Latest Ordovician-earliest Silurian chitinozoans from the Puna region, north-western Argentina (Western Gondwana)

Lower Palaeozoic basins of Western Gondwana (Robin et al. 2004), because of its geographical extent and fossil record. The southern end of this basin is located in north-western Argentina and its outcrops are exposed in the geological provinces of Puna, Cordillera Oriental, Sierras Subandinas and Sistema de Santa Bárbara (Fig. 1A) (Ramos 1999). In the eastern and central part of the basin, the Upper Ordovician is classically represented by the “Zapla Glacial Horizon” (Schlagintweit 1943) of late Hirnantian age (Monaldi & Boso 1987, de la Puente & Rubinstein 2013, Benedetto et al. 2015), which is locally called the Mecoyita Formation (Turner 1960) or Caspalá Formation (Starck 1995). This glacial horizon is deposited above the Ocloyic regional discordance, which separates it from the Lower and Middle Ordovician strata. The Zapla Glacial Horizon includes three facies associations: massive matrix-rich diamictites, finely stratified diamictites, and graded sandy and conglomerate beds covering the diamictites facies (Astini 2008). It is overlain by transgressive deposits of the Silurian Lipeón Formation (Turner 1960). In the eastern part of the basin, chitinozoan assemblages document a late Hirnantian age for the glacial deposits and support a basal Silurian age assignment for the strata representing the postglacial sequences in a condensed section (de la Puente et al. 2012, de la Puente & Rubinstein 2013, Benedetto et al. 2015). In the central part of the basin, chitinozoan assemblages document a Hirnantian age for the glacial deposits and a Telychian age for the postglacial sequences (Rubinstein et al. 2016). Within the Lower Palaeozoic deposits of western Puna region, in the western part of the basin, no direct sedimentological evidence for glaciation has been found.