Ana Maria Góes

Palaeodrainage on Marajó Island, northern Brazil, in relation to Holocene relative sea-level dynamics

A large area in northeastern Marajó Island, northern Brazil, has been characterized geomorphologically, applying information acquired from Landsat imagery. This study was combined with detailed sedimentologic analysis of continuous cores, which provided a record of depositional settings developed in this area through the Holocene. The results revealed well-preserved, meandering to anastomosed drainage networks of wide palaeochannels that were superimposed by a narrower palaeochannel system. In both cases, the sedimentary record consists of sands, heterolithic deposits and muds, locally rich in plant debris. The strata are organized into fining upward successions that reach approximately 18 m thick in the wide channels and 4 m thick in the narrow channels. Sedimentary features suggestive of a coastal location for the wider palaeochannels and reworking of sediments by tidal currents include the prevalence of well to moderately sorted, rounded to sub-rounded, fine- to medium-grained sands displaying foreset packages separated by mud couplets, suggestive of tidal cycles. The data presented herein point to a rise in relative sea level reaching the Lake Arari area during the early to late/mid Holocene. This event was followed by a relative sea level drop. Tectonics seem to have contributed to an overall lowering in relative sea level in the study area since the mid-Holocene, which does not follow the same pattern recorded in other areas along the northern Brazilian coast.

Marine influence in the Barreiras Formation, State of Alagoas, northeastern Brazil

Tidal processes were important for deposition of the Barreiras Formation located in northern Brazil, while correlatable deposits in northeastern Brazil have been traditionally related to continental environments. Facies analysis in southern Alagoas revealed that the Barreiras Formation consists of cross-stratified conglomerates and sandstones (facies Sx and Cgx), compound cross-stratified sandstones (facies Cx), and heterolithic beddings (facies H). A significant portion of these deposits occurs within channel morphologies displaying fining and thinning upward successions. An abundance of sedimentary features is comparable to those from the northern Brazilian counterpart. These include: tidal bundles; herringbone cross-stratification; heterolithic beddings with sandstone and mudstone beds in sharp contacts; and ichnofossils mostly consisting of Ophiomorpha nodosa, Skolithos and Planolites. Altogether, these features point to a marginal marine depositional setting dominated by tidal processes, which are related to an estuarine system, an interpretation also provided for the Barreiras Formation in northern Brazil. The widespread occurrence of deposits with unambiguous evidence of tidal processes in the Barreiras Formation of northern Brazil, and now in the State of Alagoas, leads to argue that the early/middle Miocene worldwide marine transgression might have left a much more widespread sedimentary record along the Brazilian coast than currently regarded.

Contribution to the stratigraphy of the onshore Paraíba Basin, Brazil

Several publications have contributed to improve the stratigraphy of the Paraiba Basin in northeastern Brazil. However, the characterization and distribution of sedimentary units in onshore areas of this basin are still incomplete, despite their significance for reconstructing the tectono-sedimentary evolution of the South American passive margin. This work provides new information to differentiate among lithologically similar strata, otherwise entirely unrelated in time. This approach included morphological, sedimentological and stratigraphic descriptions based on surface and sub-surface data integrated with remote sensing, optically stimulated luminescence dating, U+Th/He dating of weathered goethite, and heavy mineral analysis. Based on this study, it was possible to show that Cretaceous units are constrained to the eastern part of the onshore Paraiba Basin. Except for a few outcrops of carbonatic rocks nearby the modern coastline, deposits of this age are not exposed to the surface in the study area. Instead, the sedimentary cover throughout the basin is constituted by mineralogically and chronologically distinctive deposits, inserted in the Barreiras Formation and mostly in the Post-Barreiras Sediments, of early/middle Miocene and Late Pleistocene-Holocene ages, respectively. The data presented in this work support tectonic deformation as a factor of great relevance to the distribution of the sedimentary units of the Paraiba Basin.

Late Quaternary drainage dynamics in northern Brazil based on the study of a large paleochannel from southwestern Marajó Island

Marajó Island shows an abundance of paleochannels easily mapped in its eastern portion, where vegetation consists mostly of savannas. SRTM data make possible to recognize paleochannels also in western Marajó, even considering the dense forest cover. A well preserved paleodrainage network from the adjacency of the town of Breves (southwestern Marajó Island) was investigated in this work combining remote sensing and sedimentological studies. The palimpsest drainage system consists of a large meander connected to narrower tributaries. Sedimentological studies revealed mostly sharp-based, fining upward sands for the channelized features, and interbedded muds and sands for floodplain areas. The sedimentary structures and facies successions are in perfect agreement with deposition in channelized and floodplain environments, as suggested by remote sensing mapping. The present study shows that this paleodrainage was abandoned during Late Pleistocene, slightly earlier than the Holocene paleochannel systems from the east part of the island. Integration of previous studies with the data available herein supports a tectonic origin, related to the opening of the Pará River along fault lineaments. This would explain the disappearance of large, north to northeastward migrating channel systems in southwestern Marajó Island, which were replaced by the much narrower, south to southeastward flowing modern channels.

