Maria Augusta Martins da Silva

Sedimentary architecture and depositional evolution of the Quaternary coastal plain of Maricá, Rio de Janeiro, Brazil

The coastal geomorphology of Marica (Rio de Janeiro state) is characterized by a large lagoon and by two sandy barriers that confine a series of small isolated chain-like lagoons. Data collected from ground-penetrating radar and boreholes from the central coastal plain of Marica provided information on the sedimentary architecture and evolution of this area in the Quaternary. Six lithological units were identified comprising three depositional sequences limited by erosional surfaces, related to barrier-lagoon systems that migrated onshore, offshore, and longshore, giving rise to a sedimentary deposit 25 m thick or more. The data reveal a retrograding barrier overlying a basal mud unit which rests in unconformity upon Precambrian basement, thus characterizing an important Pleistocene transgression. A second Pleistocene barrier of 45,000 cal years BP migrated over a lagoonal mud unit (48,000-45,000 cal years BP) reaching over the previous barrier. A progradational phase followed due to a fall of sea level. A long interval of erosion of the barrier created an unconformity that represents the Pleistocene-Holocene boundary. A beachrock in nearby Itaipuacu, 100 m offshore from the present-day beach, dated as 8,500 cal years BP marks the onset of Holocene sedimentation due to gradually rising sea level, which continued until at about 5,000 years ago. This promoted the retrogradation of the barrier-lagoon system. A brief episode of progradation is observed as a series of paleobeach scarps. Todays rising sea level is causing the retrogradation of the barrier.

Surface modification of a granite building stone in central Rio de Janeiro

In order to evaluate environmental controls on the soiling formation and decay of building stones a set of mapping and physical and chemical analyses were carried out on granite from a historical church in the polluted centre of Rio de Janeiro. These techniques highlight the increasing of threatening damage on generally perceived as a durable building material, caused by granular disaggregation and contour scaling in areas close to ground level. Mapping also indicated the formation of black crusts over entire building façades, concentrated on areas sheltered from rain-wash. Analyses demonstrated the influence of marine aerosols, rock and mortar composition and mostly of the atmospheric pollutants on the decay and soiling of the granite. Much of the decay is associated specifically with the presence of halite (NaCl) and gypsum ((CaS04)2H2O). The fact that black, gypsum crusts are able to develop over entire façades in a humid subtropical environment is testimony to the high levels of local pollution, especially particulate deposition. Reduced rainwash, in sheltered micro-environments of narrow, canyon-like streets, overcomes the gypsum tendency to be washed away from buildings façades. These observations further highlight that decay processes are primarily controlled by microclimatic conditions.

A utilização do Ground Penetrating Radar (GPR) em estudos de estratigrafia na praia de Iataipuaçú - Maricá (RJ)

The association between data obtained from ground penetrating radar (GPR) and from boreholes at the Itaipuacu coastal plain (Marica-RJ) revealed information regarding the evolution of this coast. It has been possible to recognize a paleobarrier-lagoon environment and its associated inlets. The results indicated that the sedimentary sequence above basement is about 3 to 4 meters thick on the old sandy barrier and it is 10 meters thick towards the younger sandy barrier. The sedimentary sequence contains several horizontal to subhorizontal reflectors in addition to seaward dipping layers 15 to 40o, similar to the present-day Itaipuacu beach storm scarps. A bore hole trough the younger sandy barrier showed that the main sediment is coarse sand; however, at about 7,5 meters down the section an organic-rich lagoonal mud is present and 14C dating indicated the age of 5.900-6.400 years before present for such lagoonal environment. The water table is just 1 meter below surface on the plain between the two barriers deepening to 4 to 5 meters towards the younger sandy barrier. Water collected from this reservoir presents good quality for human consumption.

Comportamento morfológico e sedimentar da praia de Itaipuaçú (Maricá, RJ) nas últimas três décadas

The main objective of the present study, carried out in the Itaipuacu beach (Marica, Rio de Janeiro state), is to understand the morphological and sedimentary behaviour of this coastline along the last three decades. The methodology consisted of a continuous series of beach profiles employing Emerys method (1961) obtained seasonally along 8 years, associated with grain size analysis of 40 samples collected from the beach surface. The data was then compared to work published in the 70s. The results indicate that the western part of the beach arc is the most dynamic as consequence of local intense energy of S-SW storm waves. Morphology and sediment trends indicate that this dynamics decrease towards the east. The most abundant sediment is very coarse sand, however grain size decreases towards the east where a better sorted coarse sand is the most characteristic beach sediment. Comparison between this 8-year long series of profiles to profiles collected in the 70s shows that, despite the great dynamics of the entire beach and in particular, the western extreme, the beach has maintained the same morphology, more important, its width, which is not an indication of beach erosion at the present time.

