Teses em Geologia e Geoquímica (Doutorado) - PPGG/IG

URI Permanente para esta coleçãohttps://repositorio.ufpa.br/handle/2011/6341

O Doutorado Acadêmico pertence ao Programa de Pós-Graduação em Geologia e Geoquímica (PPGG) do Instituto de Geociências (IG) da Universidade Federal do Pará (UFPA).

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Fácies, petrografia e geoquímica da Formação Codó neo-aptiano, bacia de São Luís - Grajaú
(Universidade Federal do Pará, 2005) PAZ, Jackson Douglas Silva da; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136; ROSSETTI, Dilce de Fátima; http://lattes.cnpq.br/0307721738107549
The Codó Formation is an important geological unit in Brazil, representing the only record of Neoaptian rocks exposed along the Brazilian equatorial margin. This unit consists of bituminous black shales, limestones and evaporites, which are particularly well represented in the south and east margins of the São Luís-Grajaú Basin, around the towns of Codó and Grajaú, State of Maranhão. These areas were investigated in order to: 1. improve the depositional system, discussing the hypothesis that the Codó Formation was produced in a lacustrine setting; and 2. reconstruct the paleohydrological conditions with basis on the integration of facies, stratigraphy, petrography and isotope (C, O,Sr and S) data. Hence, the field data presented herein confirmed a lacustrine system for the Codó area, where prevailed stable, well-stratified, saline lakes characterized by periods of closure, anoxia and salt precipitation in the central saline lakes. On the other hand, ephemeral conditions with development of a sabkha/saline pan complex prevailed in the Grajaú area, where salts precipitated mostly in the marginal portions of the system (i.e., marginal saline pans and mudflats). Studies focusing facies and stratigraphy also revealed that in both areas the Codó Formation is arranged into several shallowing-upward cycles formed by progradation of marginal into central lake deposits. Three types of cycles were distinguished, referred to here as lower, intermediate and higher rank cycles. The lower rank cycles correspond to millimetric interbeddings of: a) bituminous black shale and evaporite; b) bituminous black shale and calcimudstone; c) bituminous black shale and peloidal wackestone-packstone; d) grey/green shale and calcimudstone; e) grey/green shale and peloidal wackestone-packstone; f) grey/green shale and ostracodal wackestone/grainstone; h) ostracodal wackestone/grainstone and/or calcimudstone with cryptomicrobial mats and ooidal/pisoidal packstone. These are attributed to seasonal deposition with basis on their regular nature forming very thin cycles resembling varves. The intermediate rank cycles average 1.7 m thick and are formed by complete and incomplete cycles. Complete cycles show an upward transition from central to intermediate and then marginal facies associations, and include two types: C1 cycles with central lake deposits consisting of evaporites and black shales; and C2 cycles with central lake deposits formed by gray/green shale. Incomplete cycles are those formed by successions lacking at least one of the facies associations, consisting of either central and intermediate lake deposits (cycles I1) or intermediate and marginal lake deposits (cycles I2). The higher rank cycles average 5.2 m thick and consist of four depositional units, which display shallowing-upward successions formed by both complete and incomplete, intermediate rank cycles that vary their distribution upward in the section, and are bounded by sharp surfaces. Unit 1, the lowermost one, averages 2.7 m in thickness, being entirely composed by thin I1 cycles. Unit 2 averages 5.2 m thick, and displays all of the aforementioned intermediate cycles, especially complete ones. Unit 3, averaging 2.6 m thick, consists of 80% of cycles I2. Finally, unit 4, which averages 2.2 m in thickness, displays only incomplete cycles, though its uppermost part was not preserved due to erosion during the development of the Aptian sequence boundary. The detailed sedimentological characterization and the stratal stacking patterns of the intermediate and higher rank cycles support a genesis linked to syn-sedimentary tectonic activity, particularly suggested by high facies variability, limited lateral extension, as well as frequent and random thickness changes of the intermediate-rank cycles. Additionally, the matching between the four higher rank cycles with four stratigraphic zones having different styles of soft-sediment deformation structures previously described in the literature as resulting from seismic activities, is a further argument to corroborate