The age of the unroofing, paleoenvironmental evolution and climate framework prior to the lower Choiyoi volcanism in the Permian of the San Rafael Block, Mendoza Argentina

PAZOS, P.; REY, F.; MARSICANO, CLAUDIA; OTTONE, EDUARDO; DE LA FUENTE, M.

Buzios

Congreso; 14 Gondwana Conference; 2011

The Embalse los Reyunos Formation is a classical Permian unit of the San Rafael Block, recently assigned to the Cisuralian by SHRIMP U-Pb zircón geochronology. The unit is formally divided in three members being the lower one mainly silicliclastic with volcaniclastic subordinated deposits, a middle one entirely siliciclastic and aeolian in origin, and an upper one dominantly volcanic and volcaniclastic. The last one is the Toba Vieja Gorda Member the record the extensive lower Choiyoi volcanism. The lower member (Psephitic Member) is an unit that received secondary attention in order to elucidate the tectonic evolution of the block, mainly due to the regional extension of the volcanism of the Toba Vieja Gorda Member, intensively studied in the last decades. However, the sedimentary evolution of the lower member is crucial to understand the tectonism and climate framework during the uplift of the San Rafael Block. This member is composed of coarsening upward conglomerates and aglomerates deposited overlying ignimbrite and volcaniclastic deposits. They record the redeposition and canivalization of the youngest volcanic deposits in the basin. The psfites composition, exhibit a clear evidence of unroofing in the región being dominantly composed of Carboniferous, Devonian and Ordovician lithologies progressively, due to the uplift of the San Rafael Block. Ordoviacian volcanic lithologies are easily confounded with Devonian lithoclasts but they clearly are dominant in the upper part of the succession. From a palenvironmental approach the succession evidence the passage from a multiepisodic record of proximal alluvial fan facies that pass upward to middle to distal parts. Interestingly, in the distal sand aeolian dunes tetrapods ichnfossils are been documented in both dunes and interdune deposits. Fluvial and aeolian interaction as well growing strata have been documented as an indisputable record of compression during all over the interval. Centimeter-length inverse faults documented in crossbedding are also frequent. A sharp surface separate the lower member from the middle one named “Areniscas Atigradas” which is completely aeolian in origin and was intensively studied due to the richness in uranium mineralization, that give the name to the lithostratigraphic unit. These deposits are also very rich in ichnofossils, both vertebrates and invertebrates that suggests not extreme climates, regardless the exclusive eolian origin of the deposits. Remnants of a mega-size dunes are observed overlying the middle member but underlying the Toba Vieja Gorda Member. This aeolian interval knows as “Toscal Rosado Sandstones” shows a dramatic change in the basin with the first intercalation of mud flows evidencing aridization. A relict paleolandscape is covered by the ignimbrites that characterize the lower choiyoi in central western Argentina. The ignimbritic flow recently dated correspond to the lower member and in fact is the most precise age of the unroofing rather the choiyoi volcanism. The dominance of SW paleowinds also document the reduction in the paleorelief prior to the intense Choiyoi magmatism