Resumen

GOMEZ, Eduardo A; GINSBERG, Silvia S  y  PERILLO, Gerardo M.E. Geomorfología y sedimentología de la zona interior del canal principal del estuario de Bahía Blanca. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.2, pp.55-61. ISSN 1853-6360.

By means of precision bathymetric surveys, side scan sonar and superficial sediment sampling, the morphology and sedimentology of the 15 km inner portion of Bahía Blanca Estuary Principal Channel, between its head and Galván Harbour, is studied. All surveys were made during spring high tides in order to obtain maximum coverage of the area, with a 14 m length boat on the deepest areas and other smaller boat for the shallower zones. To positioning the vessels, an electronic microwave device was used. A total of 37 surficial sediment samples, were obtained with a Dietz-Lafond grab sampler and processed according to standard laboratory procedures according to Folk (1974). The analyzed portion of the Principal Channel drains several secondary channels on both borders (Fig. 1). Considering the datum level, the Principal Channel has a width of the order of 400 m at Galván Harbour, diminishing progressively headward where it reaches values of the order of 50 m (Fig. 2). Maximum depths vary from 12 m at Galván Harbour to values less than 1 m at the head. Principal Channel depths usually exhibit a local increment at the vicinity of secondary channel outlets. Channels borders and adjacent tidal flats are composed of a high proportion of mud (98-45 %), while samples from bottom channel exhibit large textural variations, from compacted silty clay to a shelly coarse sand (Fig. 2). Due to the bottom hardness, at some places it was impossible to obtain samples.  . On both channel borders, the side scan sonar survey showed a series of steeped parallel scarps (FigS. 3, 4 and 5). These scarps are produced by present differential erosion on the horizontal muddy strata deposited during the Holocene transgression (Aliotta et al., 1992; Aliotta & Farinati, 1990). From Bahía del Pejerrey mouth, and based on a geomorphological criteria, it was possible to divide the reach in inner and outer sectors. The external portion is covered by shell deposits and irregular erosive features with a complex distribution carved on lithified materials (Fig. 5). The inner deepest areas show an extended dune field conformed by shelly sandy sediments (Fig. 4). These features have a mean wave length of 4 m, which according to Ashley (1990) could be classified as 2D and 3D median dunes. These differences between inner and outer sectors are attributed to an outward current velocity increase along the channel. This means that outward Bahía del Pejerrey mouth, the current velocity increment is such that the volume of sediment transported as bed load becomes insufficient to generate cumulative features. The lithified materials are from the Pleistocene, and they are at 7.5 m below the datum plane at Galván Harbour (Aliotta et al., 1992), diminishing their depth westward and outcropping at the intertidal zone of El Caño Channel, being this the unique intertidal Pleistocene outcrop recognized in the area until present. The presence of Pleistocene litified materials would control Principal Channel deepening.

Palabras clave : Geomorphology; Sedimentology; Bedforms; Bahía Blanca Estuary.

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