The marine interval of the Tupe Formation in western Paganzo Basin and its implication in the definition of the Carboniferous-Permian boundary in South America.

CISTERNA, G.A., GUTIÉRREZ, P.R., STERREN, A.F., DESJARDINS, P.R. Y BALARINO, L.

Mendoza, Argentina

Congreso; Gondwana 12; 2005

The marine horizons of the Tupe Formation have been interpreted as a marine incursion from the Paleo-Pacific into the western Paganzo basin that suggests a brief, although areally extensive, breach of the Proto-Precordillera. The marine Tupe fauna belongs to the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone (TS Zone), recently considered to be early Permian. A detailed study in different Precordilleran localities (La Herradura Creek, La Delfina Creek, La Ciénaga and Paslean), allowed to understand the evolution of the transgression from the west, as well as the compositional variation of the TS Zone and its relationship with the new palynological and paleofloristic assemblages from the associated horizons. In La Herradura Creek the marine fossiliferous horizons are located in the upper part of the Tupe Formation in a stratigraphical interval about 20 m thick composed of mudstones interbedded with calcareous beds. The fossil assemblage is dominated by brachiopods accompanied by the bivalves Nuculavus levatiformis (Walcott), Phestia sp. aff. P. bellistriata (Stevens), Modiolus (Modiolus) sp., Septimyalina sp., Palaeolima retifera (Shumard), Aviculopecten sp., Streblochondria sp., Schizodus sp., and Pleurophorella? sp. Brachiopods identified are Streptorhynchus inaequiornatus Leanza, Tivertonia jachalensis (Amos), Kochiproductus sp., Costatumulus sp., Coronalosia argentinensis Archbold & Simanauskas, Tupelosia paganzoensis Archbold & Simanauskas, Trigonotreta pericoensis (Leanza), Septosyringothyris sp. aff. S. jaguelensis Lech, Crurithyris? sp. and Orbiculoidea sp. Immediately above of this fauna new palynological data (Converrucosisporites confluens (Archangelsky & Gamerro) Playford & Dino, and Weylandites lucifer (Bharadwaj & Salujha) Foster) suggest an early Permian age (Pakhapites fusus-Vittatina subsaccata (FS) Zone). In La Delfina Creek the marine fossils of the TS Zone are located in the middle part of the section, concentrated in an interval about 20 m thick composed of alternating sandstones and claystones. This fauna is fragmentary and not abundant, and it essentially includes brachiopods, bivalves (Heteropecten? sp., Myofossa sp., Sanguinolites? sp. and Anomalodesmata indet.), gastropods and a few crinoids. Although the brachiopods that characterize the TS Zone have been recognized, the dominant species are Costatumulus sp. and Orbiculoidea sp. and this assemblage could be equivalent to the lower one described from La Herradura Creek. In the lower part of the Tupe Formation in La Delfina Creek, the NBG megafloristic assemblage consists of Fedekurtzia argentina (Kurtz) Archangelsky, Nothoracopteris argentinica (Geinitz) Archangelsky, Rinconadia archangelskyi Vega, Coniferae ind. and Eusphenopteris sp. Other assemblage was obtained from 55 m above the NBG horizons and 60 m below the TS Zone. It is composed of scarce remains that can be referred to Kraeuselcladus sp. cf. K. argentina Archangelsky, which suggests the presence of the Uppermost Carboniferous Interval megafloristic Zone. Also, in the lower 75 m of the Tupe Formation, 10 m below the TS Zone, nine palynological associations referred to the Upper Carboniferous palynological Raistrickia densa-Convolutispora muriornata (DM) Zone have been identified. This transgression has been also recognized in La Ciénaga locality where the first marine deposits are located in the lower part of the Tupe Formation, immediately above coal beds. A coquina horizon composed of large gastropods, which appear to be very close with those from the middle part of the fossiliferous interval in La Herradura Creek, occurs in the middle part of this section. However other diagnostic elements of the TS Zone have not been found yet at La Ciénaga. From the study outlined above, we can suggest that the Tupe Formation in the Precordillera of the San Juan province (western Paganzo basin), encompasses the Latest Carboniferous assemblages (NBG and Interval megafloristic zones; DM Palynological Zone) and the earliest Permian FS Palynological Zone and TS invertebrate Zone. The occurrence of these biostratigraphical units could restrict the Paleo-Pacific transgression to the Latest Carboniferous-Earliest Permian interval.