THE CONODONT-GRAPTOLITE BIOSTRATIGRAPHIC SCHEME FOR THE LOWER ORDOVICIAN OF THE EASTERN CORDILLERA, NW ARGENTINA

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

Mendoza

Congreso; 4 International Palaeontological Congress; 2014

International Palaeontological Association

The records of conodonts and graptolites in the Lower Palaeozoic of the Eastern Cordillera, Salta and Jujuy provinces are the best known from northwestern Argentine basins, and are herein organized to follow a chronostratigraphic scheme for the various recognized faunas. Recent studies on the historical geology of the Eastern Cordillera are based on sequence stratigraphy within sedimentologic and biostratigraphic frameworks. This has allowed us to verify the actual extent of the different facies, areally restricted and bounded by discontinuities and incised valleys. In this morphostructural unit, the major conodont and graptolite records are grouped into a sedimentary cycle controlled by tectonoeustatic factors of third and fourth order represented by the Santa Victoria Group (Upper Cambrian - Middle Ordovician), which is bounded by two major unconformities, named the Iruya at the base that separates the Meson Group, and the Ocloya, which separates it from the Upper Ordovician glacigenic deposits. In the complex palaeoenvironments of the Eastern Cordillera, the conodont and graptolite faunas evolved and their records are utilized for defining the following biostratigraphic units. The Tremadocian conodont zones are named in ascending order, from the chronostratigraphic Cambrian/Ordovician boundary, the Iapetognathus, Cordylodus angulatus, Paltodus pristinus, Paltodus deltifer, and the Acodus apex zones. Accordingly, the Floian is characterized by the Acodus deltatus s.l. zone, followed by the Trapezognathus? primitivus and Gothodus andinus zones. Similarly, the defined graptolite zones for the Tremadocian are the Rhabdinopora flabelliformis parabola, Anisograptus matanensis, Rhabdinopora flabelliformis anglica, Adelograptus, Bryograptus kjerulfi, Aorograptus victoriae, Kiaerograptus supremus, Araneograptus murrayi, and Hunnegraptus copiosus zones. The Floian is represented by the Tetragraptus phyllograptoides, Tetragraptus akzharensis, Baltograptus cf. deflexus, and Didymograptellus bifidus zones. Using the combined records of conodonts and graptolites in critical intervals, precise correlation ties for all named units are verified. In particular, the Tremadocian/Floian boundary coincides with the base of the T. phyllograptoides Zone and is close to the base of the A. deltatus s.l. Zone. The global standard schemes adopted in our study allow direct links withthe contemporary faunas of different continents with assemblages of the northwestern Argentine basins, and their distinctive palaeobiogeographic and evolutionary patterns in the Lower Ordovician.