The age of the Watch Hill Grits (Ordovician), English Lake District: structural and palaeogeographical implications

The Watch Hill Grits, in the lower Ordovician Skiddaw Group of the Lake District, are dated as latest Tremadoc or earliest Arenig (older than the Didymograptus deflexus Biozone) by means of acritarchs and graptolites. This places them lower in the Skiddaw sequence than previous authors supposed, and they are thought to have been thrust southwards over younger Arenig rocks. The interval inferred for the Watch Hill Grits brackets the Tetragraptus approximatus Biozone and its equivalents in Australasia, S China, N America and Scandinavia. The dating of the Watch Hill Grits suggests that equivalents of the Tetragraptus approximatus Biozone are present in N England and that sedimentation continued in the Lake District during a period of global eustatic regression. Other beds in the Lake District and on the Isle of Man have yielded similar acritarch assemblages and are probably correlatives. Five new acritarch taxa are described: Caldariola gen. nov., Acanthodiacrodium? dilatum, Stellechinatum sicaforme and Striatotheca prolixa spp. nov., and Tetraniveum arenigum cumbriense subsp. nov.; one new combination, Caldariola glabra (Martin) comb, nov., is proposed.