Middle Devonian microfloras from the Chigua Formation, Precordillera region, northwestern Argentina

Abstract

Abstract Rich palynological assemblages have been obtained from the Middle Devonian Chigua Formation (Chinguillos Group, San Juan Province), western Precordillera, Argentina, at two new localities (Del Chaco and Don Agustín creeks). From the palyniferous levels at Del Chaco Creek, there are two assemblages: one is very rich in microplankton (assemblage 1) whereas the other is dominated by spores (assemblage 2). The level obtained from Don Agustín Creek, poorest in overall taxa, is also rich in microplankton (assemblage 3). Compared with coeval microfloras elsewhere, a late Emsian–early Eifelian, and an early Givetian age are proposed for the first and the second assemblages respectively. A third assemblage (assemblage 3) tentatively represents a timespan around the Givetian–Frasnian boundary. The proposed age based on the palynomorphs reinforces previous palaeontological records. Variation in the microplankton diversity, fluctuations in the microplankton/spore ratio and differences in acritarch morphology are used here to interpret changing proximity to palaeoshoreline and so to recognize fluctuations in relative sea level that occurred during the deposition of the Chigua Formation.