Preliminary Tephrochronology and Tephrostratigraphy from Caviahue

VILLAROSA, GUSTAVO; OUTES, VALERIA

Caviahue, Neuquén, Argentina

Workshop; International Workshop on Geogenically Acidic WaterSystems: Volcanic Waters, Mining Lakes and Rivers; 2004

Univ. Nacional del Comahue - UFZ Centre for Environmental Research

Tephras are excellent time markers over long distances. These unconsolidated pyroclastic deposits are very well preserved in lacustrine records and they become relevant in environments where tephras show low preservation potential. In Patagonia surface records are incomplete compared with lacustrine records especially at non-proximal facies. This can clearly be seen in Bariloche area (southern Nahuel Huapi) where only 2 highly weathered pyroclastic layers are present in several outcrops but over 33 explosive events can be identified in cores from laguna El Trébol. Next to this lake, in a very well preserved sequence from an archaeological site covering almost the same time period as the core, only 4 tephras were individualized, confirming that the best tools to study explosive events are lacustrine records. A good Tephrochronology can be obtained through the fingerprinting and dating of relevant tephra layers. This fingerprinting is achieved by analyzing the morphology of individual glass fragments, their geochemical analysis and petrography. Indirect dating of post-glacial tephras can be obtained by AMS radiocarbon dates of organic matter directly associated to the pyroclastic layer. With several dates obtained for a single core a good chronological model can be established. This information has various applications. It is especially valuable for volcanic hazards studies as the periodicity/episodicity of a certain volcano, the spatial patterns and, eruptive processes can be established and therefore hazard maps and contingency plans can be elaborated. It also constitutes a powerful tool for paleolimnology and paleoclimatology as it can be important to establish correlations between paleoenvironmental records and it allows interpretations of effects of volcanic events on the environment. Caviahue Lake core consists of six sections, about one half of it corresponds to pyroclastic material and it shows a high variation in sedimentation rates. Due to the presence of abundant pyroclastic material and their important width it is deduced that the time span covered by this core will be much less than the expected as the average sedimentation rate will be high as a result of these recorded ash fall episodes and probably thick layers of remobilized pyroclastics (lahars, density currents). Thirty three representative layers were sampled. Reworked material will be identified through detailed microscopic description, texture analysis and geochemistry.