Evidence of microbial activity on early Tertiary sediments, Andes foothills, Argentina

TUNIK, MAISA ANDREA; AGUIRRE-URRETA, M.B; PAZOS, PABLO

Denver, USA

Simposio; Microbial mats in siliciclastic deposits (Archean to today); 2010

SEMP

The Pichaihue section is located in the inner Agrio fold and thrust belt in the retroarc NeuquénBasin of Argentina (36º40º South lat.). It is composed by volcanic and volcaniclastic rockscaped by calcareous sediments that were interpreted as being deposited in a lacustrine toshallow marine environment related to a Maastrichtian-Danian Atlantic transgression. Thefacies of the Pichaihue Limestones include massive bioclastic mudstones with calciespheres,laminated stromatolites, oncolites and macrophytes. The microbial activity was suspected fromoutcrops and through thin sections, and SEM analysis showed undoubtedly the presence ofcyanobacterial filaments, nannobacterias and coccoids microbes that clearly support themicrobial origin. We identified smooth to lumpy surfaces and forms that range from tinyspheres to stubby ellipses and long filaments. The filaments and some of the tiny spheres arecoated by aggregated of small (less than 0.5 um) anhedral equant calcite crystals. It is knownthat the microbial films contribute to stabilization of the sediment and preservation of biogenicstructures. In our study we propose that the microbial films avoid the destruction of the reliefunder intertidal conditions and also preserve the macrophytes stems. Recent works in theNeuquén Basin described a microbial reef in sediments of the same age cropping out 20 km tothe northeast of our study area, but no evidence on nannobacteria was mention there. Thiscontribution supports their proposal and also the idea that after a major extinction (K-P in ourcase) special environmental characteristics are suitable for the development of microbialcommunities (Ezaki et al. 2008). Those communities are quickly disrupted when normal marineconditions are re-established.