The Carboniferous evolution of the Central Coalfield Basin, Midland Valley of Scotland : implications for basin formation and the regional tectonic setting
The Midland Valley of Scotland was a major Carboniferous depocentre containing volumetrically significant intrusive and extrusive magmatic suites. The area has long been a target for economic activity providing large datasets and a well-constrained stratigraphy. This study assesses the sedimentological and structural development of the Central Coalfield Basin, one of four important Carboniferous basins within the Midland Valley. A tectonostratigraphic framework for the Central Coalfield is established by an integrated analysis and interpretation of 2D reflection seismic profiles, borehole and well data and sedimentological studies. Based on this research, the wider implications for the regional tectonic setting and general concepts of basin formation are also examined. The Midland Valley has a distinctive Carboniferous sedimentary fill. Temporal and spatial facies changes are minimal; depocentres were dominated by shallow-marine, deltaic and alluvial environments and no Carboniferous alluvial fans are recorded along basin margins. Therefore deposition appeared to have kept pace with accommodation generation. In contrast, deep marine Dinantian and Namurian sediments are preserved in the basins of central and northern England reflecting underfilled depocentres and in the Northumberland Basin. Accommodation was generated by the syn-depositional evolution of the NNE-SSW orientated Clackmannan-Falkirk-Stane Syncline supplemented by local fault generated accommodation. Syncline growth occurred during the mid- and late-Visean, while during the Namurian regional subsidence was accompanied by slower syncline growth and periodic fault movement. During the Westphalian folding occurred across the Midland Valley post-dating the majority of deposition. The largest, intrabasinal fault-bounded structure recognised is the 25-km-long, ENE-trending Forth graben in the northern Central Coalfield. E-W orientated faults offset syndepositional Carboniferous folds and faults and are therefore interpreted as Late Westphalian to Early Permian. The occurrence, orientation and timing of major fold and fault systems in the Central Coalfield are consistent with dextral strike-slip movement along the Southern Upland and Highland Boundary faults. This dextral motion generated ENE-WSW directed compression and NNW-SSE extension resulting in NNE- orientated folds and the fault orientations recognised. The synclinal depocentres and groups of faults observed in the other coalfield basins of the Midland Valley also have orientations and syndepositional histories consistent with this dextral transtensional model.