Clastic marine facies in the Ludlow of the Central Welsh Region
A wide range of clastic marine facies have been studied in the Ludlow sequences of the Central Welsh Region. By considering these sequences in a regional context it has been possible to review previous lithostratigraphic and biostratigraphic correlations used in the Central Welsh Region. This review has led to the production of a comprehensive stratigraphic framework for the Central Welsh Region, and has revealed the need for a significant revision of the biostratigraphic correlations presently in use. Analysis of facies patterns in Gorstian (Lower Ludlow) sequences has led to the recognition of the main active sediment transport pathways during this interval. Although the main sediment transporting processes vary across the study area, they are all interpreted in terms of the processes active at different points in a storm-driven sediment distribution system. The effects of storm related processes are also predominant within Ludfordian (Upper Ludlow) sequences. The major lateral and vertical facies changes present in Ludlow sequences are attributed to a number of causes, all of which can be related to the relatively high sea-level prevailing in the Central Welsh Region during much of the Ludlow. They include; the development of anoxia in surficial sediments as a result of low levels of dissolved oxygen in the overlying water-column; the positions of storm and fairweather wave base relative to the regional (west-facing) palaeoslope; and the effects of changes in sea-level. These changes in sea-level probably reflect a combination of eustatic and tectonic-driven events. It is concluded that the 'shelf and basin' model of existing palaeogeographic interpretations is an artificial one. Instead, it is suggested that Ludlow facies patterns record the existence of a broad shelf-slope system, and reflect relative sea-level, the topography of the shelf-slope system, the location of sediment source areas, subsidence rate, and position within the main shelf sediment transport system.