Oxfordian microbial laminites from la manga formation: remarkable nanobacteria preservation, Neuquén Basin, Argentina

Abstract

The Callovian-Oxfordian of the Neuquén Basin (Argentina) is characterized by an extensive marine carbonate system (La Manga Formation) with a predominance of shallow and middle ramp deposits. The intertidal-supratidal deposits are interesting because of the abundance of stromatolite beds developed during the middle Oxfordian (Perisphinctes-Araucanites zone). A detailed study of these beds through analysis of growth fabric, laminae variations and exceptional preservation of a rod-like bacteria community has been done in the localities of Arroyo Los Blancos, Arroyo La Manga and Yeseras Grandes (southern Mendoza province). These facies exhibit planar and crinckle lamination 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 low energy upper intertidal to lower supratidal environments. Extensive SEM examinations of polished stromatolite samples were carried out showing bacteria filaments with subspherical microstructure and framboidal pyrite aggregates. The microbial filaments (rod-like bacteria) consist of a network of 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 EDX analysis 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 calcite lattice. Nanometer-scale spheroids are considered as nanobacteria and have been observed in microbial mats as old as Proterozoic, while the occurrence of framboidal pyrite is related to the metabolic activity of sulfate?reducing bateria and decay of organic matter. The presence of abundant filaments 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.