Petrografia de arenitos e minerais pesados de depósitos cretáceos (Grupo Itapecuru), Bacia de São Luís-Grajaú, norte do Brasil

Petrography, heavy minerals and quartz analyses, palaeocurrent data revaled the sedimentary story and potential source areas of sandstones of the Itapecuru Group in the southern margin of the Sao Luis-Grajau Basin. The studied deposits consist of quartzarenites with heavy mineral assemblage consisting of tourmaline, zircon, staurolite, rutile and kyanite with a wide variety of forms, mechanical surface textures, and colors. The predominance of rounded zircon and tourmaline grains suggests a strong contribution from recycled sediments from sedimentary and metassedimentary rocks. Quartz grains exhibit irregular forms, monocrystaline and polycrystalline types and surface textures similar to the heavy minerals grains. The modal composition in a Q-F-Lt plot suggests their origin from continental block and recycled orogen. The potential sources were mainly sedimentary rocks, as well as medium -to high-grade metamorphic and granitic rocks. The studied profile was subdivided into four stratigraphic levels A, B, C and D, through palaeocurrent data and RuZi index, which indicate change in source areas. Palaeocurrent pattern suggests that the Zone A, at the base of the succession, were sourced from the northern and northeastern regions, including the Sao Luis Craton, Gurupi Belt, and northwestern portion of the Borborema Province. Sediments of the zones B, C and D were supplied from the southern, southwestern and southeastern regions, including the Amazonian Craton, Araguaia Belt, and Borborema Province.

Heavy mineral as a tool to refine the stratigraphy of kaolin deposits in the Rio Capim Area, Northern Brazil

Studies of heavy minerals in kaolin deposits from the Ipixuna Formation in the Rio Capim area (Northern Brazil) showed a mature to super mature assemblage dominated by zircon and tourmaline, and subordinately rutile, kyanite and staurolite. These minerals do not change much throughout the whole section; however, each kaolin unit displays a particular signature, defined by differences in the proportions of the whole assemblage of heavy minerals, as well as of their textural characteristics. This work revealed that the lower and upper kaolin units can be definitely considered as distinct depositional sequences. A higher proportion of opaque minerals and higher zircon values characterize the lower unit. The higher volumes of anhedric, rounded to sub-rounded grains of zircon and tourmaline in the upper unit suggests that this includes grains that were undergone to a higher degree of reworking. The increased volume of unaltered staurolite and kyanite in the upper unit leads to conclude that, even considering sediment reworking, a distinct source must be invoked. The results also show that the characteristics of the heavy mineral assemblage from the intermediate unit are comparable with those from the upper unit, which suggests they might record a same stratigraphic sequence.

Discriminação dos depósitos cenozoicos da parte emersa da Bacia Paraíba (NE, Brasil) por meio de minerais pesados e granulometria

Controversies about the stratigraphy of the Cenozoic deposits of the Paraiba Basin outcropping onshore motivated the analysis of the heavy minerals assemblage and the morphology of zircon and tourmaline grains with the purpose of improve the stratigraphic framework of the basin. These studies revealed strong similarity among the heavy mineral assemblage of the outcropping stratigraphical units (Barreiras Formation and Post-Barreiras I and II units), with the predominance of zircon, tourmaline, kyanite and rutile. An improved stratigraphic discrimination was obtained using morphological analysis of zircon and tourmaline grains, with increasing grain roundness towards younger units. Moreover, the fact that the ZTR index increases gradually towards younger units, and that the RZ index remains constant suggests that older sedimentary units and, subordinately, metamorphic rocks from the Transversal Zone of the Borborema Province are the main sources of the studied Cenozoic sediments.

