Estratigrafia e evolução da barreira holocênica do Rio grande do sul no trecho Tramandaí-Cidreira

Abstract

The Holocene coastal barrier (barrier-lagoon system IV) in Rio Grande do Sul evolved differently in the last 6 - 5ka. For instance, to the north (between Tramandaí an Torres, figure 1) the barrier prograded, while to the south (between Mostardas and Estreito) it receded. The studied segment (Tramandaí - Cidreira) is located immediately to the south of Tramandaí. In 2000 and 2001 a drilling campaign was carried out in this segment aiming to figure out its geological nature and evolution. The drill holes were performed along three cross-shore profiles (Tramandaí, Jardim do Éden and Cidreira, figure 2). Based on morphological, lithological, 14C dating, and computer modeling of coastal evolution it was possible to define a non-uniform transgressive nature for this barrier segment (30km long), as well as the barrier evolutionary chronology. In Tramandaí the barrier shows, on its western part, stratigraphic records of a transgressive phase that represents the maximum landward position of the barrier during the final stages of the Postglacial Marine Transgression (Fig. 4). On its eastern part the barrier shows a very discrete regressive stratigraphy, but in general the barrier stayed relatively stabilized during the last 6 - 5ka. Also in Tramandaí the oldest sedimentary record of the Holocene lagoonal system occurs. It corresponds to lagoonal floor mud deposits dated on 11280 - 10510 cal years BR At the maximum sea-level height of the Postglacial Marine Transgression a barrier was positioned seaward of the present shoreline on both Jardim do Éden and Cidreira. In Jardim do Éden barrier, stratigraphy is essentially transgressive as a consequence of barrier landward translation during the last 6 - 5ka (Figure 5). Lagoonal muds presently cropping out at the foreshore zone indicate that this translation was controlled mainly by a negative balance on sediment budget (figure 6). In Cidreira the barrier has a transgressive nature also as a consequence of barrier translation during the last 6 - 5ka, but showing a very small and relatively recent phase of progradation (figure 7). During barrier translation at Jardim do Éden and Cidreira, a huge amount of sand was transported both landwards as transgressive dunefields and along the littoral drift system to the north of Tramandaí. These differences on coastal evolution in a relatively small stretch of coast is being accounted to slight differences on the substrate slope, which is presently represented by slopes of the inner continental shelf surface (Tramandaí: 0,062°, Jardim do Éden: 0,067° and Cidreira: 0,069°). These differences on substrate slope have determined the existence of different wave energies along the coast, with the lower energy related to more gentle slopes and defining a stable barrier as in the case of Tramandaí, and an increasing wave energy to the south related to more steeper slopes, defining transgressive barriers at Jardim do Éden and Cidreira.