Geomorphological and stratigraphic evolution of a fluvial incision in the coastal plain and inner continental shelf in southern Brazil
Graphical abstract
Introduction
A fluvial incision is a common consequence of a relative fall in sea-level, which can be produced by a negative eustatic oscillation due to tectonic uplift, tectonic tilting of an alluvial plain, glaciation (and associated relative fall in sea-level), or decrease in fluvial discharge to form underfit streams (Posamentier, 2001; Alqahtani et al., 2015). The infilling of such incisions varies in the downstream direction, with primarily fluvial deposits upstream and more marine-influenced deposits downstream (Bowen and Weimer, 2003, Bowen and Weimer, 2004). According to Amorosi et al. (2013), incised-valley fills display a distinctive succession of the coastal plain to estuarine facies. These deposits have been extensively investigated due to their great importance for prospecting oil-bearing strata (McBride et al., 1991; Kindinger et al., 1994; Weschenfelder et al., 2014). According to Kindinger et al. (1994), fluvial incisions act as sedimentary pathways analogous to ancient incised valleys representing conduits for shelfward transport. Understanding the high-resolution stratigraphic history of ancient incised fluvial valleys can contribute to the reconstruction of the palaeo-hydrology of the systems and assess their significance concerning the depositional and stratigraphic sequence evolution (Alqahtani et al., 2015; Rahman et al., 2018).
Geological and geophysical studies of fluvial channels and incised valleys, focusing on the Quaternary coastal evolution have been presented worldwide (e,g., Gulf of Mexico, Durbin et al., 1997, Blum and Aslan, 2006; the Ria de Ferrol in Spain, Cartelle and García-Gil, 2019; the incised valleys of Tuscany in Italy, Amorosi et al., 2013; KwaZulu-Natal continental shelf in South Africa, Green, 2009; and the Qiantang River in eastern China, Lin et al., 2005). In the central and northern sectors of the Pelotas Basin and the Santos Basin, in the southeastern Brazilian continental margin, such studies have been presented by Toldo Jr. et al. (1991), Abreu and Calliari (2005), Weschenfelder et al., 2008, Weschenfelder et al., 2014, Barboza et al., 2014a, Barboza et al., 2018, Biancini da Silva et al. (2014), Tomazelli et al. (2014), Leal et al. (2016), Bortolin et al., 2018, Bortolin et al., 2019, Santos-Fischer et al. (2018), and Alves and Mahiques (2019). According to these authors, the Postglacial Marine Transgression (PMT) after the LGM inundated the exposed shelf and caused the shorelines to retreated landward, causing submergence and development of transgressive systems tracts on top of the paleodrainage network.
Despite this large number of previous studies in southeastern Brazil, none of these have documented a large extension of fluvial incisions, with geophysical data, drill holes, and geochronological data. Also, most of these studies have documented and described records of fluvial channels and incised valleys related to the Last Glacial Maximum (LGM) that are presently located offshore (under water). Therefore, the geomorphological records of the LGM presently occurring above water (at the coastal plain) are unusual features here presented that could be of broad interest.
This paper presents results based on very new data (never published) of a study aimed at investigating the high-resolution stratigraphy, geomorphology, and geological evolution of a Late Pleistocene/Holocene fluvial channel from the Taim incised valley and its infilling, developed in the southernmost Pelotas Basin, southern Brazilian coast, as a response to the deglacial sea-level rise. The interpretation and the conceptual model may be applicable in comparable depositional settings.
Section snippets
Regional setting
The southern Brazilian continental margin is a rifted plate boundary formed during the Early Cretaceous times. It integrates the Pelotas Basin, which is a marginal basin with landward limits at the highlands of the state of Rio Grande do Sul, formed by igneous and metamorphic rocks of the Precambrian shield, and Mesozoic volcanic and sedimentary rocks of the intracratonic Paraná Basin. Along the coastline, it is bounded by the Precambrian rocks that outcrop at Santa Marta Cape to the north and
Material and methods
This investigation is based on the shallow portion (~30 m depth) of a high-resolution 2D seismic data, high-resolution three-dimensional 2.5D (for details see Hübscher and Gohl, 2014, pg. 6) dataset of reflections obtained with Ground-Penetrating Radar (GPR), supplemented by a high-resolution geological dataset that includes lithological data from drill-holes, AMS radiocarbon (14C) dates, and Digital Elevation Model (DEM).
Geophysical data
Two 2D seismic reflection sections, together with a 2.5D GPR block and a 2D GPR section, are presented below. Their interpretation, in the form of depositional units (I to VII), was based on reflections termination (onlap, downlap, toplap, and truncations) and geometry and reflectors pattern. And with the purpose to give a broad view of part of the Taim incised valley a large GPR section (almost 6 km long) is also presented.
The Taim swamp area (GPR and Drill Holes)
The geological context and the correspondence between radarfacies and lithologies sampled by the drill holes (Table 2) allowed interpreting the environmental conditions under which the sediments were deposited in the Taim swamp GPR section (Fig. 8). Unit II conformably overlies the Pleistocene substrate (Unit I) and represents a Late Pleistocene fluvial deposition. It shows the coarser sand grains of all units (up to 45% of medium sand and 8% of coarse sand). This fluvial interpretation is
Conclusions
The Taim swamp area provides an outstanding stratigraphic record of the LGM. Consequently, the region is an ideal area for investigating the processes of generation and sedimentation of incised valleys associated with sea-level changes.
Unlike the majority of studies that documented records of the LGM occurring under water at the continental shelf, the records of the LGM at the Taim swamp area were documented at a coastal plain domain, which is not common.
The seismic reflection surveys at Mirim
Dataset link
Declaration of Competing Interest
The authors declare no conflict interest.
Acknowledgments
E. Barboza, S. Dillenburg, M. Rosa and J. Weschenfelder thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the provision of their research fellowships. R. Lopes thank CNPq for providing the Postdoctoral Research Grant 151313/2019-9. F. Caron thank Fundação de Amparo à pesquisa do Estado do RS (FAPERGS) project 17/2551-000-812-0, for financial support. We also thank UFRGS/FAURGS 2736-7 for financial support. We are also grateful to Alessandro Amorosi, Andrew
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