Triassic pebbles, derived fossils and the Ordovician to Devonian palaeogeography of Europe

Abstract

Papers published by Salter (1864) and Davidson (1870) on the faunas from pebbles in a Triassic conglomerate at Budleigh Salterton, Devon, are reviewed. After modern reassessment, these pebbles, although of apparently similar quartzites, have been found to be of four different ages, two Ordovician (mid-Arenig and late Llandeilo) and two Devonian (Lochkovian-Pragian and Frasnian). By comparing these four faunas with those contemporary in adjacent palaeocontinents, it can be shown that, apart from the earliest one, they have affinities closer to those of the Armorican peninsula of Brittany and Normandy than to the rest of Britain and that these Armorican faunas are in clasts which were transported northward by Triassic rivers. Consideration of all the various faunas in the whole of northwest Europe reflect the earliest Ordovician of southern Britain as part of the vast Gondwanan continent, from which it became detached by the mid-Ordovician, with a widening Rheic Ocean between the two palaeocontinents; and the subsequent merging of Avalonia with Baltica and Laurentia to form Laurussia by Mid-Devonian times. New palaeogeographical maps depicting phases from the Ordovician to the Devonian are presented.