Paper No. 2
Presentation Time: 8:00 AM-5:30 PM
PALEO-SCARP GRADIENTS WITHIN THE OUTER NAVEGANTES STRANDPLAIN, BRAZIL: SUBSURFACE SIGNATURES OF CYCLONES OVER THE PAST 1,000 YEARS
Navegantes strandplain is a 3.5-km-wide ridge-and-swale sequence in Santa Catarina State, Brazil that archives geological signatures of tropical south Atlantic cyclones. Two shore-normal 250 MHz georadar (GPR) transects of the seaward 600-m section were used to identify and map subsurface indicators of erosion. Prominent seaward-dipping, tangential-oblique truncations within the upper 1.0-1.5 m, which typically coincide with heavy-mineral concentrations in sediment cores, are interpreted as erosional paleo-scarps. In radargrams, the apparent, migrated angle value (ψ) of the upper (truncating) reflection exceeds that of the subjacent (truncated) paleo-surface by 1.7-3.8 times. Accounting for minor deviations of survey lines from ridge-normal trend, the maximum gradient increase (Δψ) is 6.2° and 8.2° for the northern and southern transects, respectively. Based on a growing number of radiocarbon dates on basal swale organics (minimum age of the seaward ridge) and optical chronology of ridges themselves, by 1,000 cal yBP the time-averaged progradation rate decreased from 0.7 m/yr to 0.5 m/yr as the strandplain accreted in a regime of a relative sea-level fall. The most prominent scarps are correlated between the two transects (2.3 km longshore distance) and coincide with regionally extensive ridgelines, dating between 1,100 and 100 cal yBP. This highlights the importance of subsurface investigations, as only some of the numerous morphologically expressed paleo-shorelines (beach-dune ridges) owe their origin to high-magnitude erosional events. Statistical analysis indicates that intense storms (comparable to the 2004 Cyclone Catarina) or phases of heightened erosional activity exhibit a 400-year periodicity, which combines with higher frequency low-magnitude events to generate 200-year and 60-year erosional cycles. In a regime of net accretion, this shift may partially explain the apparent decrease in time-averaged progradation rate. This study demonstrates the applicability of GPR imaging as a rapid and effective means of locating and mapping regional erosional paleo-indicators in prograding coastal sequences.