Abstract

GOMEZ, Fernando J.  and  ASTINI, Ricardo A.. Renalciform cyanobacterias in the Middle Cambrian of the argentine Precordillera: morphology, stratigraphic position and paleoenvironmental meaning. Ameghiniana [online]. 2005, vol.42, n.1, pp.221-232. ISSN 1851-8044.

Renalcis group cyanobacteria are recorded in La Laja Formation, the lowermost unit of the Cambro-Ordovician passive-margin carbonate succession of the Argentine Precordillera, considered to be an exotic terrane derived from Laurentia. This is the first finding of this group in lower Paleozoic deposits of South America. Two distinct meter-scale beds within the carbonate section in the uppermost Las Torres Member (upper Middle Cambrian) in the La Laja Formation yield abundant Renalcis , appearing as dispersed aggregates with dominant saccate and chambered morphologies within a micrite matrix. These beds can be considered as biostromes and the internal fabric and scarcity of broken and reworked forms suggest that they are essentially in situ deposits. According to the textural and geometric features, these bodies would classify as "cluster reefs" and may be interpreted as formed in relatively quiet subtidal environments. The finding of Renalcis in these deposits emphasizes the differences in between the Cambrian of the Precordillera and the coeval surrounding successions in Gondwana.

Keywords : Renalcis; Cyanobacteria; Middle Cambrian; La Laja Formation; Argentine Precordillera.