Midland Valley of Scotland Permo-Carboniferous Workshop

Date:
10th - 11th April 2003

Venue:
British Geological Survey, Murchison House, Edinburgh.

Convenors:
Alison Monaghan & Mike Browne (BGS Edinburgh), Sarah Davies (University of Leicester) and Ruth Robinson & Tony Prave (University of St. Andrews).

Report:
This Workshop was convened to discuss the current research on the tectonostratigraphic evolution of the upper Devonian to Permian Midland Valley of Scotland (MVS). The MVS is an important basin in terms of economic resources and can be utilised to research the mechanisms that drive basin formation. The Workshop was attended by over 30 people, covering the broad spectrum of research experience of Midland Valley geology. The four sessions aimed to promote the integration of both recent research and extensive databases built over many years of study: 1) Seismic studies, basin evolution and prospectivity. 2) Radiometric dating and biostratigraphy. 3) Depositional and stratigraphic frameworks. 4) Regional correlations and implications.

Presenters in the opening session used seismic and field-based studies to propose that a dextral strike-slip control was important for post-Viséan Carboniferous basin development in the eastern (John Underhill et al.), central (Matthew Hooper et al.) and western (Alison Monaghan et al.) MVS. These talks also highlighted the differences in structural styles across the MVS. In the eastern and central MVS, syn-depositional NE-SW and N-S orientated synclines developed, which were later tightened during Late Carboniferous Variscan deformation and were offset by E-W fractures produced during early Permian tension. In contrast in the western area, no major synclines were developed and NE- and E-trending Caledonian lineaments were the dominant control on graben and half-graben geometries that influenced Carboniferous sedimentation and volcanism (Alison Monaghan et al.). Varying stress regimes during basin evolution, amongst other factors, may have produced a range of structural styles (John Rippon). The variability in coal cleat orientation in the some of the MVS subbasins reflected the local structure whereas the dominant NE cleat orientation from English coal basins reflects the Variscan orogen (John Rippon).

These structural models have implications for MVS prospectivity. The exposed Lower Carboniferous Oil Shale source rocks are immature but may be buried to a sufficient depth beneath the Firth of Forth and hydrocarbons may have migrated into age-equivalent reservoir rocks in adjacent growth folds (John Underhill et al.). The MVS sedimentary successions are also excellent reservoir analogues both directly for the Carboniferous of the North Sea and for hydrocarbon-bearing Jurassic tidally-influenced successions in Siberia (Patrick Corbett).

The abundant Permo-Carboniferous magmatism and volcanism is one of the key differences between the MVS and other UK Carboniferous basins. The timing, distribution and controls of Midland Valley magmatism over 80-100 Myr (David Stephenson) occurred in four main, distinctive phases with the three alkaline-transitional phases attributed to an extensional model, producing lithospheric attenuation and partial melting in the athenospheric mantle. Recent 40Ar/39Ar dating of intrusive and extrusive Permo-Carboniferous igneous rocks (Alison Monaghan & Malcolm Pringle) has significantly revised the absolute dates, successfully integrating the volcanic events with palynological age constraints. The new dates have also demonstrated that key phases of volcanism, such as the outpouring of the Clyde Plateau Lavas, occurred over a much shorter time period than previously determined.

Reanalysis of palynology from key Lower Carboniferous sections in Fife revised the ages and correlation framework for these Dinantian sections (Bernard Owens et al.). These results and revisions to the MVS Carboniferous tectono-stratigraphical framework (Mike Browne) demonstrate how the palynostratigraphy must be revisited to improve the biostratigraphical framework. A new approach, integrating biostratigraphy and palaeoenvironments of palynomorphs and ostracods, has been successful in dating Dinantian structural and volcanogenic events, such as a regionally important lower-mid Viséan hiatus (Mike Stephenson & Mark Williams). The excellent, anatomical preservation of plants in Dinantian cementstone facies, in ashes and in lavas or peat layers within lavas could also have major correlation potential if used in conjunction with palaeontological and lithostratigraphical frameworks and for constraining volcanic events (Andrew Scott).

Sedimentological, petrographical and biostratigraphical studies of Lower Carboniferous sections along the Fife coast demonstrated that these sediments represent only the Asbian interval as opposed to recording deposition during the entire 12-15 Myr Viséan interval (Phillip Shell et al.; Bernard Owens et al.): there may be an unrecognised unconformity at the base of the East Fife sections. The timing of onshore deposition is only a few million years after the Garleton Hills volcanic episode (Alison Monaghan and Malcolm Pringle) and thus a link between volcanism, uplift and subsequent thermal subsidence could be established. Major Namurian marine deposits, such as the Top Hosie and Castlecary limestones, may be related to low frequency sea-level rises and the general dominance of flooding surfaces probably reflects the rapid subsidence of the Namurian basin (Bill Read). Some of the major multistorey sandstones, interpreted as valley fills or tidally-influenced deltas, may reflect significant sea-level falls (Bill Read). In the Westphalian, exotic palynomorphs (including Lower Palaeozoic marine acritarchs) and heavy mineral ratios from particular green-coloured Westphalian fine-grained sediments in Ayrshire suggest an additional, intermittent contribution from a western source, possibly as distant as Illinois/Ohio in the US (Duncan McLean et al.).

Across the UK and Canada, an early Carboniferous, broadly extensional phase was succeeded by regional subsidence in the Viséan. In the Westphalian and Stephanian, periods of uplift and erosion alternated with thermal subsidence and accommodation generation across this region. The dominance of dextral motion along major Carboniferous fault systems in eastern Canada and the MVS questions a purely extensional model for the basins of the northern UK (Sarah Davies).

The transtensional and extensional basin models proposed at this meeting and in previous publications were also challenged (Tony Prave et al.). The Workshop reflected the importance of a syn-depositional tectonic control on sedimentation and volcanism; including the thinning and thickening into growth folds, minor syn-depositional faulting and location of volcanic vents. However, the driving mechanism behind Carboniferous basinal subsidence and the extent of volcanism remains unclear (Tony Prave et al.). The future of research lies in improving the geochronological framework constraining sedimentation and undertaking a major multidisciplinary project involving scientists from a range of disciplines within the Earth Sciences.