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A landslide suceptibility zonation was performed in a 10 km wide corridor along the RECOPE pipeline between the cites of Siquirres and Turrialba. The total study area is 330 km². Analysis was based on aerial photointerpretation, bibliographic research and the application of the Mora-Vahrson method (Mora et al., 1992), in which slope was used instead of the internal relief parameter. A landslide inventory map was included in the evaluation of the lithological suceptibility parameter. Zonation resulted in a predominance of areas with low landslide susceptibility (33,9%), mostly located where Tuis Formation is present, south east of the study area, followed by Suretka Formation and partially Andesitas Poás. Keywords: susceptibility, landslides, pipeline, Siquirres, Turrialba, RECOPE, Mora-Vahrson. RESUMEN:Se presenta una zonificación de la susceptibilidad al deslizamiento en el sector entre las localidades de Siquirres y Turrialba, específicamente para un corredor de 10 km de ancho sobre la ruta del poliducto de RECOPE, que cubre una extensión aproximada de 330 km². El estudio se realizó a partir de la interpretación de fotografías aéreas, información bibliográfica y la aplicación del método Mora & Vahrson (Mora et al., 1992), sustituyendo el relieve relativo por un mapa de pendientes e incorporando un mapa inventario de deslizamientos en la evaluación del parámetro de susceptibilidad litológica. Como resultado de la zonificación, se obtuvo que las áreas correspondientes a susceptibilidad baja abarcan la mayor parte del mapa (33,9%) y se presentan mayormente en sectores donde afloran las unidades litoestratigráficas Tuis, al sureste del área, Suretka y en algunos sectores de la formación Andesitas Poás. Las áreas correspondientes a susceptibilidad media (25,3%) se presentan principalmente en rocas de las formaciones Andesitas Poás, Uscari y Guayacán; mientras que las áreas correspondientes a susceptibilidad alta (13,4%) se presentan principalmente en sectores donde afloran las rocas de las formaciones Andesitas Poás, Uscari, Brechas La Unión y Río Banano.

ABSTRACT:The geoarchaeology evaluation of the Monumento Nacional Guayabo (M.N.G.) shows the following problems: the water, with pH 5,5-6,0 or 7, flows over the rocks paths, it is a chronic problem, the cause could be springs, aqueduct collapse, poor soil infiltration. The low pH, could cause colored precipitates. The abundant flora, like lichens, have covered the rock and other geoarchaeological features. The lichens could damage the lithic and glyphs. The geological study, shows regional and big landslides, but the M.N.G. seems to be stable and there are not in the Monument itself. The petrography shows a basaltic-andesite rocks with glass matrix. It means that the rock is easily eroded. The fluidal texture produces natural laminations. It is means no harmful rocks just it is a natural feature. Key words: geoarchaeology, petrography, basaltic-andesite, Turrialba volcano, mud flows, landslices, lithology RESUMEN: La evaluación geoarqueológica del Monumento Nacional Guayabo (M.N.G.) mostró, lo siguiente: problemas con aguas de inundación pasiva, con pH bajos de 5,5-6,0 hasta 7, en las calzadas, lo cual parece ser un problema crónico, la causa puede ser descargas de manantiales, colapso del acueducto o pobre infiltración de suelo. Proliferación descontrolada de líquenes que ya han cubierto por completo rocas y objetos arqueológicos, probablemente los líquenes son encostrantes deteriorando la lítica. Desde el punto de vista geodinámica externa, se han observado deslizamientos de tierra y roca. El sitio M.N.G, no tiene evidencia de deslizamientos recientes y se encuentra asentado en un área estable. La petrografía muestra que las rocas son andesitas-basálticas con mucho vidrio en la matriz y de textura fluidal, lo cual generan laminaciones y lajamientos, tales rasgos no deben ser necesariamente interpretados como lítica en deterioro.

ABSTRACT:The studied area belongs to the Térraba basin. The stratigraphy is represented by the following units: The Tulín Formation is composed by gabbros and pillow lavas with geochemistry similar. The pillow lava has vesicular and microdolerite texture. There are basaltic lavas interbedded with epiclastic sediments. The age of this formation is Upper Cretaceous to Basal Eocene. Descartes Formation was defined by first time in this area. The formation is composed by interbedded sandstones and mudstones. The colors of the rocks are black, red and greenish pink. The strata beds are good with centimetric laminations, but they are massive strata beds; locally the sequence present hydrothermal alterations and the rocks become light green color. The age is Late Paleocene using the Morozovella velascoensis foraminifera. Caraigres Formation belongs to the Valle Central Basin. It is composed by interbedded sandstones and mudstones, with black and gray color, the volcanic influence is common. The age of the formation is Oligocene to Basal Miocene. The geologic contact between Caraigres Formation and Descartes formations probably transitional. Curré Formation is composed by fine and coarse black sandstones, mudstones and dark grey volcaniclastic conglomerates. The conglomerates has metric packages with good stratification or interbedded with sandstones. The age is Middle Miocene. Grifo Alto Formation consists of porphyritic andesites with plagioclases, pyroxenes, amphiboles phenocrysts and porphyritics basalts with augite. It is possible to find fine and coarse weathering tuffs, and hydrothermal alteration. The age of this formation is Pliocene to Pleistocene. Altos Chiral Lacustrines Deposits are defined by first time in this study like a new geologic unit. It is composed by orangized mudstones. It is not consolidated and it overlays Descartes Formation by angular unconformity. The outcrops are near Altos Chiral and Alto San Juan, to the west of San Marcos de Tarrazú. The age of this new unit probably is Plio-Pleistocene? Alluvial Terraces consist of alluvial deposits, with metric blocks of different kind of volcanic rocks. Some blocks have hydrothermal alteration. The outcrops are near San Marcos, Sta. María, Copey and Londres and Naranjito near Quepos. The study area presents a strong tectonic influence, the deformation started with reverse faults which were subsequently cut by transcurrent faults. The most important fault is Paquita-Chonetera fault, which expose the Descartes Formation.

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