The conodont-graptolite Biostratigraphic scheme for the Lower Ordovician of the Eastern Cordillera, NW Argentina.

ORTEGA, GLADYS; ALBANESI, GUILLERMO L.; ZEBALLO, FERNANDO J.; VOLDMAN, GUSTAVO G.; GIULIANO, MARÍA EUGENIA

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

International Paleontological Association-CONICET

The records of conodonts and graptolites in the Lower Palaeozoic of the Eastern Cordillera, Saltaand Jujuy provinces are the best known from northwestern Argentine basins, and are hereinorganized to follow a chronostratigraphic scheme for the various recognized faunas. Recentstudies 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 asedimentary cycle controlled by tectonoeustatic factors of third and fourth order represented bythe Santa Victoria Group (Upper Cambrian - Middle Ordovician), which is bounded by two majorunconformities, named the Iruya at the base that separates the Meson Group, and the Ocloya, whichseparates it from the Upper Ordovician glacigenic deposits. In the complex palaeoenvironments ofthe Eastern Cordillera, the conodont and graptolite faunas evolved and their records are utilizedfor defining the following biostratigraphic units. The Tremadocian conodont zones are named inascending order, from the chronostratigraphic Cambrian/Ordovician boundary, the Iapetognathus,Cordylodus angulatus, Paltodus pristinus, Paltodus deltifer, and the Acodus apex zones. Accordingly, theFloian is characterized by the Acodus deltatus s.l. zone, followed by the Trapezognathus? primitivusand Gothodus andinus zones. Similarly, the defined graptolite zones for the Tremadocian are theRhabdinopora 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 thecombined records of conodonts and graptolites in critical intervals, precise correlation ties for allnamed units are verified. In particular, the Tremadocian/Floian boundary coincides with the baseof the T. phyllograptoides Zone and is close to the base of the A. deltatus s.l. Zone. The global standardschemes adopted in our study allow direct links withthe contemporary faunas of different continentswith assemblages of the northwestern Argentine basins, and their distinctive palaeobiogeographicand evolutionary patterns in the Lower Ordovician.