Registros estratigráficos e morfológicos da variabilidade eustática e paleoambiental pós-último máximo glacial na plataforma continental leste-sudeste do Brasil
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Data
2022-07-04
Autores
Cetto, Paulo Henrique
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Universidade Federal do Espírito Santo
Resumo
Several morphological and stratigraphic data around the world have evidenced the variable character of the rates of sea-level rise during the last post-LGM marine transgression, exhibiting periods of acceleration, known as "melt water pulses," usually preceded by stillstands, in compass with the retreat and advance of the polar ice caps and ocean-atmosphere rearrangement phenomena in the NH. Passive and tectonically stable far-field continental margins, away from focal points of glaciers melting, like those in the South Atlantic, tend to properly record the sea-level rise process during the last deglaciation as a function of minimal isostasy components; however, studies on this subject are still relatively scarce in this region. In order to explore this lack, the present work investigates the influence of post-LGM eustatic/paleoclimatic phenomena triggered in the NH in the geomorphological and paleoenvironmental conditioning of the east-southeast Brazilian continental margin. In a first moment, the influence of sea-level variation processes was inferred by comparing depths of the bottom and/or subbottom morphologies of 3 different sectors of the shelf (“Abrolhos Depression - DA,” “Central-Northeast outer shelf of Espírito Santo”- PECNES and “California Reef” – RC) with the eustatic curves available in the literature. The results suggest that the deceleration of sea-level rise during the Younger Dryas (YD) favored the development of coastal barrier-lagoon systems in the vicinity of river mouths, as observed in DA and PECNES. In sectors dissociated from this terrigenous influence, erosion processes would dominate the shoreline with the formation of abrasion terraces and cliffs, such as on the flanks of the prominent marginal banks of the DA. Low gradient regions, which would behave like open shallow marine environments during the YD, would potentially favor the development of fringing reefs, such as in the northeast sector of PECNES. All these features established by the end of YD would have abruptly drowned due to the MWP-1B. Towards the present inner shelf, the successive establishment/drowned of reef formations were probably influenced by the cooling events of 8200 years B.P. and by the MWP-1C, respectively, as inferred in the RC sector. In a second moment of the work, the transgressive paleoenvironmental evolution of the main sector, the AD, was investigated through seismostratigraphic, sedimentological, geochronological, and geochemical approaches. It is revealed that the sedimentary fill of sand-quartz of drowning paleochannels in the AD dates from the Bølling-Allerød period, having been deposited probably in a drier climatic context. The chronological data corroborate that the subsequent establishment of a lagoon, with distal emerged and/or shoreface features, typical of an intralagoonal delta, occurred during the YD. The geochemical proxies for continental weathering and evaluation of the balance between terrigenous X marine input applied to terrigenous muddy sequence (CaCO3 contents <20%) reveal that palaeolagoon evolved during the YD under abundant continental input, rich in organic matter mainly from C3 plants, probably in a more wet climatic context. Concurrently, a slow but growing secondary component of marine influence would be observed, attributing an estuarine characteristic to the coastal system along the YD. The dating of a stratigraphic ravinement surface at the upper limit of the lagoonal terrigenous deposit confirms that the drowning of the entire coastal system was due to the MWP-1B onset, immediately after the end of the YD. In the cores, the scope of this event is identified by the sudden inflection of the CaCO3 content verified between ~20-80% along the mixed facies, with an abrupt replacement of terrigenous inputs containing C3 plants OM by a gradient of a substantial increase of MO inputs and autochthonous sedimentation from marine origins, probably in a relatively drier climate context in the Early Holocene. This interval would range from the sudden drowning episode of the lagoon system to the completion of the process of transforming the DA into an embayment during the MWP-1B, with the expansion of marine communication through the Besnard Channels. In this resumption of the transgressive process, new clastic features resulting from the hydrodynamic increase would have developed over the ravinement surface, including over the distal remnant of features formed until the end of YD. In the same context, possible backstepped reefal features were established on the marginal western flanks of the DA during MWP-1B. The reentrant conformation of DA was probably abandoned when the sea level reached ~40 m quota on the marginal banks, shortly after the MWP-1B, being submitted after that to an open shelf regime until the present days, with sedimentary deposition essentially based on biogenic carbonate (CaCO3>80%) and OM inputs from marine primary productivity.
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Plataforma continental , Variabilidade eustática do nível do mar , Registros estratigráficos