0.2 and 17.9 ± 0.5 Ma. A 500 ± 4 Ma (U–Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic interpretations of the Bhutan Himalaya. Supplementary materials: Zircon U–Pb geochronological data, sample locations and descriptions, features of analysed zircons, sample processing method and detailed analytical conditions are available at http://www.geolsoc.org.uk/SUP18876.

Exploring the geology of the central Arctic Ocean; understanding the basin features in place and time

Abstract: The history of the deep Arctic Ocean is largely unwritten. The means to understand this history may now be at hand. Bathymetry and potential field data have accumulated to the point it is possible to ask specific questions about the origins and evolution of the individual ridges and basins. The two primary basins have contrasting histories and deficits of understanding. The Eurasia Basin, formed during the Cenozoic, is well understood as far as the kinematics are concerned. The dynamics of ultraslow spreading, as observed on the Gakkel Ridge, are not well understood. It is widely thought that the Amerasia Basin formed during the Mesozoic. Most previous work has begun with a large-scale model of the basin tectonics and fit the ridges and basins it into this pattern. Enough is now known to establish the internal structure and relations from specific observations rather than a priori assumptions. This paper reviews knowledge about the central Arctic Ocean and proposes what should be done to develop a historical understanding of basin history. Ground truth from Arctic Ocean sediments is necessary. It will not be possible to establish the history of events, in geological time, until the sediments have been sampled and dated.

Quantitative dating of Pleistocene deposits of the Kyrenia Range, northern Cyprus: implications for timing, rates of uplift and driving mechanisms

Abstract: The Kyrenia Range underwent tectonically driven uplift during the Pliocene to Pleistocene in response to the interaction of various tectonic processes. To understand the tectonic processes driving the uplift and how this is related to uplift of other areas of the Eastern Mediterranean, uranium-series disequilibrium and optically stimulated luminescence dating were applied to marine and non-marine terrace deposits exposed on the northern flank of the range. Palaeomagnetism and strontium isotope dating were used in conjunction to date the final stages of the marine environment adjacent to the Kyrenia Range prior to major surface uplift. Uplift rates range from >1.2 mm a −1 , inferred during the Early Pleistocene, to −1 during the Late Pleistocene. The new data show that the Kyrenia Range was uplifted contemporaneously with the Troodos Massif in southern Cyprus. The uplift of the Kyrenia Range appears to have been significantly faster than that affecting other comparable regions in the easternmost Mediterranean during the Pleistocene (e.g. Lebanon coast; southern Anatolian plateau). The driving mechanism for the uplift of both the Kyrenia Range and the Troodos Massif is inferred to be the collision of the Eratosthenes Seamount with the Cyprus trench to the south of Cyprus. Supplementary material : Details of the methods used are available at https://doi.org/10.6084/m9.figshare.c.3260977.

Dealing with discordance: a novel approach for analysing U–Pb detrital zircon datasets

Abstract: Detrital zircon U–Pb geochronology is a rapidly expanding and useful technique for addressing the sedimentary rock record. However, because of difficulties in evaluating discordant analyses, common in most datasets, a high proportion of results may be discarded. These analyses, if interpreted correctly, can provide valuable information regarding the deformation, alteration or metamorphism of the zircon population, as these events are all capable of producing U–Pb discordance. A novel modelling procedure permits analysis of probabilistic relationships within U–Pb datasets to deconvolve the age information present within discordant analyses by assessing the relative likelihood of potential discordia lines. Additionally, because we retrieve useful information from discordant data, a stricter filter can be used to assess concordance, increasing confidence in those distributions. The validity of this modelling method is demonstrated using two previously published cases from the Caledonide orogen where clearly discordant analyses exist. In the Southern Uplands of Scotland, these analyses indicate resetting of the U–Pb systematics in metasedimentary rocks in the Grampian orogen prior to Ordovician erosion and redeposition. In the second case, from the Greenland Caledonides, discordant data provide additional constraints on previously proposed in situ resetting during Scandian deformation and fluid flow events. Supplementary material: The code used for writing the model, along with an instruction guide, is available at http://doi.org/10.6084/m9.figshare.c.2182197.

