Salvador Aliotta

A sand wave field in the entrance to Bahia Blanca Estuary, Argentina☆

Abstract Sand waves in the mouth of the Bahia Bllanca Estuary show differences in wavelength, crestlength and crest shape characteristics between two sectors of the field. These differences may be explained by assuming that the residual ebboriented currents are stronger in the southern sector. Crest bifurcations on the centerline are suggestive of intermediate energy conditions and the development of secondary flows oblique to the main current direction. The large lee slopes and the presence of megaripple fans are indicative of flow separation, a condition seldom mentioned for subtidal sand waves. Boundaries of the sand wave field may be controlled by the thickness of the surficial sand layer, rapid reduction of the water depth as produced by a large marginal shoal, and variations in grain size. An average migration rate of 33 m yr−1 was calculated from repeated bathymetric surveys. Assuming that sand waves preserve shape and size, the established net sediment flux over the crests is 0.025 cm3 cm−1 s−1. Current measurements made during four individual tidal cycles yielded a net bedload sediment transport of 0.048 cm3 cm−1 s−1.

Stratigraphy of Holocene sand-shell ridges in the Bahia Blanca Estuary, Argentina

Abstract The Holocene marine transgression has left important testimonies to itself in the Bahia Blanca Estuary, with, mainly in the inner part of the estuary, set almost parallel to the isohypses between 8 and 10 m, sand-shell ridges being found. These structures are composed of a medium to fine sand with a high percentage of shells (47 species of mollusc). Radiocarbon dating of the shells in the oldest ridge gave a 14 C age of 4615 ± 110 yrs B.P. The sand-shell ridges present a parallel bedding with slight inclination toward the sea. The two ridges studied stand discordantly on clay sandy silt containing laminated and flaser structures. These sediments change transitionally with depth to clay silt and silty sand. The marine sediments are distributed on a calcrete of late Pleistocene age. The types of sediments making up the ridges and their stratigraphical relationships are similar to those of cheniers. However, the term chenier is not used here because the sand-shell ridges and their associated deposits have been formed by transgressive-regressive sedimentary processes during the latest postglacial.

Paleochannels related to late quaternary sea-level changes in Southern Brazil

A evolucao geologica da plataforma continental e zona costeira do sul do Brasil no periodo Quaternario e aqui enfocada. Registros sismicos de alta resolucao da Lagoa dos Patos revelaram canais que dissecaram profundamente a regiao antes da formacao da planicie costeira atual. Dois periodos de incisao de canal sao inferidos, baseados principalmente em registros sismicos. Os cursos fluviais mapeados por sismica podem ser conectados aqueles reconhecidos na plataforma continental adjacente por estudos anteriores. a montante, a incisao da drenagem ocorreu principalmente no prisma sedimentar costeiro depositado durante eventos preteritos de nivel do mar alto. A rede de paleodrenagem, reconhecida na planicie costeira e na plataforma continental, representa um sistema de ligacao da bacia de drenagem continental aos depocentros da bacia marginal, sobrepassando a plataforma continental exposta durante uma regressao forcada. A incisao da drenagem na planicie costeira e plataforma continental do Rio Grande do Sul desempenhou um papel importante na arquitetura marginal, distribuicao de facies e no espaco de acomodacao durante as flutuacoes do nivel do mar no Quaternario.

Storm dispersal of volcanogenic sands from Buenos Aires, where heavy-metal concentrations are heavy-mineral segregations

The southeastern beaches and inner shelf of the Buenos Aires coastline are dominated by storms coming from the south and southeast. Erosion is dominant at the coastal cliffs and abrasion platforms, while deposition is extended below the 9-m contour depth. In relation to sand abundance on the inner shelf, there is a northward transition between shelly sand sheets, a fine-sand ribbon field, and sand ridges with oblique megaripples. Side-scan records indicate the selective sorting processes that lead to grain size diminishing to the north, and heavy-mineral enrichment, either at the beach or on the shelf. These storm-induced effects should be considered when evaluating placers on the inner shelf or monitoring the heavy-metal content in sediments. Two-way analyses of variance (ANOVA) tests were used to establish the grain-size effects on heavy-metal analysis.

Sediment Transport Circulation Pattern through Mesotidal Channels System

There are three modes of sediment in motion, which are recognized as rolling and/or sliding; jumped and suspension particles motion. These particles motion include two essential sediment transport modes, which are bedload and suspended load. The first corresponds to the part of the total load which is moving very close to the bed. On this opportunity, the sediment is transported by rolling and/or sliding along the floor or jumping immediately above the bed. The suspended load comprises the particles that are carried away at suspension in the moving fluid. Bedload transport occurs in low velocity flow and/or large grain sizes, while suspended load take place at high velocity flow and/or small grain sizes. The sediment transport theory to point out that sediment capacity flow and sediment availability in channels regulates the motion of sediments. The sediment transport capacity is the maximum load that can be transported by flow and the sediment availability is the material supply that the flow can put in motion. In general, the sediment load that moves in a rolling or saltating mode is smaller compared with the suspended load transported in a natural flow. However, the bedload sediment is important for its contribution to the morphological change of the channel. Moreover, it is the most important factor in determining the stability of natural channel, the bed topography, and therefore, the cause of the hydraulic resistance and some properties of the flow. Accordingly, on this chapter we focus on the bed sediment transport pointing to the pathway migration rather than the sediment transport rate itself. The sediments transport as bedload in estuarine environment controls the bottom morphology. Therefore, estimating the transport of non-cohesive sediments, mobilized by the channel floor, has relevance to the understanding of many pressing environmental problems including eutrophication, contaminant transport, sediment bed erosion, siltation and waste disposal, which impacts on seabed stability and at times on ecosystem distribution. Also, sediments that are moving on the bed generate different bedforms such as shoal, dunes, point bar, etc., which can be hazards to navigation and to affect the flow conditions and the flanks channel stability.

