Abstract

Strongly oriented lineaments, defined by lake shores and stream and vegetation alignments, are distributed throughout more than 45,000 square miles 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. To the west the basin sediments abut abruptly against the folded and faulted Andean foothills and to the east they gradually overlap gently rolling crystalline outcrops of the Brazilian Shield.

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 200 miles to where the basin sediments are 10,000 feet thick. Two poorly developed sets of lineaments that trend roughly north-south and east-west are also present within the basin.

Most lakes are strongly rectilinear in outline with shores that commonly trend northwest-southeast and northeast-southwest. They range in size from about 1000 feet square to 5.4 by 12.4 miles. Only a few lakes have streams draining into or out of them. Those that have been studied on the ground are shallow with relatively flat bottoms and abrupt steep sides. The water surfaces are almost level with the surrounding flat terrain.

The two major northwest-southeast and northeast-southwest trends are probably controlled by a regionally oriented system of longitudinal and cross fractures in the crystalline basement; the poorly developed north-south and east-west sets of lineaments may reflect basement “shear” fractures. 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 of basement blocks or differential compaction of uncon-solidated deposits over basement blocks. The minor north-south and east-west sets and some of the northwest-southeast and northeast-southwest-trending stream and vegetation lineaments may reflect individual fractures in the underlying basement rather than areal subsidence over basement blocks. The youthful appearance of some oriented features indicates that they are probably forming at the present.

Because of the unique environment of the Beni basin, where alluvial deposits are accumulating over a relatively stable shield margin in an area of flat terrain and high water table, many of the subtle surficial depressions are clearly visible as lakes with orthogonally oriented shore lines. Lakes of comparable size, shape, and degree of orientation are known to occur elsewhere only in the Old Crow Plain area of Canada.

It is postulated that prelithification fractures, such as those in the Beni basin, must play a significant role during compaction, jointing, faulting, and folding of sedimentary sequences. Criteria are needed to distinguish between fracture trends that are inherited from basement rocks, and those formed by postdepositional deformation.

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