INVESTIGADORES
ALIOTTA Salvador
congresos y reuniones científicas
Título:
DELTAIC SEQUENCES AND PLEISTOCENE PALEOCHANNELS IN THE BAHÍA BLANCA ESTUARY, ARGENTINA
Autor/es:
SALVADOR ALIOTTA; SILVIA SUSANA GINSBERG; VECCHI, LAURA; MARTA E. MINOR SALVATIERRA
Lugar:
Viñadel Mar
Reunión:
Congreso; International Geological Congress on the Southern Hemisphere; 2013
Institución organizadora:
UNIVERSIDAD ANDRÉS BELLO y SOCIEDAD GEOLÓGICA DE CHILE
Resumen:
Ancient morphological-sedimentological conditions and coastal marine dynamics prove to be the main variables in determining different coastal environmental systems. The Bahía Blanca estuarine system (Fig. 1), located in a large coastal area in the south of Buenos Aires province (Argentina), has a morphological configuration resulting from hydrological and sedimentary processes related to late Quaternary sea level changes. Its most distinctive morphological feature is the presence of a network of interconnected tidal channels of different dimensions and depths which are separated by islands and large muddy intertidal flats. Its channels are, in general, meandering and form a mesotidal system. Little is known about of the sedimentary units of the estuary marine sub-bottom. Therefore, a seismic- stratigraphical analysis of the sedimentary sequences of the bottom and marine sub-bottom that characterize the inner zone of Bahía Blanca Estuary (Fig. 1) was carried out using high resolution seismic (3.5 kHz and sparker system) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. To this end, both the sedimentological and morphological features of the estuary were analyzed by means of side scan sonar, echo sounding and bottom sampling. The seismic data analysis allowed us to identify five seismic stratigraphic sequences (S1, S2, S3, S4 and S5) in the study area. The correlation of seismic profiles with borehole data showed the lithological characteristics of the sub-bottom. Sequence S1, the oldest seismic sequence identified, forms a large acoustic basement and is formed of highly compacted, fine-to-medium sand as well as of some silt layers. Its internal structure is characterized by scarce and isolated seismic reflectors. Sequence S2 shows a semitransparent seismic configuration, with horizontal reflectors with weak acoustic response and lateral continuity. This sequence is formed of cemented, silty clay sand. S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age (Arroyo Chasicó Formation-Pampiano Formation). The seismostratigraphic column shows variations in the paleoenvironmental conditions occurring between the late Pleistocene and the early Holocene, which were considerably influenced by a large fluvial drainage network developed in the southwestern area of Buenos Aires province. Thus, the sequences identified as S3 and S4 are indicative of the development of a fluvio-deltaic environment. Deposition of clayey sand layers with inclined bedding sedimentation structures is predominant in sequence S3, oblique prograding, divergent and shingled configurations being the most outstanding seismic facies. In contrast, sequence S4 Fig. 1 - Bahìa Blanca Estuary. Study area.210GEOSUR2013 25-27 NOVEMBER 2013 ? VIÑA DEL MAR (CHILE)evidences a period of reactivation and redistribution of the drainage system during which paleochannel structures with different seismic configurations formed on its floor. The geometric-stratigraphic analysis of the channel-filling structures (symmetric, asymmetric, chaotic) reveals the sedimentary and evolutionary process of the channels. Thus, based on the seismic arrangement of reflectors, a lateral accretion domain can be inferred in those paleochannels whose sedimentary structures display a complex pattern. On the other hand, the seismic reflectors showing relative symmetry between each other indicate that vertical aggradation could have been dominant. It can also be observed that both symmetric and asymmetric filling structures are combined in several of the paleochannels identified. The data collected from the integral analysis of these paleochannels also indicate that during the process of sedimentary deposition, the transversal section of the ancient streambeds became gradually smaller and that they eventually filled up. After having identified these ancient fluvial structures it was possible to reconstruct the paleodrainage pattern. A northwest-southeast orientation could be identified for the streambeds, thus making it possible to associate these runoff pathways with the continental area located to the west of our study area.  The general and distinctive feature of seismic sequences S3 and S4 is its stratification which is marked by reflectors inclined towards the east-southeast. Both the seismostratigraphy and lithology of these sequences confirm their association with old deposits of deltaic lobes which formed part of a large regional Pleistocene drainage system (Colorado River paleodelta?).  Towards the middle Holocene, sea level rise began to affect the Bahía Blanca coastal region. Sequence S5 (medium-sand with variable proportions of biogenic remains and soft-to-moderately compacted silty clay sediments) was formed during the Holocene transgressive-regressive process, covering, in general, the deltaic deposits and completing the seismostratigraphic column defined in the present study.