this interpretation. Therefore, the several episodes of lake shallowing recorded in the intermediate and higher rank cycles of the Codó Formation are attributed to fluctuations in the lake water level, triggered by seismic pulses alternating with sediment deposition. The petrographic analysis of the evaporites from the Codó Formation allowed to better defining both the lake-sabkha-saline pan depositional system and the post-depostional histories. Seven evaporite morphologies were recognized: 1. chevron (selenite) gypsum; 2. nodular/lensoidal gypsum/anhydrite; 3. acicular gypsum; 4. mosaic gypsum; 5. brecciated gypsum/gypsarenite; 6. pseudo-nodular anhydrite/gypsum; and 7. rosettes of gypsum. Despite of this large variety of evaporite phases, the chevron gypsum, the nodular/lensoidal gypsum/anhydrite and the brecciated gypsum/gypsarenite record the preservation of primary features. The association of these morphologies with deposits displaying cyclic horizontal bedding, attributed to lake level fluctuations eventually culminated with subaerial exposure, reinforces this interpretation. Even acicular gypsum and mosaic gypsum, which replaced the chevron and brecciated gypsum/gypsarenite, respectively, formed under the influence of the depositional surface. Burial phases of gypsum are only recorded in the pseudo-nodular anhydrite/gypsum, attributed to salt mobilization induced by halokinesis. In addition, rosettes of gypsum, which crosscut the other evaporite morphologies, diagenetic in origin, have probably formed as the latest evaporite phase of the study area, under the influence groundwater and/or surface weathering. In the present research, isotope studies aiming paleoenvironmental purposes were motivated by both confirmation of strong depositional influence for at least great part of the evaporites from the Codó Formation (i.e., primary and eodiagenetic gypsum), and the low diagenetic modification recorded for the limestones. Results of these approaches show that expansion/contraction cycles in both studied areas were accompanied by significant changes in isotope values. The wide dispersion of Sr and S isotope data within individual depositional cycles reinforces the lack of significant diagenetic modification as suggested by the petrographic analysis, and confirms the utility of these isotopes as environmental tools. Additionally, a non-marine brine source is suggested by 87Sr/86Sr ratios ranging from 0.707824 to 0.709280, which are higher than those from late Aptian seawater (i.e., between 0.70720 and 0.70735). The δ34S varies from 16.12 to 17.89 %o(V-CDT) in the Codó area, which is also in disagreement with late Aptian marine values (ranging from 13 to 16 %o(V-CDT)). Both geochemical tracers were influenced by facies characteristics, and thus a model is provided where expansion of saline pan/lake systems led to decreasing 87Sr/86Sr values due to the inhibition of the 87Sr from clay minerals originated during the internal draining of mudflats. During expansion peaks, the 87Sr/86Sr values were lower due to submergence of mud flats and introduction of external 87Sr-depleted waters related to weathering of Permian to Neocomian marine limestones and evaporites, as well as Triassic to Neocomian basaltic rocks. Furthermore, the sulphur isotope values decrease in the southern margin of the basin from 14.79 to 15.60 %o(V-CDT) probably due to increased evaporation in shallower water settings. While the studies of Sr and S isotopes emphasized the evaporites of the Codó Formation, the analysis of C and O isotopes were carried out on the carbonates. The data revealed a wide distribution of dominantly low δ13C and δ18O values, ranging from –5.69‰ to –13.02‰ and from –2.71‰ to –10.80‰, respectively. It was also observed that these ratios vary according to seismically-induced shallowing-upward cycles, in general becoming lighter in their bases, where central lake deposits dominate, and progressively heavier upward, where marginal lake deposits are more widespread. In addition to confirm a depositional signature for the analysed samples, this behavior led to introduce a seismic-induced isotope model. Hence, lighter isotope ratios appear to be related with flooding events promoted by subsidence, which resulted in the development of a perennial lake system, while heavier isotope values are related to ephemeral lake phases favored through uplift and/or increased stability. Furthermore, the results show that a closed lake system dominated, as indicated by the overall good positive covariance (i.e., +0.42 to +0.43) between the carbon and oxygen isotopes, though open phases are also recorded by negative covariance values of –0.36.
