Trace Fossils from Lower Palaeozoic Ocean-Floor Sediments of the Southern Uplands of Scotland M
Transactions of the Royal Society of Edinburgh: Earth Sciences, 73, 67-87, 1982 Trace fossils from Lower Palaeozoic ocean-floor sediments of the Southern Uplands of Scotland M. J. Benton ABSTRACT: The Ordovician and Silurian rocks of the Southern Uplands of Scotland have been interpreted as sediments deposited on the northern margin of the Iapetus Ocean. Trace fossils are abundant at many localities in ocean-floor turbidites and mudstones that usually lack all other evidence of life. Twelve ichnogenera are present, and they are mainly meandering locomotion and feeding trails and burrow networks: Dictyodora, Caridolites, Helminthoida, Neonereites, Nereites, Protovirgularia, Gordia, Megagrapton, Paleodictyon, Chondrites, Plano- lites and Skolithos. The trace fossils occur in at least five distinct assemblages and the composition of these was probably controlled by the frequency and nature of the turbidity currents, and possibly by the oxygen content of the mudstones. Where turbidity currents were weak, abundant Dictyodora, together with Caridolites, Neonereites, Nereites, Protovirgularia and Gordia occur in various combinations. Where currents were stronger, traces such as Gordia, Paleodictyon and Megagrapton may be exhumed and cast on turbidite soles, and the sand may contain Skolithos. The 'deep-sea' Nereites trace fossil facies is divisible into several assemblages, presumably environmentally controlled. KEY WORDS: Iapetus Ocean, ichnology, mudstone, Nereites Facies, Ordovician, shale, Silurian, turbidite. Deep-sea trace fossil assemblages of the Ordovician and 1. Geological setting Silurian are poorly known. The Lower Palaeozoic turbidites and associated mudstones of the Southern Uplands of Scot- 1.1. Structure and history of deposition land preserve at least 12 ichnogenera and there are several The Ordovician and Silurian rocks of the Southern Uplands distinct assemblages that are associated with particular (basalts, cherts, graptolitic shales, greywackes and red or sedimentary conditions.
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