Nereites ichnofacies in the Río Seco de los Castaños Formation, Mendoza, Argentina: age, facies and trace-fossil content

PAZOS, P.J.; HEREDIA, A.; CINGOLANI, C.

Santa Rosa

Simposio; II SLIC; 2013

Universidad Nacional de La Pampa

     The aim of this communication is to analyze different aspects related to an abundant ichnofauna recorded in the Río Seco de los Castaños Formation in the Bloque de San Rafael, Mendoza Province, Argentina. These aspects include: i) the discussion of the age of the unit from both biostratigraphic and absolute dating methods; ii) the analysis of the facies scheme previously suggested for the unit; iii) the documentation of the trace-fossil content; and iv) the analysis of the impact of the re-consideration of the age of the unit in ichnostratigraphic schemes.     The units that traditionally were denominated the Lower Paleozoic "flysch" facies of the central western Argentina were intensely folded, faulted and low-grade metamorphosed during the latest (?) Devonian, Chanic Orogeny, and are well distinguishable, in the field, from the Pennsylvanian units, not metamorphosed. However, it is always a reason of controversy to assign some of them, in certain areas, to the Silurian or to the Devonian. In our case, the unit has been assigned to the Upper Silurian-lowest Devonian taking into account that palynomorphs and plant remains (primitive Lycophyta) suggest this age, but the unit is intruded by rocks of Lower Devonian age, indicating the minimum possible age for the unit. The paleontological evidence is controversial: while palynomorphs suggest an Upper Silurian age, plant remains and a lost coral remain point to a Devonian one. An Upper Silurian age is the most plausible; because the "Devonian remains" deserve further study.     The flysch units were traditionally interpreted as deep-water turbidites, particularly the Devonian units, but later, a particular case of muddy platform-deltaic system developed in a foreland basin was suggested. The studied formation was interpreted as a deltaic system deepening from the east to the west. In this facies scheme the studied area is located to the west, where the Nereites-Mermia ichnofacies was previously suggested contrasting with the Cruziana in the eastern outcrops. Abundant organic matter (coaly levels) with plant remains decreases form the east (Atuel Cañon) to the west (N°144 road) but also turbidite-like deposits decrease in thickness and grain-size.     The ichnofauna documented in the N° 144 road locality contains dominantly Nereites irregularis Schafhäutl, Helminthopsis and less abundant arthropod trackways and grazing traces. Microbial mats are very abundant but clearly not related to Nereites. At the Atuel Cañon locality Nereites is also the most abundant ichnofossil but it contains more than one ichnospecies: Nereites irregularis and one beautifully preserved specimen of Nereites cambrensis Murchinson, the type ichnospecies of the ichnogenus. Other specimens could be assignable to the questionable Nereites delpeyi Borrouilh, a dubious valid ichnospecies that probably is a junior synonym of Nereites missouriensis Weller. Other well preserved trace fossil is Dictyodora in upper relief expressions. This ichnogenus contains several ichnospecies, with an apparently ichnostratigraphic value. The more abundant ichnospecies here are Dictyodora scotica Mc´Coy, Dictyodora isp. and Dictyodora ?tenuis Mc´Coy only one specimen of cf. Taenidium isp. was observed. Microbial mats are also abundant in levels containing Dictyodora, and the co-occurrence of Nereites and Dictyodora is not very frequent. Dictyodora is less abundant than Nereites.     The abundance of organic matter, plants remains, coaly levels and microbial mats are in agreement with a relatively shallow depositional setting in a prograding deltaic system. This ichnofauna is very important because of the uncommon record, for the Upper Silurian of South America, containing the classical components of the Nereites ichnofacies for the Palaezoic, particularly in those records not attributed to basinal depositional settings.