First platyspermic seeds in the Jejenes Formation, Quebrada Grande, western Argentina dated with palynology and 206Pb/238U volcanic zircons

MILANA JUAN PABLO; CHEMALE JUNIOR FARID; DI PASQUO, MARÍA DE LAS MERCEDES; KNELLER BENJAMIN; VALDEZ BUSO VICTORIA; PAIM PAULO SÉRGIO GOMES; FALLGATTER CLAUS

Salvador de Bahia

Congreso; XIV International Palynological Congress and X International Organisation of Palaeobotany Conference; 2016

IOP-IFPS-ALPP

Platyspermic seeds and palynomorphs are found together in a tuffaceous deposit of the Jejenes Formation (Bashkirian) at Quebrada Grande, Paganzo Basin, Precordillera Argentina. The Jejenes Formation represents the glacial to postglacial fill of a palaeovalley carved in the Eastern Precordillera. The tuffaceous level bearing these fossils yielded first-cycle volcanic zircons that were analysed by SHRIMP. An absolute age of 321.3±5.3 Ma confirms a Bashkirian age. The platyspermic seeds Cordaicarpus cesariae and Cordaicarpus famatinensis were recorded for the first time in this location associated to undertermined stems of lycophytes and gymnosperms. It is known that platyspermic seeds are related to cordaiteans and primitive coniferaleans gymnosperms globally known since the Serpukhovian (late Mississippian). In Argentina and Bolivia they are mainly documented in Bashkirian assemblages of the NBG Zone from western Argentina and Bolivia. This age is confirmed herein by the novel isotopic calibration. Palynoassemblages are characterized by poorly preserved trilete spores and monosaccate pollen grains along with a high amount of terrestrial phytoclasts. Main species indicating the SubZone A of Raistrickia densa-Convolutispora muriornata Zone (DMa SZ) are Vallatisporites ciliaris, Cristatisporites rollerii, C. stellatus, C. chacoparanensis, C. inconstans and monosaccate pollen grains such as Circumplicatipollis plicatus. Below the DMa SZ it is defined the Reticulatisporites magnidictyus-Verrucosisporites quassigobbetti (MQ) Interval Zone in the Cortaderas Formation akin to the late Visean ? early Serpukhovian. The absence of monosaccate pollen grains in this zone and the presence of sparse records in northern Bolivia supports an age not older than late Serpukhovian for their inception in the DMa SZ of western Argentina. The sedimentary succession at Quebrada Grande includes five stages. This tuffaceous and fossiliferous deposit is part of the Upper Stage III, which is interpreted as the main interglacial transgression that occurred after the Serpukhovian-Bashkirian glacial event here documented as Stages I and II, and recorded elsewhere in Paganzo Basin. The palynofacies analysis of the productive samples in Stage IV (turbidites deposits) yielded palynomorphs with degraded walls due to pyritization that support the interpretation of marine settings linked to the transgressive event. A new local glacial event (Stage V) is recorded in the upper section of the area with the presence of erratic boulders and dropstones with Precambrian affinity suggesting a possible ice advance and subsequent retreat from Sierras Pampeanas located to the East.