A.A. Monaghan

Geochronology of Carboniferous-Permian magmatism in the Midland Valley of Scotland: implications for regional tectonomagmatic evolution and the numerical time scale

Four new U–Pb isotope dilution thermal ionization mass spectrometry dates from Carboniferous igneous rocks of the Midland Valley of Scotland are integrated with 16 recently published 40Ar/39Ar dates. The numerical and stratigraphical ages of the dated samples are compatible to stage level, allowing improved intrabasinal and regional correlations. Early Carboniferous extension-related volcanism in the Midland Valley was pulsed, with a date of 343.4 ± 1.0 Ma from the Garleton Hills Volcanic Formation and dates of 334.7 ± 1.7 and 335.2 ± 0.8 Ma from the Clyde Plateau Volcanic Formation. The magmatic pulses are synchronous with synrift phases in the Northumberland–Solway Basin. Volcanism continued in mid- to Late Carboniferous time contemporaneous with dextral strike-slip tectonics, in contrast to a post-rift tectonic setting in northern England. After Late Carboniferous basin inversion, tholeiitic magmatism dated at 307.6 ± 4.8 Ma provides a maximum Westphalian D age for post-Variscan magmatism and extension–dextral transtension across Scotland and northern England. In the Early Permian, as rift systems developed across NW Europe, alkaline extension-related magmatism resumed in the Midland Valley from c. 298 to 292 Ma. The improved geochronology provides important data for poorly constrained parts of the numerical Carboniferous time scale such as stages within the Viséan, and provides a well-controlled tie-point that supports a Tournaisian–Viséan boundary age of c. 345.4 Ma.

Biostratigraphy and palaeoenvironments of the Ballagan Formation (lower Carboniferous) in Ayrshire

Synopsis A local palynostratigraphical subdivision for the Ballagan Formation in Ayrshire is established, based on the Heads of Ayr coastal section. The lower part of the formation is characterized by a low-diversity CM Biozone Colatisporites-dominated assemblage, while the upper part is characterized by high-diversity assemblages containing Acanthotriletes cf. macrogaleatus and Radiizonates mirabilis, and assemblages dominated by Leiosphaeridia algal palynomorphs. Sedimentological analysis of the lower part of the formation suggests deposition in an arid coastal floodplain environment. Up sequence, there is a gradual reduction in sandstones, suggesting reduced fluvial influence. The Ballagan Formation yields ostracodes indicative of marginal marine and brackish environments, which supports the sedimentological interpretation. The lower part of the formation contains species that are typical of the Tournaisian Beyrichiopsis glyptopleuroides-Eriella Biozone, providing a stratigraphical tie with the CM Biozone assemblage. The upper part of the formation yields ostracodes that elsewhere have been reported from the Viséan, but Lycospora pusilla, the index taxon for the Viséan Pu palynomorph Biozone, was not recovered. Two age interpretations can be offered for this part of the section: either late Tournaisian or early Viséan. A late Tournaisian–early Viséan age for the Ballagan Formation would imply that L. pusilla is so rare in the lower parts of its range that dense sampling failed to recover it. By contrast, a Tournaisian age for the entire Ballagan Formation at the coast implies recalibration of some ostracode ranges. Above the Ballagan Formation, mid-Viséan palynomorph assemblages from the Lawmuir Formation allow the Heads of Ayr volcanic vent to be dated in the range late Tournaisian-mid-Viséan, implying correlation with the Clyde Plateau Volcanic Formation.

3D geological models and their hydrogeological applications : supporting urban development : a case study in Glasgow-Clyde, UK

Urban planners and developers in some parts of the United Kingdom can now access geodata in an easy-to-retrieve and understandable format. 3D attributed geological framework models and associated GIS outputs, developed by the British Geological Survey (BGS), provide a predictive tool for planning site investigations for some of the UKs largest regeneration projects in the Thames and Clyde River catchments. Using the 3D models, planners can get a 3D preview of properties of the subsurface using virtual cross-section and borehole tools in visualisation software, allowing critical decisions to be made before any expensive site investigation takes place, and potentially saving time and money. 3D models can integrate artificial and superficial deposits and bedrock geology, and can be used for recognition of major resources (such as water, thermal and sand and gravel), for example in buried valleys, groundwater modelling and assessing impacts of underground mining. A preliminary groundwater recharge and flow model for a pilot area in Glasgow has been developed using the 3D geological models as a framework. This paper focuses on the River Clyde and the Glasgow conurbation, and the BGSs Clyde Urban Super-Project (CUSP) in particular, which supports major regeneration projects in and around the City of Glasgow in the West of Scotland.

