Review of Tremadocian (Lower Ordovician) biostratigraphy from northwestern Argentine basins

ORTEGA G., ALBANESI G.L., ZEBALLO F.J.

Brisbane

Congreso; 34 International Geological Congress; 2012

IUGS

The biostratigraphy of the Tremadocian Stage is well documented in the Puna and Cordillera Oriental basins of northwestern Argentina. Index fossils are recorded in the Santa Rosita Formation and equivalent units representing estuarine to shelf and slope facies. Nine graptolite zones are documented; Rhabdinopora flabelliformis parabola, Anisograptus matanensis, R. f. anglica, Adelograptus, Bryograptus kjerulfi, Aorograptus victoriae, Kiaerograptus supremus, Araneograptus murrayi and Hunnegraptus copiosus zones. The basal R. praeparabola Zone of the global scheme was still not identified. The trilobite units are represented by the upper part of the Jujuyaspis keideli Zone, the Kainella spp. and Bienvilla tetragonalis zones, an unnamed interval, the Notopeltis orthometopa Zone, and the lower Thysanopyge fauna. The Iapetognathus, Cordylodus angulatus, Paltodus deltifer (with respective subzones) and most of the Acodus deltatus/Paroistodus proteus conodont zones, were recorded throughout Tremadocian strata. The major information corresponds to the Cordillera Oriental, where the conodont fauna includes a mixture of typical forms from the Baltoscandian and Laurentian provinces, as well as endemic species. Graptolites dispersed worldwide during the Early Tremadocian, while the Central Andean Basin incorporated Baltic taxa in the Middle and Late Tremadocian. An important number of trilobite taxa are endemic of the basin, although during the Early Tremadocian olenid trilobites dominated the faunas. Since the Middle Tremadocian the asaphids became more abundant instead. Some conodonts (e.g., Hirsutodontus), trilobites like Onychopyge and the gastropod Peelerophon oehlerti suggest an oceanic corridor linking low latitude regions (Australia and Laurentia) with western Gondwana during the late Cambrian and Tremadocian times.