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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360

Resumen

MELO, Walter D; SCHILLIZZI, Roberto; PERILLO, Gerardo M. E  y  PICCOLO, M. Cintia. Influences of the continental Pampa area on the morphological evolution of the Bahía Blanca Estuary. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.1, pp. 39-52. ISSN 1853-6360.

The characteristics of the drainage basins are one of the most important factors defining the origin and evolution of estuaries. In the case of the Bahía Blanca Estuary (Fig. 1), the presence of morphologic units such as the Colorado River and a series of depressions in the SW of La Pampa Province is connected to the genesis and evolution of the estuary since the Pleistocene. The aim of the present article is to establish the possible paleomorphologic relationships between the estuary and the adjacent continental environment. Bahía Blanca Estuary is the reception basin of a drainage basin located in the Southwest of the Buenos Aires Province, having a surface of the order of 3000 km2. The coast of the estuary can be divided in two sectors. To the north, it has a NW-SE orientation, whereas, to the west, the coast trends approximately N-S. The former has over 110 km in length between Salitral de la Vidriera and Baliza Monte Hermoso and the latter is located between the salitral and Laberinto Pt. with 130 km. To study the evolution of the area, visual analysis of satellite image charts from the IGM (scales 1:100.000 and 1:250.000) were employed. Once the morphologic unities were identified, they were digitized and assembled in a Geographic Information System (GIS) supported on the Landsat 7 satellite image ETM Path/Row 227 - 087 provided by CONAE. All IGM topographic charts of the area were also digitized and transferred to the GIS. A total of 8 field surveys were made to define the geomorphologic and geologic conditions and topographic surveys of the valleys and depressions were also performed. Furthermore, an aerial survey of the estuary and its surroundings were made. Two units were defined as Continental and Transitional domains (Fig. 1). In the former, the morphology is defined by the subaerial sedimentation processes associated to the presence and evolution of the hydrographic basin. The Transitional Domain includes the coastal and estuarine sectors. On the Continental Domain we determined the presence of geomorphologic units such as alluvial, aeolic and fluvial deposits, dune fields, sand strandflats and valleys and depressions. The latter are aligned and related to previous faulting which may be directly related to the origin of the estuary. Part of these alignments are below mean sea level (-20 m) and formed by canyons and valleys containing dunes of varied shapes. These valleys have been arranged in three groups. Group I starts on the NW of Lihué Calel Hills and formed two alignments having a NW-SE and E-W trends. Both follow the Chasicó - Salinas Chicas depression, continuing into the Salitral de la Vidriera and the Canal Principal. The second group is 120 km in length formed by the alignment passing through the Blanca Grande and Callaquéo lagoons, ending in the Chasicó - Salinas Chicas depression. The alignments in Group II are mostly trending in the E-W direction starting at the confluence between the Colorado and Salado rivers ending in the Chasicó - Salinas Chicas depression. Group III is independent of the other two and its alignments are related to the Colorado River directed towards the southern portion of the estuary. The most significant axis has a length of 140 km and relates the Anzoátegui and La Gotera salt lagoons with Falsa and Verde bays. The fluvial discharge along the three groups provided the hydric connexion between the continent and the littoral environments developing the initial stages of the formation of the Bahía Blanca Estuary. On the other hand, the morphologic unities in the transitional unit are: tidal flats (active and inactive), salt marshes, tidal channels, islands, sandy beaches and spits, and cliffs (Fig. 2). Since the Pleistocene, Bahía Blanca Estuary was affected by complex transformations due to climatic changes, alternation of dry and humid periods, and changes in the mean sea level (Aguirre, 1995). Between 9.500 and 7.500 yrs BP mean sea level increased to - 25 m (Aguirre, 1995) or between - 12 and -18 m (Aliotta and Perillo, 1990) (Fig. 3). The area presently occupied by the estuary was subaerial (Fig. 4a). Later on, the climate changed to warmer and more humid conditions (Aguirre, 1995) and the mean sea level increased to approximately the present conditions. Rivers introduced a large sediment load coming from the three groups and the Ventana Hill System developing a delta. Sediments were redistributed by the estuarine dynamics developing tidal flats, shoals, spits and islands. Due to the low slope and wide valleys, river velocities in the Group I were very low allowing only the transport of fine materials. About 6000 yrs BP the humidity and temperature was still increasing (Quattrocchio and Borromei, 1998), which induced further increase of the mean sea level reaching values of 5 to 12 m above present conditions (González, 1984). These values are further related to similar studies made by Cavallotto et al. (1999) for the Río de la Plata and Aliotta et al. (2000) for Arroyo Parejas. Because of the higher sea level, the delta was fully covered by the sea reaching the continent through the Salitral de la Vidriera. Although between 5.000 and 4.500 yrs BP the climatic conditions were stable, fluvial discharge reduced significantly inducing a sea level reduction to 2 - 3 m above present level and some of the higher islands became dry. Towards 3000 yrs BP, climate changed to temperate-dry, similar to the present conditions (Aguirre, 1995), which determined the disappearance of the rivers being replaced by aeolic environments. Consequently, the Colorado River only discharged in the depressions and to the axis La Adela - Algarrobo - canal del Embudo, while it progressively migrated southward. In this migration, the river occupied and later released the axis Salinas de Anzoátegui - La Gotera - Bahía Falsa. About 2000 yrs ago, the connexion between the continental and transitional environments (Salitral de la Vidriera) was transformed in a coastal lagoon. Wind action transported eastward the original sand deposits from southeast of La Pampa Province, forming extensive dune fields. This material also covered the Verde, Ariadna and Monte Is., located to the south of the estuary. Recently, Spalletti and Isla (2002) found at the mouth of the Colorado Nuevo River, to the south of the indicated islands, the presence of a lobular prograding shelf delta originated by marine reworking.

Palabras llave : Estuaries; Continental geomorphology; Bahía Blanca Estuary; Morphogenesis.

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