Biostratigraphy of the Santa Rosita Formation (Furongian-Tremadocian) in its type area, Eastern Cordillera, NW Argentina

VOLDMAN, G.G.; ALBANESI, G.L.; ORTEGA, G.; MONALDI, C.R; ZEBALLO, F.J.; GIULIANO, M.E.

Viena

Encuentro; EGU General Assembly 2012; 2012

European Geological Union

The Cambrian - Lower Ordovician stratigraphic units from the Eastern Cordillera were originally defined in theSanta Victoria Range, Salta Province, NW Argentina. At the southern margin of the Santa Victoria River, nearthe homonymous locality, the Santa Rosita Formation is conformably overlain by the Acoite Formation. Thesestratigraphic units are dated by means of conodont, graptolite and trilobite biostratigraphy in the type sections.The upper interval (ca. 700 m) of the Santa Rosita Formation consists of sandstones interbedded with purplishto grayish shales and occasional coquinas, which were sampled for microfossils. 15 carbonate samples (35 kg)were processed following standard laboratory techniques for conodont recovery. A number of species from thegenera Acanthodus, Acodus, Decoriconus, Drepanodus, Drepanoistodus, Iapetognathus, Kallidontus, Paltodus,Utahconus, Teridontus and the protoconodont Phakelodus were recovered from 5 productive samples. The conodontelements exhibit a CAI 3 and correspond to the Paltodus deltifer deltifer Subzone of the P. deltifer Zone(middle Tremadocian, Tr2). Pelites of the Acoite Formation, at 300 m from its base, bear Araneograptus murrayiand Thysanopyge sp., whose ranges span the Tremadocian – Floian boundary. Conodonts and graptolites werealso yielded by outcrops of the Santa Rosita Formation at the Nazareno area, 30 km to the south of the SantaVictoria type locality. The conodont associations were recorded from calcareous levels of the Alfarcito and Rupascamembers, including Drepanodus arcuatus, Drepanoistodus chucaleznensis, Teridontus gallicus, Utahconushumahuacensis, Acanthodus sp. and Utahconus sp. They also integrate the eponymous subspecies from the Paltodusdeltifer pristinus and P. d. deltifer subzones of the P. deltifer Zone (middle Tremadocian, Tr2). The SantaRosita Formation correlates with a thick heterolithic succession at the Zenta Range, 120 km to the southwest of theSanta Victoria type area. This mostly siliciclastic succession intercalates calcareous concretions, coquinas and calcarenitesbearing a significant conodont and graptolite fauna. Exposures of gray shales and subordinated coquinasnear Abra de Santa Ana, produced a low diversity conodont fauna that incorporates Acodus apex, Drepanoistoduschucaleznensis, and Drepanoistodus costatus, which are referred to the lower part of the Acodus deltatus –Paroistodus proteus Zone. The Araneograptus murrayi Zone is suggested by a graptolite assemblage that includesA. murrayi, Kiaerograptus cf. K. supremus, and Kiaerograptus? sp., which was collected from strata overlying theconodont-bearing levels. These conodont and graptolite faunas indicate a late Tremadocian age (Tr3). Another localityon the road to Abra de Zenta, west of Santa Ana, contains Hunnegraptus cf. H. novus, Hunnegraptus spp. andParadelograptus sp., allowing for the recognition of the Hunnegraptus copiosus Zone that typifies the uppermostTremadocian. The studied assemblages include elements of wide intercontinental distribution as well as endemicforms from the Central Andean Basin, characterizing a faunal province with a particular signature.