Navegando por Assunto "Estratigrafia de sequências"

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Acesso aberto (Open Access)
O Cambriano no Sudeste do Cráton Amazônico: paleoambiente, proveniência e implicações evolutivas para o Gondwana Oeste
(Universidade Federal do Pará, 2018-06-15) SANTOS, Hudson Pereira; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
Transgressive events recorded in many cratonic regions marked the Cambrian period, hypothetically related to the glacioeustasy and/or the progressive opening of the Iapetus ocean (~600 Ma). Such events influenced the paleoceanography of this period, including the progressive biota evolution – the ‘Cambrian Revolution’. Although the Gondwana Supercontinent margins, entirely amalgamated in the Lower Cambrian (540 Ma), were flooded, the inner part of this supercontinent was emergent, probably triggered by postcollisional epirogenic uplifts. Epeiric seas covered subsiding areas with projections towards the interior of the Western Gondwana, developing shallow platforms that covered ancient colisional suture zones. In the southeastern Amazon Craton, the recurrence of platform environments dates from Upper Cryogenian (~635 Ma) until the Cambrian with the installation of glacial deposits, overlaid by carbonatic and siliciclastic successions. Despite the previous insertion in the context of a foreland type basin related to the evolution of North Paraguai Belt (650-640 Ma), these deposits have been included in an inverted intracratonic basin in the Ordovician. The bottommost deposits of the Cambrian sequences, here presented, are comprised dominantly by siliciclastic rocks. These consist in the Upper and Lower members of the Raizama Formation and the base of Lower Member of the Sepotuba Formation, Alto Paraguai Group, exposed in the central and northeast portions of the inverted intracratonic basin, Mato Grosso state. Two depositional sequences (DS1 and DS2) characterize the Cambrian successions of the base of Alto Paraguai Group. The DS1 presents as a sequence boundary (SB1) an erosional hiatus previously interpreted in the southwestern basin. This stratigraphic surface becomes a correlative conformity towards the central and northern portions, where this covers the Araras carbonates and Cryogenian glacial deposits from Puga diamictites. The SB1 represents an erosional or non-depositional period of approximately 80 Ma developed over the carbonates of the Lower Ediacaran Araras Group, related to the epeirogenic uplifts of the basin. A second thermal subsidence phase would have led to the installation of a siliciclastic platform during the Cambrian, characterized by DS1 composed by two facies associations denominated FA1 and FA2. FA1 consists of subarkoses, quartz-wackes and pelites dominated by wave and storm processes, inserted in the offshoretransition, lower-middle shoreface and upper shoreface zones. The presence of infaunal vertical trace fossils belonging to the Skolithos Ichnofacies (Skolithos linearis; Diplocraterion parallelum; and Arenicolites isp.) at the base of the lower-middle shoreface deposits indicated a Lower Cambrian age, or younger, to the Raizama Formation, previously considered as Ediacaran. The FA2 comprehends subarkoses, quartzarenites, sublitarenites, quartz-wackes and sandstone/pelite rhythmites interpreted as complex tidal plain deposits, unconformably overlaid (SB2) by braided fluvial channel deposits of (FA3), which belong to the DS2. The DS1 would have been deposited during lowstand to transgressive system tract, organized in progradational parasequences. This stacking pattern is not compatible with the traditional stratigraphy sequence for TST, which is attributed to a slow subsidence rate concomitantly to a high sediment supply indicated by the Skolithos Ichnofacies. Subsequently, a less expressive drop in the sea level promoted a progradation of distal braided deposits (FA3) over the DS1, related to the lowstand system tract (LST) characterized by an abrupt change of the tidal heterolitic deposits to medium and coarse-grained quartzarenites from fluvial deposits. Paleoflow data oriented preferentially to NE and SE obtained in coastal beds from FA2 and FA3 allied to the Paleo- to Mesoproterozoic U-Pb detrital zircon ages have indicated provenance exclusively from SW and NW sources from Amazon Craton. Besides that, the detrital quartz grains analysis of sandstones of the bottommost Cambrian deposits indicate mainly igneous and metamorphic sources. Previous works indicated that the fluvial deposits of DS2 were succeeded by a transgressive system tract, marking this as the last transgressive event that influenced the Cambrian deposits of the intracratonic basin. Slowly, the ocean connection was interrupted as a consequence of the closing of Iapetus Ocean (~500 Ma) as a result of basin uplift. In this way, Cambrian epeiric seas were confined and consequently started the lacustrine phase of the basin in the Ordoviacian, represented by the Diamatino Formation deposits. Posteriorly, the intracratonic basin of the southeast Amazon Craton would have been inverted by the transtensional tectonics which propitiated the implantation of post-Cambrian intracontinental basins of the Western Gondwana.
Acesso aberto (Open Access)
Reconhecimento dos elementos deposicionais da lagoa do Violão na Serra Sul de Carajás: um estudo a partir de registros sismoestratigráficos
(Universidade Federal do Pará, 2016-11-21) ARAÚJO, Lidiane Cristina Lima de; SOUZA FILHO, Pedro Walfir Martins e; http://lattes.cnpq.br/3282736820907252
The sismoestratigraphy is one of the main tools used for the characterization of sequences and seismic facies, whose interpretation is based on the concepts of stratigraphic sequences. This study aims at the recognition and mapping of earthquake-depositional elements of Violão Lake - Serra Sul de Carajás - seeking to understand their sedimentary fill, considering the importance of lacustrine environments in the paleoenvironmental aspects, especially in tropical regions. The Violão Lake is part of the S11D plateau, built on lateritic ferruginous crusts at an altitude ~ 720 m above the sea level. It is a watershed of 1,836 km2, elongated in the NW-SE orientation, morphologically similar to a guitar. The research steps include acquisition, processing and interpretation of shallow seismic data. For the acquisition was used Bathy 2010PC ™ brand system SyQwest frequency 12 kHz and 3.5 kHz operating simultaneously with the aid of DGPS. They were collected 22 days profiles between 08/09 and 12/09/2014, treated in seismic software Reflex Win and Kingdom® Suite. 12 kHz profiles guaranteed greater quality information in relation to 3.5 kHz, and the six selected as the most representative elements earthquake-depositional patterns and ecocaráter. Stratigraphics three units were recognized (USI, USII, USIII), classified in chronological order from bottom to top. The US's rest on the rocky basement / acoustic that outcrop in the lake edges, extending to 25 m in the subsurface. Their morphology can be likened to a bowl of washing ("washing basin") of steep margins and relatively flat bottom. Their internal reflectors, sometimes discontinuous, made up of low to medium signal amplitude, unlike their neighboring surfaces. The USI is characterized by a progradational succession deposited from gravitational flow forming delta fronts for high lake conditions. The USII and USIII represent facies planeparallel, consistent with each other relatively soft reflectors. The recognition of 7 ecofacies allowed the relationship between the present acoustic signatures and the data of the distribution of superficial bottom sediments performed in previous works, which allowed an overview of the surface deposition of the Violão Lake, being the echoes of greater reflection related, predominantly, lamosos sediments with presence of peat and, in some moments, organic or oxidized sludge, favoring the occurrence of gases in the sedimentary interstices resulting from the decomposition of organic matter. In contrast, medium reflections low impedance contrast to refer to very fine sand with considerable mud content and good spread. Recognition and interpretation of stratigraphics standards identified different sedimentary filling processes of Violão Lake. The USI is characterized by a progradational succession of strata and, as they approach the depocenter, the strata of USII and USIII if have more uniform, as drapes in on a partially uneven surface under conditions of low energy influenced by flows shallow and intermediate, whose deposition occurs by suspension.