The marine inteval of 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. AND BALARINO, L.

Mendoza, Argentina

Otro; Gondwana 12 "Geological and Biological Heritage of Gondwana"; 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 Tupe fauna has previously been known as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone (TS Zone), recently considered to be early Permian. A detailed study of 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 variations of the TS Zone and its relationships with the new palynological and paleofloristic assemblages from the associated horizons. At La Herradura Creek the marine fossiliferous horizons are located in the upper part of the Tupe Formation, within stratigraphical interval about 20 metres thick composed 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 (Shaumard), Aviculopecten sp., Streblochondria sp., Schizodus sp., and Pleurophorella? sp.). The identified brachiopods 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) have suggested an early Permian age (Pakhapites fusus-Vittatina subsaccata (FS) Zone). In the La Delfina Creek marine fossils of the TS Zone are located in the middle part of the section, concentrated in a 20 m thick interval alternatin sandstones and claystones. This fauna is fragmentary and scarce, and it is essentially includes brachiopods, bivalves (Heteropecten? sp., Myofossa sp., Sanguinolites? sp., Anomalodesmata indet.), gastropods, and a few crinoids. Althought the brachiopod that characterize the TS Zone have been recognized, the dominant species are Costatumulus sp. and Orbiculoidea sp.; this assemblage could be equivalent to the lower one descrided from La Herradura Creek. In the lower part of the Tupe Formation at La Delfina Creek, the NBG megafloristic assemblage consits of Fedekurtzia argentina (Kurtz) Archangelsky, Nothoracopteris argentinica (Geinitz) Archangelsky, Rinconadia archangelskyi Vega, Coniferae indeterminate, and Eusphenopteris sp. Other assemblages was obtained from 55 m above the NBG horizonts 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 suggest 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 assaciations referred to the Upper Carboniferous palynological Raistrickia densa-Convolutispora muriornata (DM) Zone have been identified. The transgression has been also recognized in the La Ciénaga locality, where the firts marine deposits are located in the lower part of the Tupe Formation, inmmediatly above coal beds. A coquina horizon composed of large gastropods, which appears to be very close with those from the middle part of the fossiliferos interval at La Herradura Creek, occurs in the middle part of this section. However other diagnostic elements of the TS fauna have not been found yet at La Cienaga. From the study outline above we suggest tha 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 biostrigraphical units could restrict the Paleo-Pacific transgression of the latest Carboniferous-earlierst Permian interval.