Laguna Carilaufquen Grande

The closed basins of Lagunas Cari-Laufquen Grande and Chica (Fig. 1C) lie in a tectonic depression surrounded by basalt plateaus or mesetas of the Mesozoic to Tertiary age (Coira, 1979). In contrast to Lake Mascardi, this region was not affected by the last glaciation of the Andes. Laguna Cari-Laufquen Grande is located at an elevation of 800 m and is an ephemeral, brackish water body with an average depth of 3.0 m during the rainy season. Mean annual precipitation is ca. 200 mm, falling primarily in the winter months (May to August), when the southern westerly storm tracks shift toward the equator. Mean annual temperature is 4°C, and the prevailing winds are from the west. Paleo-shorelines have been observed and mapped at elevations of up to 68 m above the present lake level (Coira, 1979), although older shorelines up to 100 m above the present lake level have apparently been found (Galloway et al., 1988). During these lake-level high stands, the smaller lake, Cari-Laufquen Chica, merged with Cari-Laufquen Grande to form a large paleolake. Today, they are connected only by the Rio Maquinchao (Fig. 1C). Results of dating paleo-shorelines indicate that higher lake levels than exist today occurred ca. 19,000 14C B.P. (Galloway et al., 1988) and between 14,000 and 10,0008000 B.P. (Bradbury et al., 2000). Fine-grained lacustrine deposits underlying the upper two shorelines contain diatoms and ostracodes, which suggest deposition in a deeper, saline, and alkaline lake.

A seismic survey using a 3.5-kHz pinger system was undertaken in both lakes. With a maximum water depth of approximately 4 m, both lakes present major challenges to seismic surveying because strong multiple reflections and ringing usually obscure seismic data in such shallow water. Although the seismic record of Cari-Laufquen Chica did not show clear subsurface in

FIGURE 3 Lake Mascardi: uninterpreted (left) and interpreted (right) seismic profile in position C-D (refer to Fig. 1 for location). In contrast to profile A-B (Fig. 2), which lies in shallower water, this section is characterized by only minor unconformities. However, a distinct increase in the amplitude of internal reflections in the Holocene can be recognized.

FIGURE 3 Lake Mascardi: uninterpreted (left) and interpreted (right) seismic profile in position C-D (refer to Fig. 1 for location). In contrast to profile A-B (Fig. 2), which lies in shallower water, this section is characterized by only minor unconformities. However, a distinct increase in the amplitude of internal reflections in the Holocene can be recognized.

formation, seismic sections obtained in Laguna Cari-Laufquen Grande yielded good acoustic stratigraphy (Fig. 4) because of the high-amplitude sediment impedance contrasts. The lines are characterized by a very weak water-bottom multiple because receiver gain can be kept low. This allows imaging of subsurface geometries in great detail to a depth of more than 15 m. Such good-quality seismic data, however, were obtained only in parts of the lake. In other areas, the seismic quality was limited, probably due to the patchy distribution of gas-rich sediments in which exolved gas bubbles presumably dissipate the seismic energy (Fig. 4).

In spite of the limitations of acoustic stratigraphy, subsurface geometries imaged by seismic profiling clearly document major structures related to paleolake levels. Several reflectors show strong relief (Fig. 4). These moundlike features could unlikely form by deepwater processes in the middle of a flat, 10-km-wide basin. We interpret such mounds as having formed by either erosional or constructional processes at or near desiccation levels in the lake. Potentially, the acoustic character could also correspond to barlike features of fluvial gravels or sands. The lowest of these prominent unconformities is labeled seismic horizon A, which defines the acoustic basement. The surface is a barrier to reflected acoustic energy, likely indicating a well-indurated lithology below reflector A. The relief on reflector A is almost 10 m and is subsequently infilled by the lacustrine deposits that eventually form the modern flat-lake bathymetry. This geometry documents a period of very low lake levels or even subaeri-al exposure during formation of reflector A. Periods of higher lake levels yielded the overlying, relief-infilling lacustrine strata.

These overlying strata display numerous on-lapping geometries, as indicated by arrows in Fig. 4. Within the package, two horizons (B and C) form the base of mounds, approximately 1 m high, that are located on top of flat lacustrine substrates. These moundlike structures are most likely formed in a shallow lake as evidence of paleo-shorelines or sand / silt bars and spit features. The mounds probably comprise mainly silty

FIGURE 4 Laguna Cari-Laufquen: uninterpreted (top) and interpreted (bottom) seismic profile (refer to Fig. 1 for location). Reflection A defines the acoustic basement, which is not penetrated by the acoustic signal. It documents a paleorelief of up to 10 m in height and is either the result of erosion of a completely desiccated lake or an accumulation of river-transported gravels or sands. Horizons B and C mark the base of positive relief structures, which are Likely paleo-shorelines. Thus, all three horizons are a result of periods when lake levels were lower than they are today or when the lake dried out.

FIGURE 4 Laguna Cari-Laufquen: uninterpreted (top) and interpreted (bottom) seismic profile (refer to Fig. 1 for location). Reflection A defines the acoustic basement, which is not penetrated by the acoustic signal. It documents a paleorelief of up to 10 m in height and is either the result of erosion of a completely desiccated lake or an accumulation of river-transported gravels or sands. Horizons B and C mark the base of positive relief structures, which are Likely paleo-shorelines. Thus, all three horizons are a result of periods when lake levels were lower than they are today or when the lake dried out.

sand sediments rather than coarse gravels because seismic signals penetrated these structures and imaged the underlying sediments.

The example of Cari-Laufquen Grande seismic stratigraphy is encouraging because of the useful information gained in spite of what would be considered an environment too shallow for viable seismic profiling. Three seismic horizons (A, B, and C; Fig. 4) represent desiccation surfaces, representing periods during which the lake level was lower than it is today. Thus, these seismic sequences and associated unconformities can be used to support estimates of changing rates of evaporation and/or variations in water input during the late Quaternary period. Using the recent data for optimal coring would suggest that reflector C could most efficiently be cored at the highest elevation of C, where it occurs only ca. 4 m below the lake floor (Fig. 4). A core retrieved farther to the north of this basement high will be useful for a thicker, more continuous temporal reconstruction of the sedimentation history—a project that is in progress.

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