Silesian, introduction, Carboniferous, Wales
From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.
Silesian strata in Wales are dominantly clastic deposits of deep marine to alluvial fan environments. Correlation is based on the richly fossiliferous shale marine bands. Certain marine bands, distinguished by a diagnostic ammonoid (goniatite) fauna, can be traced within and between basins on a regional scale and these provide the basis of a high resolution biostratigraphical framework. Strata bound by successive marine bands represent units of related facies, which have been referred to as cyclothems (P916193). Many of the cyclothems are dominated by fluviodeltaic deposits and their variable facies patterns have been related to differing positions in the depositional systems. Recently the concept of sequence stratigraphy has augmented these methods of correlation and interpretation — facies relationships are related to key surfaces possibly caused by fluctuations in sea level. In the fluviodeltaic deposits, these surfaces include a range of transgressive and emergent surfaces, which are referred to as sequence boundaries. The methods of sequence stratigraphy are particularly applicable to successions deposited at times of glacial eustasy, as during the Carboniferous, when high fluctuations in sea level left a distinctive signature in the stratigraphy. The main phase of Carboniferous glaciation was initiated in the early Namurian and peaked in late Westphalian times when an ice sheet covered large parts of the Gondwanan supercontinent; the geographically widespread marine bands developed during periods of glacio-eustatic transgression.
The cyclothems occur on both a large scale, in hundreds of metres of strata, and on a small scale of a metre to tens of metres. The large-scale cyclothems are defined between marine bands, generally coarsen upwards and have been attributed to prograding deltas. The smaller cyclothems also coarsen upwards but culminate in a seatearth and coal seam, reflecting a phase of emergence. Within both cyclothems the vertical patterns of the lithologies are widely variable. In the small scale sequences, these can be related to local patterns of sedimentation, but the controls on the large scale patterns, whether eustatic or tectonic, remain a matter of some considerable debate.
BARCLAY, W J. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Third edition. Memoir of the British Geological Survey. Sheet 232 (England and Wales).
DINELEY, D L. 1992. Devonian. 179–205 in Geology of England and Wales. DUFF, P MCL D, and SMITH, A J (editors). (London: The Geological Society of London.)
FRESHNEY, E C, and TAYLOR, E A. 1980. The Variscides of south-west Britain. 49–57 in United Kingdom introduction to general geology and guides to excursions. OWEN, T R (editor). Proceedings of the 26th International Congress, Paris, 1980.
GAYER, R, FOWLER, R, and DAVIES, G. 1997. Coal rank variations with depth related to major thrust detachments in the south Wales coalfield: implications for fluid flow and mineralization. 161–178 in European coal geology and technology. GAYER, R, and PES EK, J (editors). Geological Society Special Publication, No. 125.
GEORGE, T N. 1970. British regional geology: south Wales. Third edition. (London: HMSO.)
HAMPSON, G J, ELLIOTT, T, and DAVIES, S. 1997. The application of sequence stratigraphy to Upper Carboniferous fluvio-deltaic strata of the onshore UK and Ireland: implications for the southern North Sea. Journal of the Geological Society of London, Vol. 154, 719–733.
JONES, D G. 1074. The Namurian series in south Wales. 117–132 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff : University of Wales Press.)
KELLING, G. 1974. Upper Carboniferous sedimentation in South Wales. 185–224 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff: University of Wales Press.)
POWELL, C M. 1989. Structural controls on Palaeozoic basin evolution and inversion in south-west Wales. Journal of the Geological Society of London, Vol. 140, 439–446.
RAMSBOTTOM, W H C. 1973. Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy. Proceedings of Yorkshire Geological Society, Vol. 39, 567–607.
THOMAS, L P. 1974. The Westphalian (Coal Measures) in south Wales. 133–160 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. OWEN, T R (editor). (Cardiff: University of Wales Press.)
WARREN, P T, PRICE, D, NUTT, M J C, and SMITH, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir of the British Geological Survey, Sheet 107 and parts of sheets 94 and 106 (England and Wales).
WATERS, C N, BROWNE, M A E, DEAN, M T, and POWELL, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (onshore). British Geological Survey Research Report, RR/07/01.
WHITE, S. 1991. Palaeogeothermal profiling across the South Wales Coalfield. Proceedings of the Ussher Society, Vol. 7, 368–374.
WILSON, D, DAVIES, J R, FLETCHER, C N J, and SMITH, M. 1990. Geology of the south Wales Coalfield, Part VI, the country around Bridgend. Second edition. Memoir of the British Geological Survey, Sheets 261 and 262 (England and Wales).
WOODLAND, A W, and EVANS, W B. 1964. The geology of the south Wales Coalfield. Part IV. The country around Pontypridd and Maesteg. Memoir of the Geological Survey of Great Britain, Sheet 248 (England and Wales).
WRIGHT, V P. 1996. Use of palaeosols in sequence stratigraphy of peritidal carbonates. 63–74 in Sequence stratigraphy in British geology. HESSELBO, S P, and PARKINSON, D N (editors). Geological Society of London Special Publication, No. 103.