Graptolites as constraints on models of sedimentation across Iapetus: a review

Graptolites as constraints on models of sedimentation across Iapetus: a rev iew. Proceedings of the Geologists' Association, 112, 237-251. The use of g raptolites (specifically, of the planktonic graptoloids) in sedimentary fac ies analysis is discussed and illustrated, with particular reference to the margins of the former Iapetus Ocean in the UK. By far the most important u se stems from the high time resolution afforded by graptolite biostratigrap hy, particularly in the Silurian, where biozones/subzones average under hal f a million years in duration. Increased resolution may be obtained where b iostratigraphy is combined with the event stratigraphy provided by repeated oscillations between anoxic and oxygenated sea floors, which yielded lamin ated ("graptolite shale') and burrowed ('barren beds') hemipelagites respec tively. The resolution thus afforded has allowed the elucidation of detaile d sedimentary architecture; examples are drawn from the Llandovery turbidit es of the Welsh Basin and the late Ordovician to early Silurian accretionar y prism of the Southern Uplands of Scotland. Use of this biozonation as a t emporal framework has also allowed an interpretation of the plate tectonic setting of individual basins via subsidence analysis and the sequence strat igraphic interpretation of the basin-fills. The use of graptolites as indic ators of broader palaeogeographic and palaeoceanographic settings shows pro mise but, in practice, is still hindered by incomplete understanding of the various factors that controlled the distribution of graptoloids within the early Palaeozoic seas. Finally, current-transported graptolites, may be us ed, cautiously, as palaeocurrent indicators.