Late Devonian–Carboniferous tectonic evolution within the Firth of Forth, Midland Valley; as revealed from 2D seismic reflection data

Synopsis Results of integrated seismic mapping, within the Firth of Forth in the offshore Midland Valley of Scotland, are presented and illustrate aspects of the subsurface structure and tectonic evolution of the Upper Devonian to Carboniferous succession. Evidence for three main phases of tectonic activity has been recognized: (1) Late Devonian to Dinantian fault-controlled subsidence; (2) basin-wide Silesian subsidence, localized inversion and growth folding; (3) Late Silesian dextral transtensional and transpressional strike-slip faulting. During the first phase of tectonic activity, the NNE-trending Mid Forth Fault is interpreted to represent a Late Devonian to Dinantian extensional fault, with a small, mainly Dinantian depocentre developed in the hangingwall block that has subsequently been inverted during Silesian times. A major Late Devonian to Dinantian depocentre also occurs in the hangingwall block of the NE-trending offshore continuation of the Crossgatehall Fault, although it remains unclear whether this mainly Dinantian depocentre was developed during pull-apart as a result of extension or transtensional strike-slip fault movement. The NNE-trending Leven Syncline and Mid Forth Anticline within the hangingwall block of the Mid Forth Fault are interpreted as Silesian synsedimentary growth folds that formed during the second phase of tectonic activity. The peak of this activity occurred during intra-Westphalian B to Westphalian C times. In the third phase, the ENE-trending Inchkeith Fault Zone is interpreted as a Late Silesian transtensional–transpressional strike-slip fault that dextrally offsets the axial trace of the Leven Syncline. Evidence from the Firth of Forth could provide support for regional tectonic models involving mainly dextral strike-slip fault activity during Devonian–Carboniferous times, or mainly sinistral strike-slip during Devonian to Early Carboniferous followed by dextral strike-slip during Late Carboniferous times, for the development of the Upper Devonian and Carboniferous succession. However, the latter model is preferred as it provides the more convincing explanation for our interpretation that the NNE-trending Mid Forth Fault represents a Late Devonian to Dinantian extensional or transtensional fault that was inverted during Silesian times. This inversion may therefore reflect a major change in the regional stress field.

40Ar/39Ar geochronology of Carboniferous-Permian volcanism in the Midland Valley, Scotland

Abstract Twenty-one new 40Ar/39Ar step-heating experiments on mineral separates from intrusive and extrusive Carboniferous and Permian igneous rocks in the Midland Valley of Scotland yielded 17 concordant experiments with a relative age precision better than 1% (2σ). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise 40Ar/39Ar dates are stratigraphically constrained to stage level and can contribute to Carboniferous timescale tie points at the Tournaisian-Visean boundary, within the Visean and at the Carboniferous-Permian boundary. Situated in the extending Variscan foreland, two distinct phases of extension-related transitional-alkaline volcanism have been resolved in the Dinantian: the Garleton Hills Volcanic Formation in the eastern Midland Valley near the Tournaisian-Visean boundary, 342.1 ± 1.3 and 342.4 ± 1.1 Ma; and the Clyde Plateau Volcanic Formation in the western Midland Valley during the mid-Visean, 335 ± 2–329.2 ± 1.4 Ma. Alkaline basic sills near Edinburgh, previously thought to be Namurian, appear to be coeval with the Clyde Plateau Volcanic Formation at 331.8 ± 1.3–329.3 ± 1.5 Ma. The new ages allow correlation between these short-lived Dinantian magmatic pulses and extensional and magmatic phases in the Northumberland-Solway and Tweed basins to the south. After late Westphalian, end-Variscan, compression and a regionally important tholeiitic intrusive phase at c. 301–295 Ma, alkaline magmatism related to post-Variscan extension occurred in the central and western Midland Valley during the latest Carboniferous or Permian from 298.3 ± 1.3 to 292.1 ± 1.1 Ma. This correlates well with post-Varsican extension and magmatism observed across the NW European foreland from 300 to 280 Ma.

Static and Dynamic Estimates of CO2 Storage Capacity in Two Saline Formations in the UK

Estimation of CO2 storage capacity is a key step in the appraisal of CO2 storage sites. Different calculation methods may lead to widely diverging values. The compressibility method is a commonly used static method for estimating storage capacity of saline aquifers: it is simple, easy to use and requires a minimum of input data. Alternatively, a numerical reservoir simulation provides a dynamic method which includes Darcy flow calculations. More input data are required for dynamic simulation, and it is more computationally intensive, but it takes into account migration pathways and dissolution effects, so is generally more accurate and more useful. For example, the CO2 migration plume may be used to identify appropriate monitoring techniques, and the analysis of trapping mechanism for a certain site will help to optimize well location and injection plan. Two hypothetical saline aquifer storage sites in the UK, one in Lincolnshire and the other in the Firth of Forth, were analysed. The Lincolnshire site has a comparatively simple geology, while the Forth site has a more complex geology. For each site both static and dynamic capacity calculations were performed. In the static method, CO2 was injected till the average pressure reached a critical value. In the migration monitoring case, CO2 was injected for 15 years was followed by a closure period lasting thousands of years. The fraction of dissolved CO2 and the fraction immobilised by pore scale trapping were calculated. The results of both geological systems show that the migration of CO2 is strongly influenced by the local heterogeneity. The calculated storage efficiency for the Lincolnshire site varied between 0.34% and 0.61% of the total pore volume, depending on whether the system boundaries were considered open or closed. Simulation of the deeper, more complex Forth geological system gave storage capacities as high as 1.05%. This work was part of the CASSEM (CO2 Aquifer Storage Site Evaluation and Monitoring) integrated study to derive methodologies for assessment of CO2 storage in saline formations. Although, static estimates are useful for initial assessment when less data is available, we demonstrate the value of performing dynamic storage calculations, and the opportunities to identify mechanisms for optimising the storage capacity.

