New palynological insights into the Middle Jurassic Challacó Formation, Neuquén Basin, northwestern Patagonia, Argentina

Abstract

An outcrop succession of the Challacó Formation preserved on the southern flank of the Huincul Arch, at the Quebrada del Sapo locality, Picún Leufú Sub-basin, Neuquén Basin, Argentina, allowed us to carry out the study of a palynobiota. Several taxa are documented for the first time in Argentina and in Jurassic deposits of the Neuquén Basin, as well as in Jurassic strata of other regions of the world. In light of these unconventional records, the diagnosis of two taxa has been extended, i.e. Trachysporites and Ovoidites tripartitus. On the basis of palynostratigraphic markers a middle Callovian age is proposed. The facies and palynofacies analysis allows us to infer that the accumulation of these deposits occurred in a tidal-dominated delta environment. A noteworthy feature of these assemblages is the presence of Gloeocapsomorpha, a taxon previously documented from the middle Early Proterozoic to Paleozoic. Also, the presence of Gonyaulacysta adecta dinocyst, widely distributed during the late Middle–Late Jurassic in Laurasia and western Gondwana, has enabled us to verify for the first time from a palynological viewpoint, an open marine connection with the western Tethys via the Hispanic Corridor in ages as young as the middle Callovian. Finally, the Podocarpacean Phase is recognised for the first time in South America, towards the top of the Challacó Formation. The beginning of this phase may be indicative of a transition from more humid and warmer to drier and, at least temperate, palaeoclimatic conditions in the northwestern Patagonia region.