El papel de la gravedad y la erosión en las estructuras del borde oriental de la Sabana de Bogotá
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Universidad Industrial de Santander
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El borde oriental de la Sabana de Bogotá está formado por un anticlinal orientado N-S: el anticlinal de Bogotá- Usaquén, cuyos flancos están formados por las areniscas de Guadalupe Inferior. Se describe la estratigrafía del Guadalupe Superior y se discuten los términos litológicos usados. Tectónicamente llaman la atención las inversiones que se presentan a lo largo de gran parte del flanco W del anticlinal de Bogotá –Usaquen; estas inversiones son de dos tipos, inversiones totales, de todo el flanco o inversiones de las capas más altas del flanco que despegan de los niveles más inferiores quedan en posición normal. Este último tipo de inversiones son fenómenos gravitativos del tipo de las “collapse structures” y deben su origen sobre todo al deslizamiento según el buzamiento, de un nivel arcilloso-silíceo muy bien estratificado que se sitúa entre las areniscas del Guadalupe Superior. El desencadenamiento de este proceso ha sido posible debido a que la erosión ha dejado este nivel plástico en posición muy superficial. En las inversiones totales no es posible de momento definir qué papel ha jugado la gravedad, pero si puede afirmarse que por lo menos las etapas finales de la evolución del anticlinal de Bogotá-Usaquén tuvieron lugar cuando la cresta de este anticlinal había sido erosionada en gran parte de modo que los dos flancos del anticlinal pudieron jugar con cierta independencia el uno del otro.
RESUME.- La bordure Est de la Sabana de Bogotá est constitué par l´anticlinal de Bogotá-Usaquén d'orientation N-S; dans les flancs de cet anticlinal affleurent les grès du Guadalupe supérieur et dans le noyau affleure le Guadalupe inférieur. On étudie la stratigraphie du Guadalupe supérieur et on fait la discussion des termes lithologiques en usage. Du point de vue tectonique il faut signaler les inversions présentes le long du flanc W de l'anticlinal de Bogotá-Usaquén, on constate deux types d'inversions: inversions totales du flanc et inversions des couches supérieures du flanc qui adoptent une position inverse se detachant des niveaux plus inférieurs lequels demeurent en position normale. Ce dernier type d'inversion est un phénomène de glissement du genre des "collapse structures" et son origine est dû surtout au glissement d'un niveau silicoargileux interestratifié dans les grès du Guadalupe supérieur. La formation de ces structures a été possible par le fait que l'érosion a laissé ce niveau plastique presque en surface. On ne peut pas encore définir le rôle joué par la gravité dans les inversions totales mais on peut affirmer qu'au moins les etapes finales de l'évolution de l'anticlinal de Bogotá-Usaquén eûrent lieu au moment où la crête de l'anticlinal avait été erodé dans sa plus grande partie, ainsi les deux flancs de l'anticlinal ont pú jouer avec une certaine independence l'un de l'autre.
The eastern border of the Sabana de Bogotá is formed by an anticline with a N-S trend, the Bogotá-Usaquén anticline, the flanks of which are constituted by the Upper Guadalupe, and in the core of which the Lower Guadalupe is exposed. The stratigraphy of the Upper Guadalupe is described and the lithologic terms used are discussed. According with tectonics, one attention is drawn to the reversals which occur along a large part of the western flank of the Bogotá-Usaquén anticline. These reversals are of two types: total reversal of the whole flank, or reversal of only the upper beds of the flank, which separate from the lower ones and take on an attitude of reversal whereas the lower beds retain their normal position. The latter type of reversals are gravitational phenomena of the collapse structures type, and owe their origin principally to the slipping, according to dip, of a well stratified silico-argillaceous horizon interstratified with the sandstones of the Upper Guadalupe. This process is apparent owing to the fact that erosion has left this plastic horizon in a very exposed position. In the case of total reversals, it is not at present possible to define the influence exerted by gravity, but at least it can be affirmed that the final stages in formation of the Bogotá-Usaquén anticline took place when said anticline had already had most of its crest eroded so that each of its flanks could act with a certain degree of independence from each other.
The eastern border of the Sabana de Bogotá is formed by an anticline with a N-S trend, the Bogotá-Usaquén anticline, the flanks of which are constituted by the Upper Guadalupe, and in the core of which the Lower Guadalupe is exposed. The stratigraphy of the Upper Guadalupe is described and the lithologic terms used are discussed. According with tectonics, one attention is drawn to the reversals which occur along a large part of the western flank of the Bogotá-Usaquén anticline. These reversals are of two types: total reversal of the whole flank, or reversal of only the upper beds of the flank, which separate from the lower ones and take on an attitude of reversal whereas the lower beds retain their normal position. The latter type of reversals are gravitational phenomena of the collapse structures type, and owe their origin principally to the slipping, according to dip, of a well stratified silico-argillaceous horizon interstratified with the sandstones of the Upper Guadalupe. This process is apparent owing to the fact that erosion has left this plastic horizon in a very exposed position. In the case of total reversals, it is not at present possible to define the influence exerted by gravity, but at least it can be affirmed that the final stages in formation of the Bogotá-Usaquén anticline took place when said anticline had already had most of its crest eroded so that each of its flanks could act with a certain degree of independence from each other.