Evaporitos como recursos minerais

Evaporites are sedimentary rocks commonly formed in sedimentary environments of low terrigenous input, under arid climate where the evaporation rates are high enough for the formation of brines from which the evaporitic sediments are generated. The economic potential of these rocks is enormous, and halite, the commonly consumed salt, is one of the most important evaporitic minerals for the human beings. In addition to that, evaporitic deposits are frequently associated to environments rich in the production of organic matter, and in the geologic record the association of evaporites and giant petroleum fields is widespread. In this article, we will discuss the processes of evaporite formation and its depositional environments, the most important deposits in the world where evaporites are economically explored will be presented, and the strong association of evaporites and petroleum. The Aptian evaporites of the Sergipe Basin, formed during the initial phase of opening of the South Atlantic Ocean, will be given as an example of exploration of sylvinite, carnallite and tachydrite.

The role of beachrocks on the evolution of the Holocene barrier systems in Rio de Janeiro, southeastern Brazil

ABSTRACT Silva, A.L.C., Silva, M.A.M., Souza, R.S., Pinto, M.L.V., 2014. The role of beachrocks on the evolution of the Holocene barrier systems in Rio de Janeiro, southeastern Brazil. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 170–175, ISSN 0749-0208. Beachrocks at different locations along Rio de Janeiro coast (southeastern Brazil) play an important role in the understanding of coastal evolution and sea-level fluctuations in the Holocene. The studied area presents a long and wide stretch of coastal plain characterized by two sandy barriers, which confine a series of small isolated chain-like lagoons in addition to large lagoons on the reverse side of the inner barrier. The beachrocks occur at different positions in relation to present-day mean sea level. These can be observed as submarine outcrops and along the beaches. The 8.100 years old B.P. beachrocks reveal a phase of retrogradation of the barrier system, and the drowning and partial preservation of this barrier during the sea transgression through the Holocene. The beachrocks are composed of medium-to coarse sand grained bioclastic quartz-rich sandstones which are strongly cemented by calcite. Three successive events of calcite cementation have drastically reduced the porosity of the rock. Calcite cement occurs as three basic forms: isophacous fringe of very fine crystalline calcite, aphanocrystalline calcite and coarsely crystalline calcite. Each textural type indicates different geochemical composition of the pore waters during the very early diagenetic evolution within the intertidal zone of the coastal barrier system.

Comportamento morfológico e sedimentar da praia de Itaipuaçú (Maricá, RJ)

The main objective of the present study, carried out in the Itaipuacu beach (Marica, Rio de Janeiro state), is to understand the morphological and sedimentary behaviour of this coastline along the last three decades. The methodology consisted of a continuous series of beach profiles employing Emery’s method (1961) obtained seasonally along 8 years, associated with grain size analysis of 40 samples collected from the beach surface. The data was then compared to work published in the 70’s. The results indicate that the western part of the beach arc is the most dynamic as consequence of local intense energy of S-SW storm waves. Morphology and sediment trends indicate that this dynamics decrease towards the east. The most abundant sediment is very coarse sand, however grain size decreases towards the east where a better sorted coarse sand is the most characteristic beach sediment. Comparison between this 8-year long series of profiles to profiles collected in the 70’s shows that, despite the great dynamics of the entire beach and in particular, the western extreme, the beach has maintained the same morphology, more important, its width, which is not an indication of beach erosion at the present time.

OCORRÊNCIA E ORIGEM DE BOLAS DE LAMA NAS PRAIAS DA ENSEADA DE JURUJUBA (BAÍA DE GUANABARA) - NITERÓI-RJ

This paper studies at the occurrence and origen of armoured mud balls on Sao Francisco and Charitas beaches - Jurujuba Sound (eastern Guanabara Bay), frequently found in the region after storms. The mud balls are mainly black, light grey and greenish, of various sizes, and covered by biodetritus, quartz or other fragments. The mud balls are similar to ones previously studied on a variety of modern environments including marine and lacustrine shorelines. Armoured mud balls can potentially become preserved in the geological record, thus the understanding of the modern environments of formation will aid the reconstruction of paleoenvironments. Mineralogical and geochemical studies allow the correlation between the mud balls and