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

Abstract

The use of graptolites (specifically, of the planktonic graptoloids) in sedimentary facies analysis is discussed and illustrated, with particular reference to the margins of the former Iapetus Ocean in the UK. By far the most important use stems from the high time resolution afforded by graptolite biostratigraphy, particularly in the Silurian, where biozones/subzones average under half a million years in duration. Increased resolution may be obtained where biostratigraphy is combined with the event stratigraphy provided by repeated oscillations between anoxic and oxygenated sea floors, which yielded laminated (‘graptolite shale’) and burrowed (‘barren beds’) hemipelagites respectively. The resolution thus afforded has allowed the elucidation of detailed sedimentary architecture; examples are drawn from the Llandovery turbidites of the Welsh Basin and the late Ordovician to early Silurian accretionary prism of the Southern Uplands of Scotland. Use of this biozonation as a temporal framework has also allowed an interpretation of the plate tectonic setting of individual basins via subsidence analysis and the sequence stratigraphic interpretation of the basin-fills. The use of graptolites as indicators of broader palaeogeographic and palaeoceanographic settings shows promise 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 used, cautiously, as palaeocurrent indicators.