Resumen

ALIOTTA, Salvador; LIZASOAIN, Guillermo  y  LIZASOAIN, Wilfredo. Facies fluvio-estuarina en el subsuelo del canal de acceso al Puerto de Ingeniero White, Bahía Blanca. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.2, pp.43-53. ISSN 1853-6360.

Along the entire Argentine shoreline, there are important and numerous geological evidences of the sea level changes during the Quaternary. Environmental controls, such as geomorphology, climatology and transgressive-regressive processes; determined different types of sediment, fossils and geoforms. Although a lot has been investigated about the deposits situated above sea level in the Bahía Blanca Estuary, very little is known about sedimentological characteristics of the materials below the sea bottom. subbotton of the navigation channel at the Ing.White Harbor, between 14 y 22 m depth,  was analyzed.  For the sedimentological characterization 14 cores were obtained with a Rotary Universal Device and referred to mean sea level. A well lithified sandy silt with low proportion of clay (<15%), constitutes the oldest sediment of the different cores. This material is the Tertiary bedrock (late Miocene-Pliocene, Chasicó Formation) of the unconsolidated sedimentary sequence at the inner area of the Estuary of Bahía Blanca. On the Tertiary basement, there is a sediment composed of fine to very fine sand, with a low percentage of silt-clay (<10%). Quartz grains surface texture characteristics ( V- shaped impact pits and silica globules) reveal a final transport phase in a subaqueous environment (fluviatile-deltaic). In this lenticular deposit, the remnants of mollusc shells are very scarce and unidentifiable. It was only possible to determine the presence of a fresh water ostracod. Diatom analysis indicated a high concentration of diatom taxa of clearly non-marine affinity, which are not very easily found in the present chemical conditions of the estuary. This allowed us to define a dominant fluvial sedimentary paleoenvironment, in which the marine conditions are subordinated. This fluvial-estuarine facies corresponds to a paleoenvironment formed during the last great post-glacial transgression, in a period when the sea level was lower than the present one. Considering its depth and in agreement with the pattern of world-wide sea level variations during the Holocene epoch the probably age of fluvial estuarine sediments is Early Holocene (10.000 - 7.500 yrs B.P.). Sea water gradually invaded the fluvial paleoenvironment during the rise of the Holocene sea, resulting in the deposition of estuarine-marine sedimentary facies. After the transgressive sand with abundant remnants of shells, the sedimentation of mud facies reflects the decreasing energy of deposition, and increasing the suspension sediment transport. These sediments belong to a Holocene tidal flat, which form a great coastal flat characteristic of the inner area of the Bahía Blanca.

Palabras clave : Sedimentology; Fluvial-estuarine deposit; Quaternary; Bahía Blanca Estuary.

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