Biostratigraphy and paleogeography of Middle-Late Ordovician palynomorphs from the Llanos Basin, Colombia

Abstract

Diverse and moderately to well-preserved palynomorph assemblages, composed of 81 organic-walled phytoplankton species, belonging to 47 genera, and two chitinozoan species, were recorded from the Ordovician rocks of the well RH-2, in the Llanos Basin of Colombia. The stratigraphic distribution of the taxa allows four assemblages to be distinguished that are, from base to top: Assemblage 1, assigned to the late Floian? – Dapingian; Assemblage 2, assigned to the Darriwilian; Assemblage 3, assigned to the Darriwilian?–Sandbian–Katian? and Assemblage 4, assigned to the Katian–Hirnantian. Assemblage 4 is considered no younger than pre-glacial Hirnantian, based on the absence of taxa typical of phytoplankton assemblages from Hirnantian glacial and post-glacial deposits. The Middle Ordovician interval shows clear affinities with the Perigondwana acritarch Province, mainly due to the presence of the diagnostic taxa Arbusculidium filamentosum, Arkonia, Aureotesta, Coryphidium, Dicrodiacrodium, Frankea and Striatotheca. Nevertheless, taxa with South Chinese and Baltic affinities, from low to intermediate latitudes, such as Ampullula and Tongzia are also present, even though Colombia was situated at a high, cold latitude during this time interval. The Katian–Hirnantian Assemblage 4 supports the cosmopolitanism of Ordovician organic-walled phytoplankton that began around the Darriwilian–Sandbian boundary, with globally occurring species such as Leiofusa litotes, Multiplicisphaeridum irregulare and Villosacapsula setosapellicula recorded. The moderate to high diversity of the Middle Ordovician phytoplankton significantly drops in the Late Ordovician Assemblage 4. The palynological analyses reported here contributes to the understanding of phytoplankton distribution on the East Gondwana margin.