The first shelly faunas from metamorphic rocks of the Sierras Pampeanas (La Cébila Formation, Sierra de Ambato, Argentina): age and paleogeographic implications

SEBASTIÁN OSVALDO VERDECCHIA; EDGARDO BALDO; LUIS BENEDETTO; PABLO BORGHI

AMEGHINIANA

ASOCIACION PALEONTOLOGICA ARGENTINA

Año: 2007 vol. 44 p. 493 - 498

0002-7014

In Argentina, Cambro-Ordovician shelly faunas are confined to the Precordillera, Famatina and Northwestern basins within the present-day Andean belt. Outside the Andes, fossiliferous rocks are known only from the Tandilia System, in the Buenos Aires province. These rocks, however, though rich in Ordovician trace fossils, are barren in body fossils. The Sierras Pampeanas of central Argentina are a series of mountainous ridges bounded by reverse faults, uplifted during the Tertiary Andean Orogeny in response to the flat-subduction of the Nazca plate at this latitude (Jordan et al., 1983). This basement complex is largely composed of low- to high-grade polymetamorphic rocks and extensive granitoid plutons whose Mesoproterozoic to early Paleozoic ages were based essentially on U-Pb isotopic data (Pankhurst and Rapela, 1998 and references therein). Here we provide the first report of a shelly fauna from metamorphic rocks of the Sierras Pampeanas, consisting of numerous, relatively well-preserved rhynchonelliformean brachiopod valves, and a few trilobite and bivalve remains. This fauna allows to better constrain the age of the La Cébila Formation and sheds new light on the lower Paleozoic paleogeography and depositional history of the Gondwana margin. Moreover, this discovery opens new and exciting perspectives for searching fossils in the metamorphic rocks formerly considered of Neoproterozoic-Cambrian age exposed in large areas between 27° S and 33° S, through Tucumán, Catamarca, La Rioja, Córdoba and San Luis provinces. Specimens illustrated in this paper are housed in the paleontological collection of Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina (prefix CEGHUNC).