Abstract

SPALLETTI, Luis A  and  ISLA, Federico I. Characteristics and evolution of the Colorado River Delta (Colu-Leuvu): Buenos Aires. Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.1, pp.23-37. ISSN 1853-6360.

The Colorado River delta was morphologically analyzed, considering both the deltaic plain and the prodelta areas. By the mean of the Landsat 7 ETM images 226- 087 and 226-088, different depositional environments were surveyed recognizing the processes involved in their evolution. The delta architecture and present coastal features were particularly assigned to relative sea level changes. The Colorado River runs from the Andes to the Atlantic Ocean limiting the Patagonia to the south and the Sandy Pampa to the north. It initiates from the confluence of the Barrancas and Grande rivers, ending as an extended deltaic plain after a distance of 900 km. Within the deltaic plain, significant amount of fine sand is supplied by two aeolian corridors (Ombucta and Mayor Buratovich) that originated from the Colorado River valley. At the coast between Bahía Blanca and bahía Anegada, three non-operative extended lobes were recognized. The formation of lobe 1 deltaic plain was assigned to a sea level lower than present, and related to a higher fluvial sediment supply. Lobe 2 is strongly eroded comprising the area close to Bahía Blanca. The original fluvial valley that discharged into this lobe is today mostly abandoned, and the area is subject to tidal reworking. Lobe 3 would have deposited during a sea level highstand, possibly related to the Sangamonian Interglacial (oxygen isotope stage 5e; about 120,000 years BP). During this highstand, low-gradient conditions explained a multichannel fluvial system of high sinuosity, and the erosion dominated by tidal and wave processes. In lobes 1 and 3, constructive and destructive phases are related to glacioeustatic sea-level changes and coeval tectonic uplift. The different availability of sediments between lobes 1 and 3 could be assigned to the interval when the Desaguadero System became isolated from the Colorado Basin. More recently, and attached to lobe 3, coastal ridges composed mostly of gravel, and oriented north to south, indicate reworking processes dominated by waves. These gravel ridges dated between 4000-3700 years BP, represent the regression from the Mid-Holocene maximum sea-level highstand. Comparing ancient maps, the destructive phase is evident at the former operative arm (called "Colorado Viejo") where barrier islands of 1795 became eroded. The Colorado Delta, as many deltas of the Southern Hemisphere, evolved interfingered to beach ridges related to Quaternary highstands.

Keywords : Colorado River; Argentina; Delta.

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