Abstract

Strongly oriented lineaments, defined by lake shores and stream and vegetation alinements, are distributed throughout more than 72,000 km² of the Beni basin in northeastern Bolivia. The area in which these features occur is a flat, poorly drained, lake-studded plain. It is underlain by flat-lying, poorly consolidated, continental clastic sediments of Late Cenozoic age which, in turn overlie crystalline basement. The main lineaments within the basin show preferred orientations perpendicular to each other in northwest-southeast and northeast-southwest directions. These two preferred directions of lineament orientation occur in sediments that extend from the margin of the Brazilian Shield westward 320 km to where the basin sediments are 3000 m thick.

The two major northwest-southeast and northeast-southwest trends appear to be controlled by a regionally oriented system of longitudinal and cross fractures in the crystalline basement. The oriented lakes and many of the major northwest-southeast and northeast-southwest-trending lineaments occupy shallow surficial subsidences that appear to form by progressive downfaulting or differential compaction of unconsolidated deposits. The youthful appearance of some oriented features indicates that they are probably forming at the present, most likely by recurrent differential movements between fracture-bounded basement blocks. It is postulated that prelithification fractures, such as those in the Beni basin, must play a significant role in joint formation, because they result in anisotropic horizontal stress gradients that could control the orientation of tension joints over large areas.