The origin and evolution of a southern Patagonian lake based on multi-channel seismic reflection methods
Abstract
As part of an ongoing study of interhemispheric correlations in climate change, 65^o km of multi-channel seismic reflection profiles were recorded with an air-gun source in the closed lake system of Lago Cardiel, Argentina (49^oS). Previous studies, using 3.5-kHz echo-sounder data, revealed six major seismic sequences throughout the lake's subsurface. Calibration of these earlier data with the dated piston-core samples permitted the reconstruction of lake levels since the late Pleistocene. The intent of using the stronger seismic source in this study was to concentrate on the deepest seismic units, Sequences V and VI, which may hold records of Pleistocene climatic conditions as well as information about the origin of the lake. Sequence VI is the oldest unit and underlies the entire lake basin. It has a parallel to wavy reflection pattern with km-scale fold wavelengths and a maximum dip of 3^o. The upper sequence boundary of Sequence VI is strongly reflective and highly irregular. A small depression, about 1 km wide and 2 km long, characterizes its deepest portion, which is over 160 ms (∼120 m) beneath the lake surface. This depression is slightly west of the lake center and oriented NNE. Sequence V unconformably overlies Sequence VI, and its extent is limited to the western part of the lake basin. It is irregular in shape and largely transparent to seismic waves, but its few internal reflections indicate an eastward progradation. The maximum thickness of Sequence V, 47 ms (∼35 m), occurs approximately in the center of the depression. Outcrops around the lake help to interpret Sequence VI as Cretaceous-Tertiary marls. Both the outcrops and the seismic profiles show fold axes that primarily strike north-south, thus indicating a generally east-west orientation of tectonic compression. Although Tertiary volcanism is widespread in the region, the nature and continuity of Sequence VI beneath the lake do not point to volcanic or impact-related origin of the lake. The irregular surface of Sequence VI represents erosion from structurally-controlled rivers flowing from the southwest and the northwest. An alluvial fan, Sequence V, fills the deepest incisions and exhibits major erosion, which occurred chiefly along its western side and prior to the onset of lacustrine sedimentation (Sequences I-IV). Thus, these data confirm that during the Late Glacial Maximum, the region was dominated by extremely dry climatic conditions, which prevented lacustrine sedimentation and ultimately caused erosion of previous deposits.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....2545B