Navegando por Assunto "Carbonate platform"

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Acesso aberto (Open Access)
Gravimetria e estratigrafia cenozoica da porção onshore da Bacia do Marajó e Plataforma Bragantina, nordeste do Pará
(Universidade Federal do Pará, 2021-12-12) SANTOS JÚNIOR, Gilberto Carneiro dos; MARTINS , Cristiano Mendel; http://lattes.cnpq.br/8303640454649778; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Northern Brazilian coast was an exceptional depositional site for siliciclastic and carbonate deposits during Oligocene to Holocene distributed in several basins and continental platforms that include Foz do Amazonas, Pará-Maranhão, Marajó and Bragança-Vizeu basins adjacent to the Bragantina and Ilha de Santana platforms. Although these geotectonic compartments are regionally well-know, their tectonic limits are generally inferred in subsurface by geophysical data. Likewise, these surface limits are poorly defined due to the extensive Cenozoic sedimentary cover. This study allowed to obtain a new interpretation for the gravimetric field related to intracrustal or residual density contrast for the eastern portion of the Marajó Basin. The presence of a deep depression at the limit of this basin with the Bragantina Platform, the Vigia-Castanhal Trough, confirm previous geophysical interpretation, as well as, attested by subsurface and surface stratigraphic studies. Furthermore, the identification of low gravity values below the -10 mGal anomaly with high thickness of sediments suggests a strong subsidence not compatible with the previous interpretation of a platform between the Marajó Basin and the Bragantina Platform denominated “Pará Platform”. This segment is frankly subsiding with depressions below -30 mGal being included here as part of the eastern edge of the Marajó Basin. The platforms are here interpreted with anomalies above -10 mGAL exemplified by the Bragantina Platform. Following this tectonic proposal and using previous stratigraphy, the sedimentary filling of the eastern part of the Marajó Basin is represented by siliciclastic deposits from the Marajó and Barreiras formations, respectively from the Oligocene-Miocene and Middle Miocene. Coastal and marine carbonate deposits of the 120 m-thick of Middle Oligocene-Miocene Pirabas Formation fill the reduced accommodation space of the Bragantina Platform. This unit is overlaid by ~40 m of fluvio-coastal deposits of the Barreiras Formation. The Quaternary is represented by the Post-Barriers unit of the Pleistocene-Holocene and recent sediments that form the alluviums and coastal sandy strand plains. The structural limits of the Bragantina Platform are re-evaluated and the inland portions to the south are exposed Silurian sedimentary and crystalline rocks. These basement rocks were a geographic barrier to the Oligocene-lower Miocene transgression. The use of residual gravity anomaly based on crustal modeling combined with geological and stratigraphic data is an effective tool to assess the geotectonic compartments limits covered by Cenozoic sedimentation and provide a new evolutionary understanding for this part of Amazonia.
Acesso aberto (Open Access)
O Neógeno da Plataforma Bragantina e da parte leste da Bacia do Marajó, norte do Brasil: Paleoambiente, Proveniência e relação com a evolução do Proto-Amazonas
(Universidade Federal do Pará, 2021-11-16) BAIA, Lohan Barbosa; SILVA JUNIOR, José Bandeira Cavalcante da
During the lower Miocene, tectonic stability on the Bragantina Platform favored the accumulation of carbonate deposits, later suppressed by a siliciclastic system associated with the Barreiras Formation. Previous works suggest that this one came from the sedimentary plume of the proto-Amazonas, while others indicate an independent evolution. Based on the above, this study applied provenance techniques to indicate the tectonic compartments that contributed as source rocks of the Barreiras Formation, discuss the influence of the proto-Amazonas in the suppression of the carbonate platform and understand the paleogeographic context. The study area covers the municipalities of Ourém (OU), Capanema (CA), Castanhal (CS), Santa Isabel do Pará (SI), Outeiro (OT), Mosqueiro island (IM), Salinópolis (SA), Aricuru (AR) and Mocooca (MO). Facies analysis techniques were performed with paleocurrent measurements, and 12 samples were collected, containing around 500 g of sediment each. Clasts were collected for morphological characterization in Ourém and Mosqueiro island. For the unconsolidated sediment, sieving was performed targeting the 250 - 125 µm and 125 - 63 µm fractions. Oxalic acid (50 g.L-1) was applied to these fractions and the light and heavy minerals were separated by density in bromoform (2.8 g/ml). Twenty-four petrographic thin sections of light minerals were prepared for cathodoluminescence (CL) and 24 thin sections of heavy minerals for identification of the mineralogical assembly, analysis of shape and elaboration of graphs. From the results of heavy minerals, it was possible to group the values with data from previous studies and apply the Inverse Distance Weighting method. The results indicate the presence of 10 facies, which were grouped into two associations, covering conglomerates, sandstones and pebbles. The clasts presented exclusively quartzous composition, predominating subangular to rounded shapes. The CL indicates a predominance of 45% of quartz with dark blue luminescence, 28% with light blue luminescence and 26% with violet luminescence. The results of heavy minerals indicate the presence of staurolite (54.45%), zircon (20.29%), tourmaline (10.02%), kyanite (7.19%), rutile (5.37%) and sillimanite (2.67%). Values above 50% of the sum of the percentages of zircon, tourmaline and rutile indicate an increase in maturity towards the coastal portion, while the distribution maps show a predominance of staurolite in the Bragantina Platform and an increase in kyanite values following those of tourmaline and zircon in the Marajo Basin. From the faciological interpretations, we were able to improve paleoenvironmental reconstruction consisting of alluvial fans, intertwined and meandering rivers, sandy plains, muddy plains and mangroves. The CL values and the high concentration of staurolite reflect sources of metamorphic rocks for the Bragantina Platform that come from southeastern Pará, possibly from the Gurupi Belt and São Luís Craton; while the CL values for the Marajó Basin, linked to the values of kyanite, zircon and tourmaline, indicate a strong influence of plutonic and metamorphic sources from southern Pará, such as the Tocantins-Araguaia Belt, Grajaú Basin and Sub -Cametá Basin. In general, the proto-Amazonas had no influence on carbonate suppression, the main phenomenon being responsible for the progradation of sediments, from metamorphic rocks with smaller contributions of plutonic and volcanic rocks, from the evolution of tectonics in the south and southeast of Pará.