Leaving no stone unturned: the feedback between increased biotic diversity and early diagenesis during the Ordovician

Abstract: Ordovician change in the nature of seafloor carbonates saw rapid decline of previously widespread flat pebble conglomerates and the Palaeozoic peak abundance of hardgrounds. The effective disappearance of flat pebble conglomerates, widely attributed to physical disruption of substrate by bioturbation, is reinterpreted as reflecting increased depth of carbonate precipitation below the Taphonomically Active Zone such that early lithified carbonates were less frequently reworked by scour. With deeper, more stable zones of cementation, exhumed limestones formed hardgrounds, whose mid-Ordovician acme supported rapid increase in epizoan diversity. Further deepening of cementation to below normal scour accompanied post-Ordovician decline in submarine hardgrounds. Supplementary materials: Database for Figure 2 is available at http://www.geolsoc.org.uk/SUP18896.

Gondwana break-up related magmatism in the Falkland Islands

Abstract: Jurassic dykes ( c . 182 Ma) are widespread across the Falkland Islands and exhibit considerable geochemical variability. Orthopyroxene-bearing NW–SE-oriented quartz-tholeiite dykes underwent fractional crystallization at >1 GPa, and major element constraints suggest that they were derived by melting of a pyroxenite-rich source. They have eNd 182 in the range −6 to −11 and 87 Sr/ 86 Sr 182 >0.710 and therefore require an old lithospheric component in their source. A suite of basaltic-andesites and andesites exhibit geochemical compositions transitional between Ferrar and Karoo magma types, and are similar to those seen in the KwaZulu-Natal region of southern Africa and the Theron Mountains of Antarctica. Olivine-phyric intrusions equilibrated at 182 1.6–3.6 and 87 Sr/ 86 Sr 182 0.7036–0.7058) that require limited interaction with old continental lithosphere. A suite of plagioclase-phyric intrusions with 87 Sr/ 86 Sr 182 c . 0.7035 and eNd 182 c . +4, and low Th/Ta and La/Ta ratios ( c . 1 and c . 15, respectively) also largely escaped interaction with the lithosphere. These isotopically depleted intrusions were probably emplaced synchronously with Gondwana fragmentation and the formation of new oceanic lithosphere. Estimates of mantle potential temperature from olivine equilibration temperatures do not provide unequivocal evidence for the presence of a plume thermal anomaly beneath the Falkland Islands at 182 Ma. Supplementary materials: Mineralogical data, XRF major and trace element data and Ar-Ar results for Falkland Islands intrusions are available at http://www.geolsoc.org.uk/sup18873.

Discussion on ‘Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation’

Abstract: The first-order importance of tectonic and environmental controlsfor terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signa ...

Orbital calibration of the late Campanian carbon isotope event in the North Sea

Abstract: A new record of carbon isotopes, nannofossil biostratigraphy, gamma-ray and Fe content variations is presented for the upper Campanian of the Adda-3 core, Danish Central Graben, North Sea. The studied interval provides a revision of previously assigned late Coniacian to early Santonian ages. New biostratigraphic data indicate a late Campanian age for the 60 m thick studied interval. The Late Campanian Event (LCE) is well recorded by a 1.5‰ negative excursion in the bulk δ 13 C, along with two stepwise pre-excursion negative shifts (defining the pre-LCE). The amplitude of the LCE appears higher in the North Sea than in other areas as seen from the correlation to Germany, the UK and France. This correlation allows identification of a new 0.4‰ negative excursion (defined as the conica event). Fe and gamma-ray variations are used to calibrate the record with cyclostratigraphy. Fourteen 405 kyr cycles identified in the upper Campanian of Adda-3 can be correlated to North Germany. The compilation of previous results from North Germany and correlation to Adda-3 shows that the Boreal upper Campanian spans a total of 17 cycles each of 405 kyr; that is, 6.885 myr. The duration of the LCE is estimated to be c . 1 myr at Adda-3 and in North Germany. Supplementary materials: Calibration of the HH-XRF data is available at https://doi.org/10.6084/m9.figshare.c.2134362.