Lake Titicaca

With a maximum water depth of 285 m, Lake Titicaca is located at the northern end of the Peruvian-Bolivian Altiplano extending from 14°-21°S at ca. 36004000 m asl (Fig. 1D). The high-resolution seismic reflection data from Lake Titicaca have been used to look at both recent lake-level change (Seltzer et al., 1998) and sedimentation in the lake during the late Quaternary period (D'Agostino, 1998). Seismic data obtained in the regions surrounding the deltas in the lake and in regions shallower than a water depth of ca. 85 m show erosional features that have been interpreted as having

FIGURE 5 Erosional channels in the Río Huancane-Ramis delta at the north end of Lake Titicaca. TWT, two-way travel time (meters per second).

formed during a low stand of the lake ~100 m below present depths during the early and middle Holocene (Seltzer et al., 1998). Channels incised on the tops of the deltas and erosional truncation of seismic reflections appear to have formed relatively recently, as there is little or no sedimentation above these features (Fig. 5). Independent confirmation and dating of the lakelevel change are provided by sediment core analyses (Cross et al., 2000). Apparently, the low stand of the lake was produced by more arid conditions during the late glacial and an early Holocene phase, followed by a rapid transgression of the lake at ca. 3600 14C B.P. (Abbott et al., 1997; Seltzer et al., 1998; Cross et al., 2000).

Lake Titicaca (18°S) provides an example of a large, deep, closed tectonic basin with large catchment influences. Although the lake is surrounded by mountain ranges that reach over 6000 m and are glaciated today, the lake itself was never glaciated. The result is a relatively continuous sediment stratigraphy that spans the last glacial and the interglacial transition. The glacial-to-interglacial transition can be readily identified in the seismic stratigraphy as a transition from a package of relatively high-amplitude, evenly spaced reflections below to a 3- to 5-m thick unit of low-amplitude, closely spaced reflections (Fig. 6). This transition can be traced throughout the basin (D'Agostino 1998) and corresponds to a relatively inorganic, gray silt characteristic of glacial sedimentation with an abrupt transition to relatively organic-rich silts that have accumulated during the Holocene. At greater depths in the sediment (~40 m subbottom), an unconformity has been imaged at the limits of the penetration of the high-resolution data. This unconformity appears to represent a major low stand of the lake during the late Pleistocene (Fig. 6) (D'Agostino, 1998). Thus, the seismic data from Lake Titicaca indicate major hydrological changes in the basin and allow researchers to determine the best sites for piston coring and potential drilling for the recovery of long records.

Seismic Stratigraphy Drilling
FIGURE 6 The glacial-to-interglacial transition and a deeper unconformity in the seismic stratigraphy as indicated with arrows in a representative cross section from Lake Titicaca. TWT (m / sec), two-way travel time (meters per second).
FIGURE 7 GEOPULSE 1- to 7-kHz profile from Laguna Miscanti in Chile showing mid-Holocene desiccation surface separating two parallel-layered lacustrine reflection packages (Valero-Garces et al. 1996).

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