Bivalvian trace fossils of the Labrado Formation (Lower Ordovician) from Zenta Range (Jujuy province): preservational controls of some push-and-pull deposit-feeder traces

ACEÑOLAZA, G. Y ACEÑOLAZA, F.

Ushuaia

Congreso; Quinta Reunión Argentina de Icnología y 3ra Reunión de Icnología del Mercosur; 2007

Laboratorio de Geología Andina, CADIC

Systematic treatment of the trace fossils in the Ordovician strata of NW Argentina started during the 70 ´s. Since then, an important amount of data has been put together with a fairly good number of traces known so far. During the last years, several papers have presented additional taxa and a reevaluation of early known material that provided some clues on the trace fossil diversity within its paleoenvironmental framework. Even though the Lower Paleozoic strata of the South American Central Andean Basin contain an abundant fossil bivalve fauna, there is an almost complete absence of the record of their trace fossils in the literature. In this opportunity, a new bivalve trace fossil association is presented for the Labrado Formation extensively cropping out in Zenta Range (Jujuy Province). The trace fossil association is represented by Lockeia isp., Protovirgularia dichotoma and several dragging forms that are accompanied by Skolithos linearis and the Cruziana rugosa group. The Labrado Formation has been classically described from clear outcrops of the southern part of the Sierra de Zapla, along the Rio Capillas of Jujuy province (type locality). Recent research to the north of this locality recognized a continuity of the Ordovician strata into the Sierra de Zenta. A 1.200 m thick succession of sheet-like and ribbon-like highly bioturbated sandstones and mudstones has been described and assigned to the Zanjón and Labrado formations, and are covered at Laguna Verde by the Caspalá and Lipeón formations (Sixth Ordovician Stage / Hirnantian – Silurian). The trace fossils in the section are better preserved in the less bioturbated intervals, that are characterized by a dominantly heterolithic sucession with the sandy beds bearing the analyzed material. Although characteristic assemblages occur restricted to certain sedimentary facies, the presence of individual trace fossils is mostly controlled by preservational factors such as mean grain size, compactation of sediment while the trace was made, and the degree of lithologic heterogeneity. Weathering of sediments has also played a secondary role on the final preservation of the association, with traces on sandstone displaying a better preservational status. Description and analysis of the ichnological association provides additional data to the early known trace fossil spectrum, helping to characterize environments and diversity of the Ordovician seas of the South American Gondwanan margin.