An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications
Juan L. Benedetto 1 and Norma Cech 1
1 CONICET. Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba. Av. Vélez Sarsfield 299, 5000 Córdoba, Argentina. [email protected]
Abstract. Rhynchonelliformean brachiopods from the Arenig outer-ramp carbonate facies exposed at the Guandacol-Gualcamayo depocenter (northern Precordillera basin) include the new species Orthidium guandacolensis , Taffia ( Chaloupskia ) minima and Leptella ( Petroria ) ventrocristata . Associated taxa are Archaeorthis sp., Ffynnonia sp. and Tritoechia ( Tritoechia ) sp. The overlying basinal black shales (Gualcamayo Formation) have yielded Pelonomia ? sp. On the basis of the age of Ffynnonia occurrences in western Gondwana it is suggested that this genus migrated from the Famatina back-arc basin to the open platform waters of the northern Precordillera basin and then to shallower water settings of the carbonate ramp located to the south (in present coordinates). The studied assemblage is characterized by low values of richness, diversity and equitability. The nature of substrate is interpreted as the main controlling factor in the distribution of brachiopods through the carbonate ramp. The muddy substrates prevailing in the outer-ramp were colonized mainly by semi-infaunal (tritoechiids) and subsidiarily by liberosessile brachiopods (plectambonitoids), other suspension feeders are rare. In the ramp-to-basin and basinal black shales the decreasing oxygen level was probably the main barrier to permanent colonization of rhynchonelliformean brachiopods.
Key words. Lower Ordovician. Argentine Precordillera. Brachiopods. Paleoecology.
Ameghiniana, Jan./Mar. 2006, vol.43, no.1, p.27-44.