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Proveniência de depósitos albianos do grupo Itapecurú (Bacia de São Luis - Grajaú) com base em petrografia, paleocorrentes, geoquímica e idades de zircão detrítico
(Universidade Federal do Pará, 2006-06-23) NASCIMENTO, Marivaldo dos Santos; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136; GÓES, Ana Maria; http://lattes.cnpq.br/2220793632946285
The Albian deposits comprise ca. 70% of sedimentary fill of the São Luís-Grajaú Basin, with 500 m thick in their main depocenters. The origin and sedimentary evolution of these deposits is related to the breakup of Africa and South America which led to the connection of the formerly separated Central and South Atlantic oceans, in Mesozoic time. Albian exposures in the southern border of the São Luís-Grajaú Basin, Grajaú region, represent a fluvial-deltaic succession whith six depositional environments, including: delta front (mouth) bar, distal bar/prodelta, upper shoreface/foreshore, interdistributary bay/crevasse, fluvial channel and distributary channel. They consist, mainly, of fine- to medium sandstones, as well as siltstones, mudstones and intraformational conglomerate. Palaeocurrent study, petrography and geochemistry of sandstones and heavy minerals, and detrital zircon ages were utilized to investigate the provenance of this sedimentary unit. This study was performed on sandstone samples whose modal compositions plot in the quartzarenite field indicating origin from recycled orogen and continental blocks. High proportion of quartz grains, monocrystaline and polycrystalline, as well as quartzite fragments, and rounded grains of zircon and tourmaline with a wide variety of mechanical surface textures, suggest a provenance from sedimentary rocks, and that at least part of the studied sediments are multicyclic. Chemical weathering processes is little evident as indicated by absence of solution features in these mineral grains. Therefore, the modal composition can be related to the sedimentary recycling or strong reworking on depositional environment. High CIA (Al2O3/Al2O3+K2O+Na2O+CaO*x100) values suggest transportation and recycling from sources located far away from the depositional basin, which is, in turn, consistent with a provenance from the recycled orogens and continental blocks. Based on palaeocurrent patterns and RuZi the Albian succession in the south region of the São Luís-Grajaú Basin was defined four heavy mineral zones, generically named A, B, C and D, in ascending stratigraphic order. Geochemistry of tourmaline grains indicate provenance from metapelites and metapsammites, with few contributions from granite and pegmatite. Zircon Hf patterns suggest changing of the source of these zones: Zone A is characterized by an Hf unimodal distribution, while in the Zones B, C and D, is bimodal. The staurolite shows a relatively limited amount of compositional variations. Involvement of metasediments is inferred from the presence of staurolite and kyanite in the sandstones, as well as by predominance of dravite in the tourmaline populations. Discriminant function analysis using major element compositions show that these deposits were deposited in the passive continental margin (PM). PM sediments are mainly quartz-rich, sourced from craton interiors or stable continental regions, which were deposited in intra-cratonic sedimentary basins or on passive continental margin. The Albian sediments are characterized by LREE enrichment, depletion in HREE, and negative Eu-anomaly. This REE pattern, measured to infer the provenance of sediments and their relationship with average post-Archean upper continental crust, is very consistent with this interpretation. Enriched HREE concentration in the some samples may be attributed to the presence of REE bearing heavy minerals, supported by the fact that these samples have higher concentration of Th, U and Zr, reflecting natural concentration of zircon grains. Pb-Pb geochronological analyses of 238 detrital zircon grains show a direct fingerprint of Precambrian terrains (Archean to Proterozoic) in the source. Three major zircon populations were detected: Archean (3103-2545 Ma), Paleoproterozoic (2460- 1684 Ma) and Neoproterozoic (993-505 Ma); small groups of Mesoproterozoic (1570- 1006 Ma), Paleozoic (440-540 Ma) and Mesozoic (141-314 Ma) grains are also present. The Neoproterozoic component shows an increase upwards with main peaks between 550 Ma and 650 Ma. A similar pattern is shown by the Archean interval, which exhibits a strong relative increase upwards, peaking between 2725 Ma and 2926 Ma, while Paleoproterozoic component has a distinct behavior, showing an evident decrease upwards. The potential source regions were deduced on the basis of palaeocurrent patterns and correlations of detrital zircon age from the sandstones studied with U-Pb and Pb-Pb zircon data from the basement. Our data suggest that the Albian deposits, specially those of the Zone A, were preferentially sourced from the northern and northeastern regions, including São Luís Craton, Gurupi Belt and northwestern portion of Borborema Province. Paleoproterozoic and Neoproterozoic zircon ages as those found in zircons from this zone, are very common in these basement. In contrast, the sediments of zones B, C e D were supplied from the areas located to the south, southwest and, possibly east, involving the eastern portion of the Amazonian Craton/Araguaia Belt, and Borborema Province. Metassedimentary and igneous rocks with similar zircon ages (mainly Archean) have been described in these regions. The ca. 1.0 Ga detrital zircon ages show a correspondence with the Cariris Velhos Event, widely recognized in the central portion of the Borborema Province. In summary, this study demonstrates the effectiveness of an integrated approach to provenance evaluation of Cretaceous sedimentary deposits using petrography, heavy minerals and bulk sediment chemistry, zircon ages, and palaeocurrent data.

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