Acesso aberto (Open Access)
O Neógeno da Plataforma Bragantina e da parte leste da Bacia do Marajó, norte do Brasil: Paleoambiente, Proveniência e relação com a evolução do Proto-Amazonas
(Universidade Federal do Pará, 2021-11-16) BAÍA, Lohan Barbosa; SILVA JUNIOR, José Bandeira Cavalcante da; http://lattes.cnpq.br/8615194741719443
During the lower Miocene, tectonic stability on the Bragantina Platform favored the accumulation of carbonate deposits, later suppressed by a siliciclastic system associated with the Barreiras Formation. Previous works suggest that this one came from the sedimentary plume of the proto-Amazonas, while others indicate an independent evolution. Based on the above, this study applied provenance techniques to indicate the tectonic compartments that contributed as source rocks of the Barreiras Formation, discuss the influence of the proto-Amazonas in the suppression of the carbonate platform and understand the paleogeographic context. The study area covers the municipalities of Ourém (OU), Capanema (CA), Castanhal (CS), Santa Isabel do Pará (SI), Outeiro (OT), Mosqueiro island (IM), Salinópolis (SA), Aricuru (AR) and Mocooca (MO). Facies analysis techniques were performed with paleocurrent measurements, and 12 samples were collected, containing around 500 g of sediment each. Clasts were collected for morphological characterization in Ourém and Mosqueiro island. For the unconsolidated sediment, sieving was performed targeting the 250 - 125 μm and 125 - 63 μm fractions. Oxalic acid (50 g.L-1) was applied to these fractions and the light and heavy minerals were separated by density in bromoform (2.8 g/ml). Twenty-four petrographic thin sections of light minerals were prepared for cathodoluminescence (CL) and 24 thin sections of heavy minerals for identification of the mineralogical assembly, analysis of shape and elaboration of graphs. From the results of heavy minerals, it was possible to group the values with data from previous studies and apply the Inverse Distance Weighting method. The results indicate the presence of 10 facies, which were grouped into two associations, covering conglomerates, sandstones and pebbles. The clasts presented exclusively quartzous composition, predominating subangular to rounded shapes. The CL indicates a predominance of 45% of quartz with dark blue luminescence, 28% with light blue luminescence and 26% with violet luminescence. The results of heavy minerals indicate the presence of staurolite (54.45%), zircon (20.29%), tourmaline (10.02%), kyanite (7.19%), rutile (5.37%) and sillimanite (2.67%). Values above 50% of the sum of the percentages of zircon, tourmaline and rutile indicate an increase in maturity towards the coastal portion, while the distribution maps show a predominance of staurolite in the Bragantina Platform and an increase in kyanite values following those of tourmaline and zircon in the Marajo Basin. From the faciological interpretations, we were able to improve paleoenvironmental reconstruction consisting of alluvial fans, intertwined and meandering rivers, sandy plains, muddy plains and mangroves. The CL values and the high concentration of staurolite reflect sources of metamorphic rocks for the Bragantina Platform that come from southeastern Pará, possibly from the Gurupi Belt and São Luís Craton; while the CL values for the Marajó Basin, linked to the values of kyanite, zircon and tourmaline, indicate a strong influence of plutonic and metamorphic sources from southern Pará, such as the Tocantins-Araguaia Belt, Grajaú Basin and Sub -Cametá Basin. In general, the proto-Amazonas had no influence on carbonate suppression, the main phenomenon being responsible for the progradation of sediments, from metamorphic rocks with smaller contributions of plutonic and volcanic rocks, from the evolution of tectonics in the south and southeast of Pará.