GEOPHYSICAL EXAMINATION OF A REGRESSIVE BARRIER: IMPLICATIONS FOR MID- TO LATE-HOLOCENE SEA-LEVEL FALL IN THE SOUTHERN HEMISPHERE

The geophysical and sedimentological examination of a regressive strandplain system in central Santa Catarina, Brazil has uncovered the existence of a paleo-barrier island system that illuminates the nature of sea-level fall in the Southern Hemisphere during the mid- to late-Holocene. Following a 2 to 4 m highstand at 5-6 ka BP, falling sea level in southern Brazil forced coastal progradation and the formation of extensive strandplain systems. At Pinheira, located immediately south of Ilha de Santa Catarina, sediment derived from the continental shelf was reworked onshore during this regression to form a 5-km wide strandplain. RTK-GPS and 11 km of ground-penetrating radar (GPR) were ground-truthed with a series of vibracores (3 – 6 m deep), drill cores (17 m deep), auger cores (1- 6 m deep), and radiocarbon dates to reconstruct the depositional history of this strandplain. This sequence contains closely spaced, arcuate low ridge crests, which are a product of refracted ocean waves and an inner shelf sediment source. Ridge crests reach 1-4 m in elevation and are separated by shallow swales containing mud deposits less than a meter in thickness. Strandplain progradation rates varied between 0.8 and 1.0 m/yr. The strandplain sequence is composed of a basal unit consisting of shallow (0.45^o - 0.55^o) seaward-dipping layers becoming nearly horizontal with depth; a middle unit exhibiting shallow to steeply (0.75^o – 1.0^o) seaward-dipping strata and relatively thin steeply landward-dipping beds (ridge and runnel facies); and an uppermost unit consisting of beach and incipient dune facies with a variety of bed orientations.

GPR and sediment cores were further used to identify a lagoon, tidal inlet, a 100-m wide barrier ridge, and numerous overwash units within the strandplain sequence. The development of a barrier island system within this regressive strandplain implies the existence of perturbations in sediment source, wave climate, or sea level trends through time. However, ridge crest morphology and sedimentological data suggest that the former two forcings were constant throughout the mid- to late-Holocene. Therefore, it is concluded that the post-highstand gradually fall in sea level was punctuated by occasional minor oscillations, allowing for the accumulation of sediment and evolution of a more complex system.