Resumen

ALIOTTA, Salvador; SPAGNUOLO, Jorge O; GINSBERG, Silvia S  y  GELOS, Edgardo M. Sedimentos de fondo-subfondo y aspectos geomorfológicos de la plataforma continental entre el estuario de Bahía Blanca y bahía Anegada, provincia de Buenos Aires. Rev. Asoc. Argent. Sedimentol. [online]. 1999, vol.6, n.1-2, pp.19-35. ISSN 1853-6360.

This investigation is a contribution to the geological kowledge of the Argentine marine shelf. The study area is a sector of the internal shelf up to 25 m of depth, along a coastal zone extending approximately from Bahía Blanca estuary to Anegada bay (Fig. 1). The purpose of this report is to define, through the analysis of surface sediments and the marine subbottom, the present sedimentological conditions and establish the coastal sedimentary paleoenvironments that characterized the last transgressive holocene period. The specific oceanographic tasks, carried out with the research vessel El Austral, comprised the extraction of sedimentological samples (surface and core samples) and 3.5 kHz seismic survey (Fig. 2). The sediments were analyzed both in their texture (sieve and/or pipette) and their mineralogical contents (microscopy and X-ray difractometry). Generally, in bottom surface sediments, the psammitic fraction (medium-fine) is predominant, albeit important concentrations of silt-clay are found on the coastal region, while the gravel-type sizes (biogenetic remains, sandstone and “patagonic pebbles”) appear in the more external sector. The 3.5 kHz seismic survey, together with the cores, indicate that the shelf subbottom adjacent to the west and northwest coast is formed by a sequence of fine sediments (silt clay-fine sand), determined acoustically up to a depth of 7 m. Both texture and mineralogy, show lateral continuity towards the inner Bahía Blanca estuary. The seismic records determine a facies with a reflective configuration of the parallel-subparallel type and hummocky, associated with conditions of uniform sedimentation and to a shallow low energy palaeoenvironment, in marked contrast with the present hydraulic regime and sedimentation. During a period when the sea level was lower than the present one, the coastal palaleogeographic configuration would have determined restricted marine conditions, with a relevant continental contribution silt-clay type sediments. This ancient river-estuary sedimentary environment, defines seismically up to Anegada bay, would be formed by channels of various dimensions, as is evident from the seismograms. The silt-clay materials found on the shelf subbottom are thus associated with a wide deltaic palaleoenvironment, which may be put into correlation with the ancient drainage basin of the Colorado river, where marine conditions were influential only due to the effect of tides. It is to be noted that south of the present Colorado river mouth an ancient river bed of it is defined in the subbottom. The transgressive holocene process originated an important increase in the energy of the transport agent. Consequently, sandy material, with pebbles and bioclasts of different sizes and fragmentation, cover in erosive unconformity the silt-clay materials of the subbottom. In the cores, the thickness of this surficial sandy stratum is on the order of 40-50 cm, although towards greater depths seismic information establishes that it increases. Terraces and topographic slopes, together with the presence of pebbles and concentrations of heavy minerals and biogenic remains, represent morphosedimentological relict elements, belonging to periods of stabilization in the marine ascent and to ancient coastal environments. The regional distribution of the bottom sedimentary facies, as well as certain morphological marine features, are thus valuable indicators for inferring the present sedimentary dynamics. On the basis of the textural and statistical results, sedimentological evaluations were carried out to infer transport and sedimentation modes. Likewise, as part of the mineralogical analysis of the sediments, the regional distribution or tendency of the diverse mineral species was also taken into account, since through their different specific weights and differential hydrodynamic behavior, they point to sedimentary sources and/or energy conditions that operate on the shelf investigated. It then emerges from these observations the predominance of the bedload sediment transport in the more external sector of the study area. Contrarily, on the more coastal zone, the influence of silt-clay coming from the inner Bahía Blanca estuary and the Colorado river contribution is evident. In the latter case, considering the textural areal analysis of the sediments, the “sedimentary plume” developed starting  on the west coast, adjacent to the Colorado river mouth, and extending northeastward, is remarkable. The distributive tendency of the heavy mineral concentration clearly points to an important source of sedimentary contribution, related to the ferrotitaniferous deposits present on the shores of the study area, from San Blas bay up to the Negro river mouth. These sediments would be affected by an intense coastal drift towards the zone of the present work. On the other hand, the bed forms, particularly sand waves, tend to reach an equilibrium state relative to the dominant flux conditions. The asymmetry of these geoforms is then an indicative feature of the sedimentary dynamics and circulation as bedload transport. The morphological evaluation of the sand waves found in the northern and southern sectors of the area investigated, in conjunction with analysis of the distribution of the sedimentary surface facies (textural-mineralogical), support the conclusion that the sedimentary bottom dynamics in the shelf environment studied is represented by a northward net bedload sediment transport.

Palabras clave : Sedimentology; Morfology; Seismic; Holocene Evolution; Argentine Continental Shelf.

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