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Revista de la Asociación Argentina de Sedimentología

Print version ISSN 1853-6360


SCHILLIZZI, Roberto; GELOS, Edgardo M  and  SPAGNUOLO, Jorge. Cliff-retraction processes in the Patagonia between the mouths of the Negro and Chubut rivers, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2004, vol.11, n.1, pp.17-26. ISSN 1853-6360.

Retraction processes in sedimentary cliffs are studied for the first time along the coast of the Argentina Patagonia between Playa El Faro (Río Negro Province) and the mouth of the Chubut River (Chubut Province). The lithology and structure of the cliffs were determined, along both exposed and protected coasts. Based on the various classification criteria for active processes the resulting cliff faces and slopes vary in relation with the mechanisms produced by the movement of particles and blocks. Also these processes can be also considered as static and dynamic. Cliff retreat is related to the lithologic and structural characteristics that act in a differential way against the exogenous agents. The hydraulic action of these agents produce, on the base of the cliffs, the formation of wave-cut notches and block falling, among others. Only in exceptional cases the marine erosion does not act efficiently. Zenkovich, (1967) classified the various kinds of cliff and the scarp movements. Based on the observations by Sanders (1981) and Sunamura (1983), Carter (1991) established experimental models of the erosive processes affecting cliff faces. Schillizzi and Gelós (1999) and Schillizzi et al., (2001) analyzed the coastal morphology of the area»Golfos Norpatagónicos» (Schillizzi et al., 2003). Cliffs, in this region, have subvertical faces with heights that vary from 20 to 100 m msl. A) Particle and block movements: they are subdivided in two clases. The former corresponds to the disaggregation or simple remotion which produces sapping and the development of notches (Gelós et al., 1992, Pta Hércules), combined with the action of the marine spray. Due to this process, as series of small, unconsolidated accumulation ridges are formed with highs up to 0.30 m, which are redistributed by the waves (Barrancas Blancas). The latter, or block dislodge, is produces by the wave erosion of the base diaclases Playa Parana and Punta Cono). The top banks are debilitated by the rain and transported gravels, wedge effect (Playa El Faro). B) Mass movements. Reptation slides or in stairs (slow flow) are effective when the cliff face is formed by fine sediments (sand and silt) with low degree of consolidation. The movement occurs in several stages as a function of the presence of dominant directional winds which attack low slope cliff faces, even with scarce vegetation. C) Complex movements: (Slip and flow combination). In these processes participate the formation of particles and their translation. Include slipping by serial fluxes and slipping by serial blocks. The surface runoff and infiltration contribute to a fast degradation of the cliff faces inducing a differential movement of the material (Pta Ninfas). Static or conditional factors. They are related to the sediment composition, structural weakness and face angles. Along the coast of Rio Negro Province where the faces have an E-W orientation, they are formed mainly by blue sandstones of the Río Negro Fm (Andreis, 1965) and have levels with Rodados Tehuelches (Weber, 1983). On the NE sector of the Chubut Province, the faces are constituted by limestones and sandstones of the Pto Madryn Fm (Haller, 1979). The inclination of the scarps varies between 20 and 70º (Gelós et al., 1992). Dynamic Factors. They indicate the type and intensity of the exogenous agents. The marine dynamics acting on the base of the cliff originate notches and particulate matter. The next step with alternate sandy and clayey strata, combined with a short wave-cut platform and strong slope produce the fall of blocks. These blocks protect temporarily the base of the cliffs from marine action. The continental hydraulic is expressed by the runoff and infitltration that originate gullies and lobulated surfaces. The combined marine and continental hydraulics occurs when the upper levels are well consolidated sandy-calcareous sedimentites with fossils (Fm Pto Madryn) and the lower ones are tuffs or sandy tuffs (Fm Gaiman). Marine and continental wetting and drying are more active on pelitic material and on areas protected from wind action. The dominant erosive process is the surficial weathering of the minerals. Vertical cliff faces adquire inclination and lobulated morphology. In summary, in the NE sector of the Chubut Province, the main processes that modify the cliff faces are mass movements through the widening of diaclases in clayey strata. The meteoric waters act in both sectors, inducing a weakness of the upper levels of the cliff, originating gullies and the fall of blocks.

Keywords : Acantilados sedimentarios; Costas patagónicas; Retraction; Cliffs; Patagonian coasts.

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