Navegando por Autor "SILVA, Pedro Augusto Santos da"

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O mar epicontinental Itaituba na região central da Bacia do Amazonas: paleoambiente e correlação com os eventos paleoclimáticos e paleoceanográficos do carbonífero
(Universidade Federal do Pará, 2019-03-14) SILVA, Pedro Augusto Santos da; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Carboniferous paleogeography of the West Gondwana was dominated by epicontinental seas connected with the Panthalassa Ocean to the west. 50m-thick Pennsylvanian mixed siliciclastic-carbonate transgressive succession of the Amazonas Basin, Northern Brazil, were studied in outcrops and cores using facies and stratigraphic analysis in combination with O- and C-isotopic stratigraphy. Thirty-four facies, representative of coastal to shelf depositional systems grouped in three facies associations (FA): FA1) coastal desertic deposits, consisting of fine to medium-grained sandstone, mudstone and fine-grained dolostone that corresponds to a complex association of aeolian dunes, sand sheets, interdunes, fluvial channels and lagoon deposits bioturbated by Palaeophycus, Lockeia, Thalassinoides and Rosselia trace fossils; FA2) mixed tidal flat setting, constituted by fine to medium-grained sandstone, mudstone, shale, siltstone, lime mudstone and fine dolostone interpreted as supratidal, tidal channel, tidal delta and lagoon deposits with some brachiopod and echinoderm body fossils; and FA3) carbonate shelf deposits, consisting of lime mudstone, wackestone, packstone and grainstone with allochems (ooids and peloids), terrigenous grains and abundant and diversified open shallow marine benthic organisms, including, fish remains, foraminifers, brachiopods, echinoderms, gastropods, bryozoans, trilobites, corals, ostracodes, and conodonts, interpreted as bioclastic bars and carbonate shelf deposits. The conodonts species Neognathodus symmetricus, Streptognathodus sp. and Ellisonia sp. in the FA3 indicate the Baskirian-Moscovian age. The dolomitization affected the limestone and sandstone of AF1 and AF2 replacing the micritic matrix and occur as saddle dolomite indicating mixed of meteoric and marine waters and late burial. The neomorphism of opaque micritic matrix and bivalve shells are indicated by the growing of xenotopic mosaic of calcite crystals. In contrast, the secondary calcite cement is equant, fiber, bladed and espatic. Micritization is found in the bioclast shells exhibiting micritic coatings. The autigenesis of quartz and biogenic pyrite is commonly found in FA2 and FA3. The mechanic and chemical compaction in limestone caused the porosity reduction, cementation, fractures and development of dissolution seams and stylolite. The sandstones were cemented by quartz, calcite and iron hydroxides/oxides and show concave-convex and sutured contacts between quartz grains. The predominance of eodiagenetic and subordinate mesodiagenetic features in the Monte Alegre-Itaituba succession indicated less modified framework by the diagenesis corroborating the pristine signature of δ13C values ranging from ~ -2 to ~+5.28‰. This enriched trend upsection coadunate with high organic productivity triggered by massive flourishment of euphotic-controlled benthic organisms mainly in the FA3. Five types of shallowing upward asymmetric cycles characterize the Monte Alegre-Itaituba succession. Peritidal cycles in coastal desertic (Cycle I) were formed by alternance of dolostone and sandstone with δ13C values ranging from -1.5 to +0.3‰. Cycles II consist in interbedded of sandstone-mudstone and sandstone-mudstone-floatstone rhythmites and the Cycle III constitute dolostone interbedded with sandstone. These cycles II and III were interpreted as tidal flat and lagoon deposits with δ13C values ranging from +3,98%₀ to +4, 62%₀. The Cycle IV is a rhythmite formed by wackestones/mudstones couplets while the Cycle V consists of alternance of grainstones, wackestones and lime mudstone (ABC cyclicity) passing upsection for cycles composed by wackestones and lime mudstone (AB cyclicity). The cycles IV and V are shelf deposits with δ13C values ranging from +3,65%₀ a +5,28%₀. The stacking of 53 cycles with average thickness of 1,1 m, combined with Fisher plot diagram, indicated an aggradational to retrogradational stacking pattern inserted in the lowstand to early transgressive system tract (Cycles I-III) and late transgressive system tract (Cycles IV and V). The succession was deposited in ~13 Ma and individual cycles accumulated in an approximately 0.25 my typical of fourth order cycles related to high-frequency fluctuations of relative sea level. These data were correlated to the global δ13C and sea-level curves that positioned the Monte Alegre-Itaituba succession in the Late Serpukhovian to Early Moscovian age. The influence of Late Misssipian glaciation was negligible in these deposits but the post-glacial transgression combined with slow subsidence of the Amazonas basin caused the generation of allogenic cycles I, IV and V. The cycles II and III were formed by autochthonous processes during a period of equilibrium between supply and glacioeustasy. The Monte Alegre-Itaituba succession is the record of a large Amazonia epicontinental sea that was directly connected with the Panthalassa Ocean during Pennsylvanian.
