PROVENANCE OF BEACH SANDS ALONG SOUTHERN BRAZILIAN STRANDPLAINS: SANTA CATARINA, BRAZIL

The most striking features of the Santa Catarina coast in southern Brazil are the large, irregular headlands and intervening strand plains that reflect an abundant sand supply and falling level during the past ~5,000 years. Extensive dune systems characterize the relatively narrow (10 - 60 km) and discontinuous coastal plain developed along this Amero-trailing margin segment. The coastal plain is bordered by a variety of Archaean to Proterozoic crystalline rocks that include very coarse-grained granites that comprise part of the Brazilian Highlands and much of the bedrock within coastal drainage basins. Rivers that enter along the 565 km Santa Catarina coast drain approximately 40% of the state. Sediment comprising the strand plains consists of fine to medium quartz sand that is considerably finer-grained than the constituent quartz of the basement. The source of the abundant sand is poorly understood but existing models suggest that the bulk of the sand was ultimately derived from the Rio de la Plata located 900 km to the south and/or it may have been discharged from local rivers.

New studies suggest that the very coarse-grained granitoids, which are highly tectonized, are an important source of sand for much of the coast. Intense weathering of this relatively high relief bedrock has produced a saprolite that is tens of meters thick. The quartz and feldspar in the granitoids are highly fractured and upon weathering in the soil profile experience significant size reduction. During intense precipitation events sand is removed from the soil profile and transported by streams to the coast, where it undergoes further size reduction. Preliminary thin-section study of the granitoids and the specific phi-size fractions of the deeply weathered soils have provided petrographic evidence to address the question of provenance of the quartz.