Thermal and burial history modelling in the Midlothian-Leven syncline in the Midland Valley of Scotland using BasinMod and HotPot

Synopsis Basin subsidence and thermal history models of the eastern part of the Midland Valley of Scotland have been constructed to characterize the development of the Midlothian-Leven synclinal basin from the mid Mississippian to the present day. Two modelling programs were used: BasinMod and HotPot. BasinMod models are based on thermal maturity at borehole sites; HotPot models operate on stacks of 2D layer grids using a mechanical compaction method. The thermal and burial history modelling indicates up to 1.9 km of additional burial of Carboniferous strata compared to present day levels: up to 660 m deposited by the end of the Carboniferous period, subsequently removed by Variscan uplift and erosion, followed by up to 1.9 km of burial by sedimentary rocks deposited during the Permian to the Palaeogene periods. Temperatures greater than 130 °C were reached by the most deeply buried Carboniferous sedimentary rocks. Mississippian (Lower Carboniferous) rocks reached the oil generation window during the mid Carboniferous, with Pennsylvanian (Upper Carboniferous) rocks reaching the oil generation window during the Cretaceous. Sedimentation rates were of 0.03–0.63 m ka−1 (metres per thousand years) with changes in the rate and the positions of depocentres consistent with both extensional and strike-slip basin formation.

Fluvial and aeolian deposition in the Siluro-Devonian Swanshaw Sandstone Formation, SW Scotland

Synopsis The recognition of fluvial and aeolian facies within the Siluro-Devonian Swanshaw Sandstone Formation of SW Scotland allows a more detailed depositional history of the Lanark basin in the southern part of the Midland Valley of Scotland to be inferred. The formation is well exposed in Ayrshire and has four main sedimentary facies associations: Channel, Aeolian, Floodplain and Mudflat. Each combines several sedimentary facies: eight facies are recognized in the channel association, three in the aeolian, four in the floodplain and two in the mudflat. These facies associations co-existed in seven different depositional settings in the western Lanark basin. Deposition occurred on a semi-arid proximal alluvial plain with an aeolian component described for the first time in the Lanark basin. Sedimentation was pulsed, largely driven by periods of source area uplift that resulted in the deposition of coarse-grained conglomeratic and sandstone-dominated facies. The principal transport was via river channels, commonly in shallow braided systems. Periods of low sediment flux are marked by fewer channels and by the preservation of floodplain successions including aeolian deposits. Abandonment of the fluvial system towards the top of the formation in Ayrshire is marked by a progression through sandflats and mudflats to the overlying volcanic formation. The relationship of the fluvial and aeolian deposits is analysed. The palaeo-wind direction is compatible with that recorded in sandstones of similar age in the Orcadian basin.

Shale prospectivity onshore Britain

Abstract The UK Department of Energy and Climate Change (DECC) commissioned the British Geological Survey (BGS) to summarize the available geological knowledge, integrate new seismic mapping and well analysis, and make preliminary in-place resource assessments for the three most prospective areas onshore Britain to foster a greater understanding of the unconventional shale resource potential in advance of the 14th Landward Licensing Round. The first study, published in June 2013, reviewed the Carboniferous Bowland–Hodder shales across central Britain where a large volume of in-place gas was assessed to be present. The second study, of the Jurassic shale of the Weald Basin in southern England, published in May 2014, concluded that owing to insufficient burial there was no significant Jurassic shale gas potential, but there could still be shale oil resources at several levels in the centre of the basin. The third study, published in June 2014, covered the Midland Valley of Scotland where both oil and gas potential in Carboniferous shales was identified. A large volume of in-place gas and oil resource has been assessed to be present. However, not enough is known at the time of writing to estimate a recovery factor or to estimate potential producible reserves. This paper summarizes the results of the BGS reports and their impact on the subsequent licensing process in England.

Can uncertainty in geological cross-section interpretations be quantified and predicted?

This work was undertaken while C.H. Randle held a joint British Geological Survey University Funding Initiative (BUFI) and University of Aberdeen, College of Physical Sciences Ph.D. Studentship at Aberdeen University. The contributions by C.H. Randle, R.M. Lark, and A.A. Monaghan are published with the permission of the Executive Director of the British Geological Survey Natural Environment Research Council. We would also like to thank all those who took part in both experiments as well as the many people who have given input on our results.