Proveniência de sedimentos da Bacia de Curitiba por estudos de minerais pesados

Sediment provenance of the Curitiba basin by study of heavy minerals. Curitiba Basin is located in the South-central portion of the first plateau of Parana. It includes the city of the same name and some towns of the metropolitan region. It is a neogean basin elongated towards ENE direction, which belongs to the Cenozoic Rift System in Southeast Brazil. Its maintained sedimentary fill extends over 3,000 km2 and includes sludge, arkosic sands, and gravels of Guabirotuba Formation. Among the basins that are part of the Cenozoic Rift System of Southeast Brazil, Curitiba Basin is the least studied in details. Until the present moment, researches on this accessory assembly were not disclosed. We have analyzed transparent, nonmicaceous and nonmagnetic heavy minerals, with fraction of particle size from 3 - 4 φ (0.125 - 0.062 mm), which were separated using bromoform (d = 2.89 g/cm³) and handheld magnet. The determination was done with permanent assemblies of grains and Canada balsam (n = 1.53), analyzed in a petrographic microscope. In the mineralogical assembly, zircon grains, epidote, tourmaline, kyanite, rutile, and traces of sillimanite and garnet were predominantly found. Zircon occurs as euhedral to sub-euhedral grains, colorless to subordinate pink and metamict. The study of heavy minerals in Curitiba Basin suggests that towards the center of the basin, zircon quantities tend to decrease while amounts of epidote increase. Two mineralogical associations were characterized: the first, called High Zircon, has zircon quantities between 51 to 97%; the second one was called High Epidote and has relative epidote amounts between 31 to 53%. Such differences in zircon and epidote values allow to infer the possibility of having more than one source area for these sediments. The main paleocurrents trends indicated the granites of Graciosa Province and metamorphic rocks of the Atuba Complex as potential source rocks. Moreover, one cannot rule out the possibility of hydraulic segregation. Since zircon is denser, it tends not to move so much the basin transport as the epidote, which presents the lowest density. Therefore, it was concluded that the relative proportion between these two minerals would be a good tracer of the dynamics of sediment transportation in the basin.

Sistema eólico úmido nas sucessões sedimentares interderrames da Formação Mosquito, Jurássico da Província Parnaíba, Brasil

Relativo a tectonica dos eventos precursores da genese doOceano Atlântico central, na Provincia Parnaiba ocorre a FormacaoMosquito, uma sucessao de derrames basalticos com intercalacoes decamadas sedimentares. Estes depositos acumularam-se sobre paleobaixostopograficos e seus contatos sao erosivos na base e no topo dassucessoes sedimentares. No vale do rio Mosquito encontra-se a maisespessa sucessao sedimentar: o Membro Macapa. Este consiste em umasucessao de arenitos com as associacoes de facies de campos de dunaseolicas com depositos de interduna encharcada, lencois de areia comfluviais efemeros e playa siliciclastica. Os pacotes de sucessoes sedimentaresinterderrames isolados e de pouca continuidade sao interpretadascomo associacoes de 1) margem de playa lake; 2) lencois deareia; e 3) campos de dunas eolicas. O nivel relativo do lencol freaticofoi o fator que mais influenciou a sedimentacao das unidades interderrames,controlando a disponibilidade sedimentar e a superficie deacumulacao. A variacao do nivel freatico foi resultante das variacoesclimaticas e, provavelmente, das condicoes da taxa de subsidencia. O nivel relativo do lencol freatico foi o fator que mais influenciou a sedimentacao das unidades interderrames da Formacao Mosquito, controlando a disponibilidade sedimentar e a superficie de acumulacao. A variacao do nivel freatico foi resultante das variacoes climaticas e provavelmente, das condicoes da taxa de subsidencia.

Faciologia e contexto deposicional da Formação Guabirotuba, Bacia de Curitiba (PR)

A Bacia de Curitiba situa-se na porcao meridional do Sis- tema de Riftes Cenozoicos do Sudeste do Brasil (SRCSB). Possui cerca de 3.000 km2 de sedimentos cenozoicos com espessura preservada de ate 80 m, denominados de Formacao Guabirotuba. As pesquisas realizadas se ocuparam da caracterizacao sedimentologica e evolucao da compar- timentacao tectonica da bacia. Porem, a distribuicao das associacoes de facies sedimentares e seu contexto deposicional nao foram analisados de modo sistematico e, ainda hoje, prevalecem interpretacoes baseadas nos levantamentos das decadas de 1960 e 1980 ou em correlacoes com ou- tras bacias do SRCSB. E nesta circunstância que este estudo investigou o contexto deposicional dos sedimentos da Bacia de Curitiba por meio da analise dos elementos arquitetonicos e sua distribuicao em exposicoes de superficie. A arquitetura deposicional e os rumos medios de paleo- correntes permitem interpretar que as associacoes A, B, C e D sao de- positos de transicao entre as zonas proximais, intermediarias, e distais de sistemas distributarios fluviais. Por outro lado, as associacoes E e F exibem caracteristicas faciologicas e sedimentologicas distintas em rela- cao as demais associacoes de facies. Sua distribuicao restrita sugere que sistemas fluviais contemporâneos aos demais, mas com sutis diferencas se formaram nos setores oeste e nordeste da bacia. Estes setores coincidem com aqueles onde foram descritas as formacoes Tinguis e Piraquara, in- terpretadas como unidades distintas da Formacao Guabirotuba. A abun- dante quantidade relativa de lama e a ausencia de facies eolicas e/ou de feicoes sedimentares de exposicao e ressecacao, sugerem que tais sistemas formaram-se em condicoes de clima razoavelmente umido.