Reports: AC846696-AC8: Strand Plain Geometry, Stratigraphy, and Evolution: Tijucas, Santa Catarina, Brazil

Duncan M. FitzGerald , Boston University

This past year we have turned our attention to data analysis from previous field campaigns and expanded the scope of our analytical capabilities on previously-collected samples. At Tijucas, we had determined that the strandplain has prograded approximately 6 km since the 5.8 ka sea-level highstand. During this time, the overall trend in sediment deposition shifted from sand-dominated facies (tightly-spaced beach ridges), to alternating sand ridges and muddy cheniers in the mid-plain, to mud-dominated facies with widely spaced sand cheniers in the past few centuries. The presence of a seaward-thickening wedge of marine mud underlying the surface ridges suggests that decreasing accommodation space controls sedimentation patterns. In this scenario, basinal filling would have gradually attenuated wave energy leading to less sediment reworking and a change from sand to mud dominance. Imprinted on this longer-term trend are a series of nearly 70 abrupt transitions between shore-parallel sand- and mud-dominated facies. These provide the background for our final stage of work at Tijucas: addressing how the geological record informs us about past climate sensitivity and the impact of past abrupt changes in climate under a variety of different boundary conditions. Preliminary stable isotope analyses of plant leaf waxes extracted from sediments within these various units, combined with correlations with published paleoclimate proxy records, reveal that these decadal- to centennial- scale transitions likely resulted from changes in the fluvial bedload / suspended load ratio. Such changes are related to climate change, which in turn produced modifications in vegetation patterns, bedrock weathering and soil formation processes, and ultimately sediment contribution in the drainage basin.

At Pinheira, located ~70 km south of Tijucas, we determined that, unlike the riverine-dominated system at Tijucas, the evolution of this strandplain was marked by continuous sediment supply from nearshore under conditions of generally falling sea-level. During the mid-Holocene highstand, sediment derived from local fluvial and shelf sources was reworked onshore and alongshore and deposited in a 20-m deep embayment to form a basal nearshore lithologic unit. Falling sea level forced strandplain progradation at a rate of 1–2 m yr-1 through the deposition of a shallow-dipping shoreface unit, an intermediate foreshore unit, and an uppermost upper beach and foredune unit containing a variety of bed orientations. All three units are composed of very fine to fine, quartz-dominated sand. The presence of these latter three units reveals that the plain developed first through the shallow water deposition of shoreface sands followed by the welding of successive beach ridges and landward-migrating bars to form the foreshore. This series of ~500 beach ridges was discontinuously topped with foredunes and a later aeolian dune facies, resulting in a foredune ridge plain. A single 100-m wide barrier ridge with associated overwash units, 3–4 m deep lagoon and 40-m wide, 3–5 m deep tidal inlet were also identified within the strandplain sequence. The isolated nature of the Pinheira strandplain buffered it from changes in precipitation patterns, wave energy, and fluvial sediment supply during the time of its formation. The lack of major storms along this coast would indicate that this barrier system could only have formed in response to a change in the rate of relative sea-level fall due to steric or ice volume effects, thus illustrating the complex nature of sedimentological response to relatively minor (decimeter-scale) variations in sea level.

Finally, at Navegantes, located ~50 km north of Tijucas, we identified a mid-Holocene barrier ridge formed penecontemporaneously with that at Tijucas. A series of radiocarbon dates confirm that an elongate, shore-normal ridge located ~6 m above modern mean sea level is the remnants of the highstand barrier. Here, it is emplaced immediately on top of a radiocarbon dead sediment surface that is interpreted as likely deposited during the Marine Oxygen Isotope Stage 5e highstand. A similar lower strandplain surface was identified at Tijucas. The Navegantes Holocene highstand ridge is backed by a muddy freshwater wetland area, analogous to the lagoon located landward of the mid-Holocene highstand at Tijucas. Furthermore, it is fronted by the repetitive, sandy seaward-dipping reflectors associated with strandplain progradation. In this manner, the wetland – ridge – strandplain sequence at Navegantes is nearly identical to the highstand barrier at Tijucas. We completed an extensive literature review as part of this study and identified a total of 27 additional sites where similar highstand features have been identified. The results of this study are currently in preparation for submission for publication in a peer-reviewed journal.

The funding provided by the ACS PRF has had a profound impact on my broader research program. The study of these chenier / strandplain systems in response to changes in sea level and sediment supply are particularly important for the future of coastal regions such as the east coast of North America that rely on both shelf and fluvial sediment inputs, but where fluvial sources are largely diminished. Our work in the Brazilian strandplain systems is demonstrating that even minor variability in sea level or climate-driven the volume or type of sediment supply can affect the mode of coastal evolution. Results from this work have led to numerous abstracts at national and international conferences. I and my students have one paper in review and two in preparation from this work. One of these papers is a major chapter in my PhD students dissertation, which he defended in September of this year. Finally, it has provided a foundation for future studies of climate-coastal interactions through the application of organic geochemical techniques. This represents a new direction for my research and has provided a foundation for my PhD student’s postdoctoral studies. Finally, studies funded by this grant have had a significant impact on the undergraduate students who have been involved with it, both in the United States and in Brazil. All five undergraduates (2 Brazilian, 3 US) who have been involved in this work are now pursuing graduate-level degrees in coastal geology, inspired by our exciting and unique work in the strandplains of southern Brazil.

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