Procesos de retracción de los acantilados patagónicos entre la desembocadura de los ríos Negro y Chubut. Argentina

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 t