Early Ordovician acritarchs of the Lierneux Member (Stavelot Inlier, Belgium): stratigraphy and palaeobiogeography

Abstract

Abstract The messaoudensis-trifidum acritarch microflora is currently considered to be characteristic of latest Tremadocian-earliest Arenigian cold-water environments on the periphery of Gondwana, at high latitudes in the southern hemisphere. Integrated biostratigraphical studies on both acritarchs and graptolites are available from two areas of peri-Gondwana, the Lake District (northwestern England) and the Sierra Morena (southwestern Spain). The assemblage was also recorded from other areas on the northern border of the Gondwana continent where macrofossils are generally not available: from southern Ireland, the Isle of Man, southern Wales, the island of Rügen in northern Germany, the Prague Basin in the Czech Republic, and southern Turkey. While it appears that the messaoudensis-trifidum acritarch assemblage is limited to cold-water environments in localities on the periphery of Gondwana, some of its elements can be found in other areas. Some taxa, but not the complete assemblage, have been identified in the eastern Cordillera of Argentina, and some species of the assemblage are cited from continents which were situated at intermediate latitudes in warmer waters: some elements of the assemblage are described from localities of Baltica (from Norway, Estonia and the St. Petersburg area in northwestern Russia) and from the Yangzte Platform in southern China. In these regions, typical representatives of the messaoudensis-trifidum assemblage co-occur with taxa typical of temperate and warm-water areas. The present paper reports the discovery of the messaoudensis-trifidum acritarch assemblage in the Lierneux Member (Stavelot Inlier, Belgium). The Lierneux Member, which constitutes the uppermost part of the Jalhay Formation (formerly Salmian 1c) in the Stavelot Inlier, was first dated as late Tremadocian by Vanguestaine [1992a]. Following recent stratigraphical conclusions based on the detailed investigations of the messaoudensis-trifidum assemblage from different sequences of peri-Gondwana, the age of the Lierneux Member recovered from the Chevron borehole in the Stavelot Massif can now be confirmed as being probably latest Tremadocian. The discovery of the messaoudensis-trifidum assemblage in the Stavelot Inlier provides further evidence for the palaeogeographical distribution of the assemblage on the northern border of Gondwana and allows tentative correlations between eastern Belgium and northern Germany.