A ROCK VOLUME ACCUMULATION CURVE FOR THE LATE ORDOVICIAN-SILURIAN WELSH BASIN

A rock accumulation curve is presented for the Ordovician-Silurian Wel sh Basin, based on the volume of sediment entering the basin during ea ch graptolite biozone. Even allowing for its inherent uncertainties, t he curve still illustrates a significant and sudden increase in the ra te of sedimentation in early Silurian time. This may be explained by s ubmarine fan migration, large scale climate change, or river capture i n the hinterland, but is most likely caused by tectonic uplift at sour ce. This would support models for either an early Silurian, or late Or dovician closure of the lapetus Ocean.