High-Resolution Seismic Reflection Imaging of Lacustrine Stratigraphy of the Salar de Uyuni, Bolivia

Results from a near-surface seismic reflection experiment across the Salar de Uyuni on the Altiplano of Bolivia will be presented. Data were collected in the summer/fall of 2000. This salar is the largest salt flat in the world and is a remnant of several Quaternary paleolakes. There are two main objectives for this project: (1) to produce a seismic stratigraphic record for Pleistocene deposits within the southern basin of the Altiplano by imaging interbedded layers of lacustrine marls and salar salts down to depths of 300-500 meters and (2) to document the extent to which the Quaternary lake stratigraphy is affected by neotectonic deformation in the form of either isostatic rebound or active faulting. The profiles were designed to cross the site of the 220 meter deep corehole drilled by Baker et al. (Nature, 2000) within the Salar de Uyuni in June 1999 to look at paleoclimate history; the seismic stratigraphy will be tied to the stratigraphy seen in the core. The profiles also provide a more regional stratigraphic framework in which to place the core data and help identify any structural or stratigraphic complexities within the salar portion of the lake stratigraphy. The seismic profiles were acquired using high-resolution seismic equipment from IRIS PASSCAL and a jackhammer source. Data were recorded to 500 ms at 0.25 ms sampling. Depth penetration exceeded 300 m with numerous reflections and surprisingly high frequency content (over 500 Hz in the shallow section). Data quality is quite good despite the 2-10 meter thick surficial salt layer and some very windy conditions. Two different types of profiles were collected to satisfy the nearly exclusive need for both high-resolution data with dense spatial sampling and very long profiles (up to 130 km). Seven standard 60-channel profiles were collected along an east-west transect of the salar at 2 m shot/receiver spacing to provide detailed stratigraphy in segments of 1 km or less. (Two of these are tied to the Baker et al. corehole.) In addition, two transects were collected E-W and N-S across the salar with 6-channel records at 1 km spacing. Despite the low spatial sampling, these long transects clearly show faulting and tilting across the basin. The subsurface stratigraphy of this paleolake region is clearly imaged to depths exceeding the core stratigraphic control.