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

VERDECCHIA, S.O., BALDO, E. BENEDETTO, J.L. Y BORGHI, P.A.

AMEGHINIANA

Asociación Paleontológica Argentina

Lugar: Buenos Aires; Año: 2007 vol. 44 p. 493 - 498

0002-7014

The monospecific assemblage from the La Cébila Formation is inadequate for paleobiogeographical comparisons. However, the record of Ffynnonia is significant as this genus is always associated with key taxa of the Celtic Realm. Ffynnonia has been recorded in the classical Celtic locality of Anglesey (Wales) and in the Famatina Range, whose brachiopods form a statistically significant cluster with other Celtic localities. In the Precordillera basin, species of Ffynnonia are also associated with distinctive Celtic Realm forms (i.e. Inversella (Reinversella), Rugostrophia, Skenidioides). The Celtic realm is a paleobiogeographic unit developed mainly at mid- to high-latitudes, along the northwest Gondwana continental margins and around Iapetus Ocean-related terranes as Avalonia, Famatina, and Precordillera . The presence of Ffynnonia in the La Cébila Formation suggests a faunal connection between the Sierras Pampeanas seaway and the Famatina basin located to the west. The shallow-water marine succession of the La Cébila Formation would record deposition near the outermost Gondwana foreland. This unit developed probably along an incipiently eroded mountain belt uplifted during the early to mid Cambrian Pampean compressive deformation. Between this eastern (in present coordinates) shoreline and the rapidly subsiding Famatina back-arc basin  located to the west, an over 150 km wide marine platform system must have developed. Astini et al. (2003), based on detrital zircon data (c. 480 Ma), proposed a tentative correlation between the La Cébila Formation and the low-grade metamorphic La Aguadita Formation exposed along the eastern slope of the Famatina Range. These authors interpreted this immature sandy unit as a relict of a clastic wedge developed in a back-arc setting during the rapid exhumation of the Famatinian granitoids, so that it should be younger than the volcanosedimentary Suri and Molles formations. However, the discovery of Ffynnonia –a typical brachiopod of the Suri Formation –in the La Cébila Formation favour the correlation of these units. The profound eastward flooding of the Gondwana plate may have been associated with a major global sea-level rise, probably the ‘Evae Drowning Event’.