Silesian, introduction, Carboniferous, Wales

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From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Idealised contrasting cyclothems (after Woodland and Evans, 1964). a–c South Wales Coal Measures Group d Warwickshire Group. P916193.

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.

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