CONTRASTING DEPOSITIONAL TRENDS ON A RECENTLY EMERGENT COASTLINE OF SOUTHERN BRAZIL: INSIGHTS FROM SUBSURFACE IMAGING

Thirty kilometers of ground-penetrating radar (GPR) surveys and sediment cores provide the first detailed picture of the stratigraphic architecture of two coastal strandplains in central Santa Catarina, Brazil. This high-energy, moderate-relief, embayed coastline experienced a relative sea-level fall since mid-Holocene. In this regime of forced regression, Itajaí and Tijucas rivers have supplied large quantities of sediment into coastal embayments resulting in morphologically and sedimentologically contrasting basin fills. GPR imaging has proven to be a successful tool for identifying and mapping sand-rich versus mud-dominated portions of the plains, while providing high-resolution images of the extent and geometry of various facies boundaries. The Navegantes coastal plain, adjacent to Itajaí River, is a relatively exposed, low-relief strandplain with a series of parallel beach ridges. The geophysical records reveal a series of shallow-dipping (1.5-2.5º) tangential-oblique reflections extending 4-5 m below present sea level. The inter-ridge swales are occupied by shallow wetlands containing freshwater peat, which attenuates the GPR signal. A modern dune system has formed along the present beach, which is distinctly different from the rest of the plain morphology and is related to a regime of stable to slowly rising sea level. In contrast, a more sheltered Tijucas plain is characterized by a recent deposition of thick liquefied muds with intervening ridges of very coarse-to-medium sand. North of the river, a 5-km-long shore-normal GPR transect shows an older generation of shoreline deposits, possibly formed during a Late Pleistocene highstand, fronted by a geophysically transparent, 8-10-m-thick, muddy nearshore-lagoonal-wetland sequence. Farther seaward, a succession of steeply-dipping (9-10º), sigmoidal-oblique reflections, representing successive beach-shoreface accretionary surfaces, decreases in thickness from more than 8 m to 4 m in a seaward direction. Erosional events are suggested by truncated bounding surfaces and buried tidal-channel incisions. The contrasting facies architecture and depositional gradients of the two strandplains are likely due to differences in fluvial bedload/suspended load ratios, the resulting sediment contribution to the beaches, antecedent basin-floor topography, and incident wave energy.