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Seismic investigation of sedimentary fill and bedrock geometry of maar lake Laguna Potrok Aike, Southern Patagonia

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Gebhardt, C. , Niessen, F. , Anselmetti, F. , Ariztegui, D. , De Batist, M. , Haberzettl, T. , Ohlendorf, C. and Zolitschka, B. (2007): Seismic investigation of sedimentary fill and bedrock geometry of maar lake Laguna Potrok Aike, Southern Patagonia , 4th International Limnogeology Congress, 11-14 July 2007, Barcelona. .
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Abstract:

Laguna Potrok Aike is a maar lake located in the Pali Aike Volcanic Field in Southern Patagonia,Argentina, at about 52°S and 70°W. The lake with a diameter of about 3.5 km is almost circular and bowl-shaped with a deep, flat plain (about 100 m water depth) in its central part. Steep flanks separate the central plain from the lake shoulders in 15 to 30 m water depth. The lake is located at the present boundary between the Southern Hemispheric Westerlies and the Antarctic Polar Front. It is highly susceptible to changes in the Antarctic Circumpolar Current controlling the regional precipitation patterns. Its sediments possibly contain a long and continuous record of several glacial and interglacial cycles, which is unique in the southern South American realm.In a first step towards continental deep drilling, four seismic surveys were carried out between 2003 and 2005 to reveal the general geometry of the maar crater as well as the internal structures and thickness of the lacustrine sediments, their spatial distribution and possible lithologies.Two major stratigraphic units were distinguished in the seismic sections, Unit I consisting of thelacustrine infill and Unit II forming the surrounding and underlying bedrock. Unit I was further subdivided into Sub-units I-a and I-b on the lake shoulders and I-ab, I-c and I-d in the central basin.The sediments of Unit I are generally well-layered. Sub-units I-a and I-b on the lake shoulders areseparated by a major unconformity and contain several paleoshoreline structures formed during a step-wise transgression after a lake level lowstand of approx. 35 m below the present lake level. In the central basin, Sub-units I-a and I-b are merged into Sub-unit I-ab, not being separated by any unconformity. Pelagic sedimentation dominates in the northern and central parts, whereas mass movement deposits were found in the southern, western and eastern parts close to the steep diatreme flanks. The boundary between I-ab and I-c is non-erosive with I-ab forming downlaps onto I-c from the eastern and western parts of the lake, pointing at a significantly lower lake level during its accumulation. Sub-unit I-d shows similar characteristics as I-ab. The bedrock (Unit II) that forms the steep diatreme flanks consists of the well-layered sandstones found in outcrops of the lake surroundings.

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