TUNIK Maisa Andrea
congresos y reuniones científicas
Evidence of microbial activity around the K/P boundary, Neuquén Basin, Argentina
Simposio; AGU 2010 Meeting of the Americas; 2010
Institución organizadora:
The discovery of carbonates in Pichaihue, Neuquén, Argentina, improves our understanding of the paleogeography of the first Atlantic transgression that reached the Andes foothills in lateCretaceous times. The locality of Pichaihue is situated in the inner Agrio fold and thrust belt in the retroarc Neuquén Basin (36°40􀏶SL). There are three isolated outcrops, composed by pyroclastic flows, ash-fall distal tuffs, and calcareous sediments of the Malargüe Group. SHRIMP U-Pb dating of volcanic zircons of the tuffs yielded an age of 64.3 ± 0.9 Ma that confirms the correlation to the Maastrichtian-Paleocene transgression from the Atlantic Ocean. The limestones are interpreted as being deposited in fresh water to shallow marine environments. The facies include bioclastic mudstones with calcispheres, stromatolites, oncolites, and macrophytes. In the eastern outcrop, the stromatolites are decimetric beds with very thin lamination. On petrographic section micritic and siliciclastic laminae were identified. The laminae are millimetric and irregular with clotted micrite, dark organic matter, and siliciclastic material. The oncolites are isolated subspherical bodies of concentrically arranged laminae up to 10 cm; the lamination consists of light-dark couplets, being the dark ones composed of micrite and organic matter, and the light ones composed of quartz, silica and admixtures of clay minerals. These oncoids have their nuclei of silt probably caused by the disintegration and subsequent infilling of plant steams. These nuclei might be indicators of the existence of former macrophytemeadows, and some oncoids have fresh water gastropods as nuclei as well. In the western outcrop, the stromatolites consist of domes of up to 1 meter high of dark to light brown color. The presence of large columns (diameter 30 to 100 cm) indicates that the shoreline was exposed to wave action. Recent studies support the relationship between morphostructures versus water depth and energy and also indicate that larger stromatolites are associated with episodic currents in deeper waters. The microbial activity was suspected from outcrops, polished and thin sections,and SEM analysis showed undoubtedly the presence of cyanobacterial filaments, nannobacteria and coccoids microbes that clearly support the microbial origin. There are forms that range from tiny spheres to stubby ellipses and long filaments. The filaments and some of the tiny spheres are coated by aggregated of small (less than 0.5 um) anhedral equant calcite crystals. It isknown that the microbial films contribute to stabilization of the sediment and preservation of biogenic structures. The presence of microbial films prevented the destruction of the relief underintertidal conditions and also helped to preserve the macrophytes stems. This contribution supports their proposal and also the idea that after a major extinction (K-P in our case) specialenvironmental characteristics are suitable for the development of microbial communities. Those communities are quickly disrupted when normal marine conditions are re-established.