Mammalian biostratigraphy and biochronology at Gran Barranca: the standadr reference section for the continental middle Cenozoic of South America.

CARLINI A.A.; MADDEN, R.H.; VUCETICH, M. G.; BOND, M.; LÓPEZ, G.; REGUERO, M.; SCARANO A.C.

La Plata. BA.

Congreso; XVI Congreso Geológico Argentino; 2005

Asociación Geológica Argentina

The Sarmiento Formation at Gran Barranca is a 285m thick composite section containing over 49 different fossil-bearing levels. Based un the stratigraphic occurrences of standard index and guide taxa, ther4 are at least six different faunal zones. The occurrences of Notostylops, Astraponotus, Pyrotherium, Colpodon and Astrapothericulus together with other mammalian guide taxa permit recognition of the stan and sequence of middle Cenozoic South American Land-Mammal Ages at Gran Barranca. In addition, clea stratigraphic relationships between fossil mammals, dated tuffs and lavas, magnetic polarity intervals, and nconformities of constrained temporal duration, permit establishing a refined biochronology for assemblages and biozones. Starting from the base of the composite section and working upward, the Barrancan is essentially as Cifelli established based on Simpson´s collections and stratigraphy, and its upper temporal limit can now be established. An assemblage of intermediate taxonomic composition at the top of the Barrancan interfral, if amenable to age control, may warrant recognition. The Mustersan is represented by two levels that pre directly dated and also bracketed by dates from overlying and underlying tuffs. An unconformity sep crates the Mustersan from overlying fossil levels. Knowledge of the "Astraponoteen plus Superieur" of Arhieghino is significantly improved and stratigraphy suggests the age of the base of a Tinguirirican Lanl Mammal Age may be more precisely constrained at Gran Barranca. A rich pre-Deseadan and postTTinguirirican level with guide taxa of both faunal units and with primitive rodents must be accomodated. The Deseadan has been one of the least well-known parts of the section at Gran Barranca. Deseadan levels can be precisely located with respect to the basalt complex and associated unconformities. The richColhuehuapian levels are the best constrained temporally. The topmost fossil levels at Gran Barran- ca yield a Pinturan assemblage older than that of the type area. Biostratigraphy and biochronology serve many useful purposes. By revealing the history of life in sequences of evolutionary and environmental events, it provides the factual basis for arguments about coincidence and causality. We recognize three additional purposes, 1) to enable correlation in the middle Cenozoic of South America by establishing the empirical facts at the standard reference sequence, andt2) to determine the limits of temporal resolution at Gran Barranca, and 3) to demonstrate the completeness of the fossil record at Gran Barranca. The basic sequence was first revealed by the Ameghinos. The discovery of a succession of faunal levels at Gran Barranca in 1899 was first announced in publication in 1901. For ease of communication, Ameghino resorted to conventional practice and selected a single conspicuous taxon to serve as standard bearer for each level. The first of Ameghino´s standard-bearers to concern us here is Asrapothericulus, a rather small taxon among the unusual and distinctive astrapotheres. These large herbivorous are conspicuous in surface collections made early in this century, and as many as three taxi may occur among material in museum collectios from the Colhuehuapian at the Gran Barranca. Originally discovered and generally associated with the distinctive fauna from the Pinturas Fm in the area of the source of Río Deseado, Astrapothericulus or a closely related taxon is common in the type Colhuehuapian at Gran Barranca. Studies presently underway suggest the small taxon from the Colluehuapian at GB may not be Astrapothericulus, but a similar-sized small astrapotheriid. Another distikctive notoungulate Colpodon is the standard-bearer for the Colhuehuapian, and remains of Colpodon distinctus are common at Gran Barranca. The Colhue-Huapi Mbr may have been the first mammal levels collected at Gran Barranca and it is certainly the most richly fossiliferous.