FIRST PALYNOLOGICAL DATA FROM LOMA DE LOS PIOJOS FORMATION SUGGESTING A REAPPRAISAL OF ITS AGE

DI PASQUO MERCEDES; JUAN PABLO MILANA

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Congreso; Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

A first palynologic study of the Loma de los Piojos Formation at the homonymous locality (5 km north of Jachal city) was carried out. The Guandacol Formation (late Serpukhovian?early Pennsylvanian) erosively overlies Lower Devonian sandstones and shales of the Talacasto Formation. To the south and as a stratigraphic wedge, Mississippian deposits of the Loma de los Piojos Formation crop out, from which floral remains were studied and named as Frenguellia eximia-Nothorhacopteris kellaybelenensis-Cordaicarpus cesarii zone (Tomiodendron, Nothorhacopteris kellaybelenensis, Frenguellia, Bumbudendron, Cordaicarpus), akin to the Serpukhovian sensu lato. This new biozone bears close similarities with the late Serpukhovian?early Pennsylvanian Nothorhacopteris-Botrychiopsis-Ginkgophyllum Zone due to the occurrence of Cordaitean seeds in both, which compare to the appearance of monosaccate pollen grains in the Raistrickia densa-Convolutispora muriornata Palynozone. The fact that Loma de los Piojos is separated by an erosive surface from the overlying Guandacol Formation, was one reason to differentiate the Frenguellia eximia-Nothorhacopteris kellaybelenensis-Cordaicarpus cesarii biozone from the Nothorhacopteris-Botrychiopsis-Ginkgophyllum biozone. This first palynologic analysis is based on four fertile samples (CICYTTP-Pl 2577, 2583, 2579, 2581; Centro de Investigaciones Científicas y Transferencia de Tecnología a la Produccion) obtained from the shales of the Loma de los Piojos Formation. We recovered 45 species of terrestrial plants. Lycophytes dominate in all samples (ca. 50?80%) being Cristatisporites crassilabratus, Cristatisporites inconstans and Cristatisporites stellatus the most frequent species (> 15 %). Cannanoropollis janakii, Cannanoropollis mehtae, Circumplicatipollis plicatus, Crucisaccite smonoletus represent the Cordaitean and Coniferalean that varied in their frequency ca. 25% in the lower two samples to 34% and 7% in the other two. Spore species with biostratigraphic value are present from the lowest stratigraphic sample (Anapiculatisporites concinnus, Cyclogranisporites rinconadensis, Lophotriletes intermedius, Lundbladispora braziliensis, Lundbladispora riobonitensis, Microreticulatisporites punctatus, Velamisporites cortaderensis, Verrucosisporites patelliformis). Most of these species appeared in the late Serpukhovian?Bashkirian subzone A of the Raistrickia densa-Convolutispora muriornata zone, supported by the presence of monosaccate pollen grains. However, two key species of the early?late Bashkirian Subzone B of the Raistrickia densa-Convolutispora muriornata zonewere found in the lowest sample: the spore Spelaeotriletes ybertii and a well-preserved specimen of the striate bisaccate pollen Illinites unicus. Besides, the overlying Guandacol Formation does not show the typical thick dark shales interval corresponding to the maximum flooding zone recognized across the Paganzo basin realm, while Loma de los Piojos Formation does. Therefore, we reinterpret a Bashkirian age for the dark shales of the latter unit and propose their correlation with the widespread postglacial shales that have been dated locally at ca. 320 Ma (Huaco section). The informal floral zone Frenguellia eximia-Nothorhacopteris kellaybelenensis?Cordaicarpus cesarii of Balseiro et al. should be considered as part of the Nothorhacopteris-Botrychiopsis-Ginkgophyllum Zone.*Financial support: CONICET (PIP 0812 2015-2017).