The Trapezongnatus diprion conodont zone and correlation of an outstandig occurrence of the Cruziana rugosa group (trace fossil) in the Lower Ordovician of Western Gondwana.

HEREDIA, S. Y ACEÑOLAZA, G.

Mendoza

Congreso; Gondwana 12; 2005

Academia Nacional de Ciencias

In addition to graptolites, conodonts are the most important group of fossils for precise, high-resolution correlation of the Series and Stages of the Ordovician System. For this reason, conodonts were used to define the Cambro – Ordovician boundary and, most likely, will be used to define the base of the Middle Ordovician Series and its lower (still to be named) stage.  The biostratigraphic zonation of conodonts has been applied to the stratigraphy the Central Andean Basin, but because of significant gaps there in the conodont succession, new zones may need to be defined in order better understand the Ordovician succession in the western margin of Gondwana. Trace fossils are valuable elements for characterizing sedimentary environments and a varied spectrum of paleoecological patterns. Although trace fossils are not considered as guide fossils, extensive study since the 60´s and 70´s demonstrated the stratigraphic value of trilobite burrows in the Gondwanan realm.  Furthermore, a “Cruziana stratigraphy” was developed, based on the association of several forms of Cruziana in shallow marine sandstone facies of the Lower Paleozoic.  As a result, Cruziana is used today as a reliable field work tool for correlation of the frequently “non-fossiliferous” sandstone facies that is typical of Gondwana. In NW Argentina, the Cruziana rugosa group is remarkably preserved in the upper levels of the Santa Victoria Group where the lack of biostratigraphic data makes very uncertain the correlation of the thick siliciclastic sequences.  This occurrence is in outcrops in the Sierra de Zenta (Jujuy Province) where the uppermost Santa Victoria Group is composed of interbedded sandstones and siltstones with very fine coquinoid carbonate lenses.  These lenses, dominated by fragmentary articulate and inarticulate brachiopod valves, contain conodonts representative of the Trapezognathus diprion Zone.  Trapezognathus diprion (Lindström) has a Baltoscandic provincial affinity and could be referred to the temperate water Baltic Province of the Atlantic Realm. The T. diprion Zone correlates with the uppermost Lower Ordovician Series and to a level that is close to the base of the Middle Ordovician Series and its lower stage.  In the Precordillera of western Argentina, the correlative biostratigraphic level is occupied by   Cooperignathus aranda, which is under consideration as the species on which to define the base of the Middle Ordovician Series. The co-occurrence in the Santa Victoria Group of conodonts of the Trapezognathus diprion Zone with Cruziana rugosa group trace fossils provides an important correlation link between the Cruziana stratigraphy of Gondwana and the zonation of a key group of guide fossils for the Ordovician System.  This calibration of Cruziana stratigraphy will lead to improved understanding of biostratigraphic correlations of Ordovician stratigraphic successions of Gondwana, which so often lack standard guide fossils.