Patterns of Sedimentation in Rhondda Beds of South Wales

Abstract

Variations in the sedimentologic character of the Rhondda Beds, an Upper Coal measures (upper Carboniferous) sequence in the South Wales coal field, are attributed to differences in tectonic and sedimentary environments and to the nature of the sediment-supplying fluvial agencies. A comparison of isopachs with the variations in total and proportionate thickness of sandstone suggests that generally there is an inverse relation between subsidence rate and sandstone thickness, although there are important exceptions, especially in the eastern part of the basin. Significantly, the proportionate distribution of sandstone is more readily compatible with the inferred current system than is the isopach pattern. The Rhondda Beds are characterized by fining-upward cycles, generally sandstone-dominant, of two basic types distinguished both by dimensions and by the detailed nature of the sequences. The sequence represents the filling of discrete channel forms, whereas the other type of cycle is interpreted as a floodplain sequence, resulting from lateral accretion. Sequential analysis of vector properties furnishes additional evidence, valuable in interpreting the succession of hydrodynamic events recorded in such cycles. Significant variations in the form of the standard cycle in parts of the basin have been related to environmental factors. Integration of several sedimentologic attributes shows that the Rhondda Beds were formed in the alluvial valley tract of a west-flowing river system, enclosed on three sides; sediment was supplied principally from the south and north, with minor contributions from the east. Streams in the east and northeast regions were small but energetic, whereas those on the south were relatively large, of slight sinuosity, and subject to abrupt abandonment. The central and north regions were characterized by more sluggish and sinuous rivers which generated thick sand bodies interfingering laterally and vertically with topstratum deposits.