Stratigraphic analysis and geomorphological reconstruction of Grumari coastal plain, Rio de Janeiro, Brazil
Introduction
The Southeastern Brazilian coastal area has been constructed by the formation of a barrier island or by a regressive beach ridge system, during Pleistocene and Holocene, being driven by the balance between paleoclimatic controlled sea-level changes and sedimentary supply (Villwock et al., 2005). In the Rio de Janeiro metropolitan region, two major coastal areas have been studied since 1940's: the Marambaia spit, separated from the mainland by the Sepetiba bay (Lamego, 1945; Ponçano et al., 1979; Dadalto, 2017; Alves Martins et al., 2020), and the Jacarepagua plain, a two-barriers island system (Roncarati and Neves, 1976; Maia et al., 1984). Between them, a secondary coastal plain has not been well defined yet, presenting features different from the surrounded areas.
Grumari plain (Fig. 1a) was anthropologically modified from the 1960s, where all superficial sedimentary deposits were devastated, and the area was flattened for residential construction. As it became part of Pedra Branca National Park, the urbanization was interrupted and the area went through a reforestation process (Guerra, 2005). Nowadays, although the area seems preserved from urbanization, its original landscape and its type of evolution is not clear: whether it is an old barrier island system or a strandplain. Pereira et al. (2012) point to the hypothesis that the area is an old barrier island system, such as the adjacent areas, however, still remains the possibility to be a strandplain up to be investigated. As there are currently no outcrops or superficial sedimentary body remnants and exposed, geophysics comes as a method to understand the area's recent development.
In coastal evolution studies, the Ground Penetrating Radar (GPR) method is commonly applied, allowing the correlation between geophysical and geological features, through inferences of paleoenvironment and sedimentary processes, enabling a geological reconstruction (Beres and Haeni, 1991; Jol and Smith, 1991; Martins et al., 2020). The interpretation of the sedimentary and geomorphological processes of the coastal plain internal architecture and its related deposits is essential to understand their evolution in geological time. Several authors have used the radar stratigraphy and radarfacies interpretation methods with this purpose (Neal and Roberts, 2000; Gandolfo et al., 2001; Neal and Roberts, 2001; Bristow and Pucillo, 2006; da Rocha et al., 2013; Barboza et al., 2014; Fernandez and Rocha, 2015; Rosa et al., 2017; Dillenburg et al., 2017; Angulo et al., 2018; Barboza et al., 2018; Montes et al., 2018; Berton et al., 2019; Oliveira et al., 2019).
As the radar reflection patterns allow the deposits characterization, as well as its related sedimentary processes, through the interpretation of GPR data, this current work aims to improve the understanding about the Grumari geological evolution during the Holocene, being a primary investigation to elucidate the plain stratigraphic and geomorphological evolution, guiding future sedimentological and geochronological studies. In order to interpret the possible latest coastal development of this area, were taken into account the mean sea level variations in Southeast Brazil (Martin et al., 1979; Angulo and Lessa, 1997; Angulo et al., 2006; Jesus et al., 2017) and in the adjacent areas (Maia et al., 1984; Dadalto, 2017). From correlations between them both, this research presents the interpretation of the stratigraphic evolution scenario added to a landscape reconstruction, contributing with other analogues investigations and complementing Rio de Janeiro's coastal plains researches.
Section snippets
Geological context
The study area is located in the Oriental Terrain of the Ribeira Belt, Santos Basin onshore northern portion, in the Cenozoic Rift System of Southeast Brazil context (Zalán and Oliveira, 2005). The geological evolution in the Southeastern Brazil is pronounced by the Paleocene tectonic reactivation responsible for the Serra do Mar and a series of continental rifts origin, carving a particular coastal geomorphology, characterized by narrow coastal plains settled between the mountains and the
Fieldwork
Two different types of GPR data acquisition were made, Fixed-offset and Common-midpoint (CMP), along Grumari plain, with a Differential Global Positioning System (DGPS), to guarantee geographic coordinates and altitude of the radargrams. The Fixed-offset section (2D) registers both vertical and lateral medium variations. The CMP (1D) measures the subsurface wave propagation velocity. Each Fixed-offset section has its corresponding CMP data, showing how velocity changes with depth in each point,
Results
In the GPR sections (Fig. 2, Fig. 3, Fig. 4) were recognized and interpreted seven radarfacies (Table 1), divided into three major units, according to their reflection pattern, and two main surfaces limiting these units.
Discussions
Mean sea level variations control the vertical translation of the shoreline equilibrium profile, while changes in sedimentary supply, on the other hand, influence the horizontal translation. Therefore, the main controlling factors for coastal plain development are the sedimentary supply and accommodation space creation rates.
The geological interpretation of Grumari's geophysical sections combined with recent mean sea level changes (Martin et al., 1979; Angulo and Lessa, 1997; Angulo et al., 2006
Conclusions
For Grumari stratigraphic evolution proposal, we suggest that at the sea level rise beginning in the region, from about 7,000 years BP, the sedimentary supply rate was higher than accommodation space creation rate, causing the shoreline tent to prograde, even though the sea level was rising. With the continual rise in sea level, the accommodation space creation rate is turning in equilibrium with the sedimentary supply, allowing a more aggradational/retrogradational deposition, in both vertical
Authorship statement
Conception and design of study: T. Mira and S.S. Martins; acquisition of data: T. Mira, S.S. Martins, S. Gouvêa and F. Dourado; analysis and/or interpretation of data: T. Mira; drafting the manuscript: T. Mira, S.S. Martins, S. Gouvêa and F. Dourado; revising the manuscript critically for important intellectual content: T. Mira; approval of the version of the manuscript to be published. The names of all authors must be listed: T. Mira, S.S. Martins, S. Gouvea and F. Dourado.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the Master Grant provided by CAPES, the Graduate Program in Geosciences (UERJ), Labsismo (UERJ), CEPEDES (UERJ), LEXMIN (UERJ) and LAMEMO (UFRJ) laboratories for equipment, software and work environment.
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