Navegando por Autor "SANTOS, Hudson Pereira"

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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.
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Depósitos flúvio-costeiros da Formação Raizama, Ediacarano-Cambriano da faixa Paraguai Norte, região de Nobres, Mato Grosso, Brasil
(2014-12) SANTOS, Hudson Pereira; SILVA JÚNIOR, José Bandeira Cavalcante da; NOGUEIRA, Afonso César Rodrigues; ABRANTES JÚNIOR, Francisco Romério
The Ediacaran-Cambrian Raizama Formation presents siliciclastic deposits exposed discontinuously along of the southern margin of the Amazon Craton and the Northern Paraguay Belt, Central-Western Brazil. These deposits are interpreted as progradational coastal succession conformably overling the carbonate platform succession of the Upper Araras Group. The facies and stratigraphic analysis of outcrop section were carried out in the Nobres region, State of Mato Grosso, allowed the individualization of seventeen facies, grouped into five facies associations (FA): FA1) lower shoreface, consisting of sandstone with parallel and wave-truncated lamination (microhummocky) parted by laminated mudstones, locally bioturbed by Skolithos; FA2) upper shoreface, composed by parallel and swaley cross bedded sandstone; FA3) subtidal, represented by sandstones with tangential and through cross stratifications drapped by siltstone/very fine sandstone laminae interpreted as channel and bar deposits; FA4) tidal flat is characterized by sandstones with tangential and sigmoidal cross bedding, even parallel stratification, low-angle cross bedding, mud cracks, siltstone/very fine sandstone rhythmites with flaser bedding, organized in shallowing-meter scale cycles; and FA5) distal braid plain consisting of sandstones with through cross-bedding and laterally discontinuous lags, parallel stratification and low-angle cross stratification partially reworked by wave. The sedimentation of the Raizama Formation suggests an increase in the siliciclastic supply linked to uplift regions of the Craton in the northwest of the studied area, succeeding the Araras carbonate platform deposits. Tubular trace fossils in the FA1 indicate, by the first time, the presence of burrowed organisms, what strongly points to an age near of the Ediacaran-Cambrian boundary.
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Fácies e proveniência de depósitos costeiros da Formação Raizama: evidências do registro Ediacarano-cambriano na faixa Paraguai, região de Nobres, Mato Grosso
(Universidade Federal do Pará, 2014-03-10) SANTOS, Hudson Pereira; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
Siliciclastic rocks from the Raizama Formation, a basal unit of the Alto Paraguai Group, from the Ediacaran-Cambrian interval (635-541 Ma), is discontinuously occur distributed along the southern margin of the Amazonian Craton within the Paraguay Fold Belt northern segment, west-central of Brazil, Mato Grosso state. This Group unconformably overlies carbonate shelf deposits of the Araras Group, where evidence of Marinoan glacial event (635 Ma) was recorded. The Alto Paraguai Group represents the final stages of the collision between the Paranapanema and Amazonian blocks, leading to the closure of the Clymene Ocean (540-520 Ma). The Raizama Formation is approximately 570 m of thickness and is composed by mudstone, fine to coarse sandstones, and sandstones with dolomitic cement previously interpreted as fluvial-coastal deposits distributed in the lower member (270 m) and upper member (300 m). The facies and stratigraphic studies of this unit in the Nobres region, Mato Grosso state, were mainly focused on the outcropping section of 600 m in the bed of Rio Serragem II, which includes the Serra do Tombador waterfall. In this stratigraphic section, 17 sedimentary facies were described and grouped into five facies association (AF), representative of a progradational coastal sequence beginning with lower shoreface deposits, overlying in correlative conformity the shelf carbonate deposits of the Serra do Quilombo Formation (Araras Group). The AF1 facies consists of sandstones with planar lamination and wave-ripple cross-lamination (microhummocky), individualized by layers of laminated pelite interpreted as lower shoreface deposits. It stands out in the AF1 the first occurrence of centimetric bioturbed levels of Skolithos in Neoproterozoic-Cambrian deposits in the Paraguay Belt. The AF2 facies is composed by sandstones with swaley cross-stratification and plane bedding interpreted as upper shoreface deposits. The AF3 facies is composed by sandstones with tangential and trough cross-stratification with drapes of siltstone/very fine sandstone representative of channel and subtidal bars deposits. The AF4 facies is characterized by sandstones with tangential and sigmoidal cross-stratification, planar to low angle cross-lamination, rhythmites very fine sandstone/siltstone with flaser bedding and mudcracks, organized in metric tidal flat shallowing upward cycles. The AF5 facies is comprised of sandstone with trough cross-bedding characterized by common lags at the base of the association, sandstone with planar to low-angle cross-stratification, interpreted as distal braided rivers, in part reworked by waves. Detrital zircon grains were obtained from AF3 and dated by U-Pb method, resulting in an age 1001±9 Ma interpreted as the age of the maximum deposition of Raizama Formation. Combined with this analysis, the NE-SE paleocurrents show that source area of these sediments would be the Sunsas Fold Belt, SW of the Amazonian Craton not being discarded contributions coming from the NW part of this Craton. The obtained Mesoproterozoic age has predominantly served to unravel the provenance of Raizama Formation. Whereas dating from the base of Araras Group, around 627-622 Ma, associated with the clear presence of the ichnogenus Skolithos, suggests that the age of this unit is closer to the limit with the Lower Cambrian. Trace fossils from the Proterozoic are characterized almost exclusively by horizontal traces, while vertical bioturbation are virtually absent throughout the Neoproterozoic. This inference is confirmed by the maximum age of 541 Ma obtained for Diamantino Formation, which overlies the studied unit. The radiometric data combined with paleoenvironmental interpretation, including the record of the first burrowing activities in Paraguai Fold Belt, opens up perspectives to understand in greater detail the sequence of events that typify the Ediacaran-Cambriam boundary strata of Brazil, still poorly known.