OXFORDIAN MICROBIAL LAMINITES FROM LA MANGA FORMATION: REMARKABLE NANOBACTERIA PRESERVATION, NEUQUEN BASIN, ARGENTINA

PALMA, RICARDO M.; KIETZMANN, DIEGO A.; BRESSAN, GRACIELA S.; COMERIO, MARCOS

Mendoza

Congreso; IV International Palaeontological Congress; 2014

International Palaeontological Association

The Callovian-Oxfordian of the Neuquén Basin (Argentina) is characterized by an extensivemarine carbonate system (La Manga Formation) with a predominance of shallow and middle rampdeposits. The intertidal-supratidal deposits are interesting because of the abundance of stromatolitebeds developed during the middle Oxfordian (Perisphinctes-Araucanites zone). A detailed study ofthese beds through analysis of growth fabric, laminae variations and exceptional preservation ofa rod-like bacteria community has been done in the localities of Arroyo Los Blancos, Arroyo LaManga and Yeseras Grandes (southern Mendoza province). These facies exhibit planar and crincklelamination often disrupted by subaerial exposure surfaces such as sheet-cracks, mud-cracks,and fenestral pores. Different lines of evidence suggest that these stromatolites are typical of lowenergy upper intertidal to lower supratidal environments. Extensive SEM examinations of polishedstromatolite samples were carried out showing bacteria filaments with subspherical microstructureand framboidal pyrite aggregates. The microbial filaments (rod-like bacteria) consist of a networkof irregular distributed filaments, which range from 150 nm to an uncommon 640 nm in length,meanwhile diameters range from 54 nm to 90 nm. Besides the excellent preservation of these rod-like bacteria there are small holes that are reminiscent of filament molds. The result of the EDXanalysis reveals that the filaments and spheroidal bodies are both composed of calcium carbonate.The presence of a K peak may be related to potassium ions occupying interstitial sites in the calcitelattice. Nanometer-scale spheroids are considered as nanobacteria and have been observed inmicrobial mats as old as Proterozoic, while the occurrence of framboidal pyrite is related to themetabolic activity of sulfate?reducing bateria and decay of organic matter. The presence of abundantfilaments of rod-like bacteria, whose sizes are very similar to those observed in modern microbialites,is a strong evidence of the role of microorganisms in stromatolite formation. [Contribution C-85 ofthe IDEAN].