Paleoecology, biostratigraphy and biogeography of late Silurian to early Devonian acritarchs and prasinophycean phycomata in well A161, Western Libya, North Africa

Abstract

Diverse and well preserved acritarch and prasinophycean phycomata assemblages were recovered from the late Silurian to Lower Devonian strata of well A161 in western Libya, and four distinct acritarch biozones are recognized, based on the stratigraphic distribution of 156 species. The palynoflora is independently dated by means of chitinozoans, and allows discussion of the evolution of acritarchs and prasinophyte phycomata across the Ludlow–Přı́dolı́ boundary in relation to probable major climatic change, as well as in the early and middle Přı́dolı́, and the lower Lochkovian. Correlations are proposed with the British Isles, Baltica, and Algerian Sahara. Sedimentation occurred in shallow high-energy conditions throughout, but with periodic rise of sea level. The changes in marine to terrestrial palynomorph ratios through the section document the relationship between marine palynomorph assemblages and sea surface conditions in these marginal marine environments. The major drift of Gondwana towards low latitudes during the Ludlow–Přı́dolı́ transition seems to have been the driving force behind homogenization of assemblages on the two sides of the Rheic ocean, and explains the similarities between phytoplanktonic assemblages of the north Gondwanan margin and the South of Baltica. The data suggest that the Rheic ocean was almost closed by the late Silurian, and had become restricted to a moderately deep sea. In the Lochkovian the microflora are strongly facies-dependent and delineate more restricted provinces such as the Ibarmaghian domain in the sense of Plusquellec (1987) including the Maghreb and Ibero–Armorican areas. Nine new species are described: Arkonia nova, Arkonia paulumstriata, ? Cymatiosphaera florida, Cymatiosphaera nimia, Dactylofusa hispidusa, Disparifusa quasibernesgae, Evittia areolata, Multiplicisphaeridium verticisum, and ? Villosacapsula steemansii. In addition, three new combinations are suggested: Visbysphaera bonita (Cramer) comb. nov., Visbysphaera jardinei (Cramer) comb. nov. and Visbysphaera albanega (Cramer et al.) comb. nov.