Acesso aberto (Open Access)
Paleoambiente e diagênese da formação Itaituba, carbonífero da bacia do Amazonas, com base em testemunho de sondagem, região de Uruará, Pará
(Universidade Federal do Pará, 2014-08-04) SILVA, Pedro Augusto Santos da; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Carboniferous Itaituba Formation represents expressive retrograding carbonate sedimentation included in the sedimentary evolution of the Tapajós Group of the Amazonas basin. These carbonate consist of fossiliferous limestones, fine grained dolostones, fine to coarse grained sandstones and subordinate reddish siltstones, black shales and evaporites. Facies and microfacies analysis of drill cores from the Uruará region, State of Pará , allowed to individualize nineteen facies grouped into five facies associations : tidal plain ( FA1 ) , tidal channel ( FA2 ), lagoon ( FA3 ), bioclastic bar ( FA4 ) and outer shelf ( FA5 ). FA1 is composed of fine grained sandstone with rip-up clasts and mud cracks , marl with quartz and feldspar grains, laminated dolomudstone with fine terrigenous grains and silicified dolostone with intercalation of mudstone and disseminated quartz grains dolomitized and locally with microcrystalline quartz. FA2 consists of medium to coarse grained sandstone with trough cross-bedding and mud drapes on foresets, very fine to fine grained sandstone with wavy bedding, laminated siltstone, synsedimentary faults and convolute lamination. FA3 consists of massive red siltstone, mudstone with fossils, brachiopods and floatstone with disseminated pyrite and massive mudstone with frequent quartz grains. AF4 and AF5 exhibit abundant bioclasts represented by spines and fragments of echinoderms, shells, spines and fragments of brachiopods, ostracods, foraminifera, red algae and bilvave shells. AF4 consists of fossiliferous oolitic grainstone and grainstone with terrigenous grains, mainly monocrystalline quartz grains and the AF5 consists of fossiliferous wackestone, wackestone with terrigenous grains and massive mudstone with monocrystalline quartz grains. Subarkoses (AF1), arkoses (AF2) and lithic arkoses (AF2) predominate in the Itaituba Formation and are composed by of polycrystalline and monocrystalline quartz grains, K-feldspar, plagioclase, pyrite, detrital muscovite, mudstone, metamorphic and chert fragments and rare bioclasts. The cement is of nonferrous calcite, iron oxides/hydroxides, silica overgrowth and intergranular, moldic and sometimes elongated porosities reaching up to 11 % of the rock. The diagenetic processes in sandstone are physical compaction, quartz overgrowth, non-ferrous calcite cementation, chemical compaction, grain replacement, pyrite autigenesis, formation of iron oxides/hydroxides and alteration of plagioclase. The diagenetic processes in carbonates are: micritization, neomorphism, dolomitization, fracturing, chemical compaction, calcite cementation, secondary dissolution and mineral autigenesis. The Itaituba succession is interpreted as a lagoon/tidal flat system linked to the marine carbonate platform. Tidal flats developed on the margins of the lagoons were periodically supplied by fine (silt) terrigenous influxes that inhibited the carbonate precipitation. Bioclastic bars were cut by tidal channels (inlet) connected the lagoon with the shallow platform rich in benthic organisms.