Identification of Bed Forms Through ERS SAR Images in San Matías Gulf, Argentina

Abstract The European Space Agency (ESA) funded two projects in 1991 and 1994 concerning the observation of the Argentine coastal environment with synthetic aperture radar (SAR). Studies were carried out for the periods 1992–1994 and 1994–1997. Several ERS-1/2 satellite SAR images were acquired over the San Matías Gulf in the Patagonian coast during these two periods. SAR is a side-looking imaging radar that operates from either a satellite or an aircraft. The instrument emits a series microwave pulses toward the earth in a direction perpendicular to the flight path. Imagery is constructed from the strength and time delay of the returned signals, which depend primarily on the roughness and dielectric properties of the surface under observation and the distance from the radar. Ocean surface roughness wave-like patterns, imaged as a series of bright and dark linear features by SAR, are persistently observed over the San Matías Gulf mouth region. A total of thirty-three (33) ERS-1/2 SAR images (100 km × 100 km) from 15 different orbits from 1992 to 2000 were analyzed. This series of observations has allowed for a detailed examination of the location, persistence, and the conditions involved in the imaging of the observed wave-like patterns. Very strong tidal currents of the order of 2 m/s characterize the gulf region. The characteristics of recurrent surface patterns in the SAR images indicate that they are caused by the interaction of the tidal currents with bed forms in the bottom topography of the gulf. The location of these bed forms is poorly documented in the available bathymetric charts of the region. The SAR images show the significant potential that satellite radar observations have as a tool for detecting unmapped coastal ocean bottom features, particularly, where bathymetric mapping activities can be extremely difficult, dangerous, or costly.

Seismic Stratigraphy of Pleistocene Deltaic Deposits in Bahía Blanca Estuary, Argentina

The Bahía Blanca estuary (Argentina) has a morphological configuration resulting from hydrological and sedimentary processes related to Late Quaternary sea level changes. This estuarine system occupies a large coastal plain with a dense net of tidal channels, low-altitude islands and large intertidal flats. Little is known about the sedimentary units of the marine subbottom. Therefore, a stratigraphical analysis of the northern coast of Bahía Blanca estuary was carried out using high resolution seismic (3.5 kHz) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. The seismic stratigraphic data collected and their correlation with drilling lithological data show five seismic sequences (S1, S2, S3, S4 and S5), of which S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age. Sequences S3 and S4, whose lithology and seismic facies (paleochannel structures and prograding reflection configurations), were defined on these materials, to evidence the development of an ancient deltaic environment which was part of a large Pleistocene drainage system. The S5 sequence was formed during the Holocene transgressive-regressive process and complete the seismostratigraphic column defined in the present study.

Short term impact of artisanal dredges in a Patagonian mussel fishery: Comparisons with commercial diving and control sites

Mussels in the San Matías Gulf fishery are targeted using artisanal dredges and diving. The main objective of this study was to assess the direct impact of artisanal dredging on the biota and sediments, and to compare the composition of the catches and the individual damage induced by fishing between dredging and commercial diving. The experimental design included samplings from dredge catches, dredge tracks, control sites and commercial diving. According to their damage level, individuals were scored as undamaged, lightly damaged and severely damaged. Sediment characteristics were analyzed using coring samples and traps. Damage of mussels, mostly corresponding to the severely damaged category, was less than 5% both in samples from dredging and diving. Conversely, mean damage of the main bycatch species (sea urchins and ophiuroids) was 75 and 65% in samples from dredging and diving respectively, being most of the individuals lightly damaged. Considering also the catch sample composition of both fishing methods, dredging affected relatively more individuals than diving. Although sediment removal in dredged areas was three times higher than that in non-dredged ones, mean grain size and gravel percentage of sea floor sediments showed subtle differences between them.

Predatory evidence in the high-latitude cold-water gastropod Buccinanops deformis (King, 1832) from the Holocene littoral sand ridges, Argentina, SW Atlantic

Catalina Gómez-Espinosa, Ester Farinati, Salvador Aliotta Predatory evidence in the high-latitude cold-water gastropod Buccinanops deformis (King, 1832) from the Holocene littoral sand ridges, Argentina, SW Atlantic

Bioerosion and encrustation rates in recent mollusk death assemblages on a supratidal siliciclastic setting, Playa Norte, Veracruz State, Mexico

This study presents both qualitative and quantitative data regarding marine mollusk (gastropods and bivalves) shell bioerosion and encrustation based on death assemblages obtained from a recent supratidal environment in Playa Norte, Veracruz State. The objectives of this study were to assess the nature of bioerosion and encrustation processes and to investigate the role of these taphonomic features contributing to the deterioration of natural shell accumulations within a tropical siliciclastic tidal environment. The assemblage comprises 31 species: 13 gastropods and 18 bivalves. The bioerosion and encrustation degrees were low to moderate for both types. The most abundant traces were predatory gastropod structures (Oichnus paraboloides and O. simplex), whereas sponge borings (Entobia isp.), polychaete dwellings (Caulostrepsis taeniola), and echinoid raspings (Gnatichnus isp.) were less frequent. The encrusting organisms include polychaete serpulids, bryozoans, and rare foraminifers (Homotrema rubrum). Because of the low bioerosion and encrustation degrees occurring in this area, accumulation is expected to predominate over biotic destruction. As deposition conditions (richly fossiliferous carbonate sandstone beds) were similar to those prevailing in the Tuxpan Formation during the Miocene (Langhian), it is suggested that this study provides an equivalent reference to interpret mollusk fossil assemblages located in this site.