New insights in the Lower to Middle Triassic Floras of Argentina (Southwestern Gondwana)

COTUREL, ELIANA P.; BODNAR, JOSEFINA; MOREL, EDUARDO M.

Paris

Congreso; 5th International Paleontological Congress; 2018

Argentinean Triassic Floras are known since more than a century, and evolution models,biostratigraphic schemes and biogeographic provinces were proposed by several authors in the last three decades. Nevertheless, in most recent years, new methodologies in the stratigra- phy and radiometric techniques oering more accurate ages have been quivered the previous palaeoFloristic models. Here we revised the biostratigraphy and the palaeoFlora evolution of the Lower to Middle Triassic from Argentina under the light of the recent stratigraphic and chronologic advances. We focus on Sorocayense Group (San Juan province), Puesto Viejo Group (Mendoza province) and Los Menucos Group (Ro Negro province). In previous schemes, the appearance of the Dicroidium Flora was regarded to the upper part of the Barrealian Stage (Anisian) which would have succeeded the "Pleuromeia Flora" recorded in the Quebrada de los Fosiles Formation (Puesto Viejo Group), the last being considered as Lower Triassic. The diverse plant assemblage (interpreted as forests with various species of Zuberia, cycads and voltzialeans) from Los Menucos Group was rst correlated to the Early to Middle Triassic Dicroidium zuberi Oppel Zone from Australia, but afterwards a radiometric age pointed it as early Late Triassic. Then again, recent radiometric analyses indicate that the Vera Formation of Los Menucos Group was deposited during the earliest Triassic. On the other hand, new fossil ndings in Quebrada de los Fosiles Formation led to interpret it as early Middle Triassic, and to infer that its sedimentation environment comprised lowlands with pleuromeid accompanied with other lycopodiopsids and sphenopsids, and uplands with corystosperms and other gymnosperms. Finally, the Barreal Formation (Anisian, Sorocayense Group) contains a preserved forest dominated by Zuberia, with a remarkably diverse Dipteridaceae fern assemblage in its understory. The evolution of Argentinian Floras is now more compatible with the Australian biostratigraphic scheme, with Zuberia in the Early Triassic and Dicroidium odontopteroides since the early Middle Triassic. Both lycopodiopsids and ferns are known as pioneer species in the ecosystem succession; their presence as lowland-dominants in the Anisian is coeval with the last stages of the Gondwanic magmatism which aected this zone. Ladinian and Late Triassic deposits show the best development of the Dicroidium Flora in the lowlands. The case of the earliest Triassic Los Menucos ora poses new questions. This is a Zuberia Flora in the lowlands suering a nearby and active volcanic system, hardly interpretable as a refugium like those recorded during stress events. One explanation is that this Zuberia Flora colonized the lowlands under stress, but also, we could be in front of the remnants of a Permianora, resisting the strikes of the volcanic stress, preparing to take refuge in the uplands to grow stronger and go back to the lowlands in the Middle Triassic.