Stratigraphic and Holocenic Evolution of the Submerged Platform of the Eastern Margin of the Lagoa dos Patos Lagoon, RS. east margin of the Lagoa dos Patos lagoon, between the spits of INTRODUCTION Cristóvão Pereira and São Simão, and close to Mostardas city (Figure 1). It was identified along the eastern margin of the Lagoa dos Patos lagoon, southern Brazil, a wide platform (up to 1 km) STRATIGRAPHIC ANALYSIS bordering the lagoonal margins, and limited by the 1 m isobath was described by VILLWOCK (1977), TOLDO JR. (1989 and 1994), TOMAZELLI (1990), VILLWOCK and TOMAZELLI (1998), The studied core is 2.7 m long (Figure 2) and was collected BARBOZA (1999), BARBOZA and TOMAZELLI (2003). These by a percussion method under a 1 meter deep water column. authors suggest that the border of the platform marks the Three lithofacies were identified: 1) 2.7 to 2.3 m - stained position of an ancient 1 m lower lagoonal level. According to yellow quartzose very fine sand, well sorted, showing roots; 2) TOMAZELLI and VILLWOCK (1989) this platform was drowned 2.3 to 2.05 m gray quartzose very fine sand, well sorted, with by a recent sea level rise started at about 1 2 ka. organic remnants and well preserved shells of Erodona By studying the subsurface stratigraphy of this platform this mactroides; 3) 2.05 to 0 m pale yellow quartzose very fine sand, paper aims to better define its nature. With this purpose a well sorted and homogeneous textural. Two shell samples from 14 shallow core (2.7 m long) was collected close to the edge of the lithofaciesshowed C ages ranging from 240 +\- 80 years to 310 platform. +\- 60 years. Lithofacies 1 is interpreted as the Pleistocene substrate (Barrier III) of the Holocene lagoonal deposits REGIONAL SETTING drowned and reworked by the Postglacial Marine Transgression. The top of this unit is limited by a ravinement surface. The Lagoa dos Patos lagoon, situated in Rio Grande do Sul The succession of lithofacies 2 and 3 is a product of coastal plain, southern Brazil, is the largest lagoon of the progradation of the lagoonal margin, with the surface of country, covering an area of about 10,000 km2 (Figure 1). Its VILWOCK TOLDO JR. lithofacies 3 corresponding to the 1 m deep platform bordering evolution was studied by (1977), (1989 the lagoonal shoreline. It is suggested here that platform and 1994). In general the long-term evolution of the lagoon ZENKOVITCH progradation is contemporaneous to the progradation and grow follows the model of (1959). This large coastal 14 water body is permanently connected to the Atlantic Ocean of the modern 2sand spits. The young C ages obtained for through an inlet situated in its southern part. Because there is lithofacies 2 indicates that lithofacies 3 was formed very only one inlet connecting the lagoon to the ocean, and because recently at the border of the platform, and that progradation is the coast is microtidal (mean tide range of about 0.30 m), tidal probably still active. effects on the lagoon-level and surface area are negligible. Thus, the Lagoa dos Patos lagoon is an open-water lagoon that SEDIMENT DYNAMICS ON THE has a relatively constant water surface whose level is basically SHOREFACE controlled by the relative variation of the sea level. The study area is placed in the area known as “Restinga da Wave statistics of the Lagoa dos Patos lagoon are used to Laguna dos Patos”, which is a complex multiple barrier calculate the mean breaker waves depth (d ), and the threshold (VILLWOCK, 1984), and whose origin is related to the relative b variations of the sea level during the Quaternary Period, of sediment motion. The results indicated an annual average associated to changes on climate an on sedimentary supply. The breaker depth changing between 0.3 and 0.9 m, based on the area selected for this study is located in the central portion of the wave height and beach slope (U.S. Army Corps Of Engineers, Journal of Coastal Research SI 39 266 - 269 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 E. G. Barboza†‡; E. E. Toldo Jr.‡; L. J. Tomazelli‡; S. R. Dillenburg‡ and R. N. Ayup-Zouain‡ † Programa de Pós-Graduação em Geociências/IG/UFRGS - PRH 12-ANP/MME/MCT Av. Bento Gonçalves, 9500 91509-900, Porto Alegre, RS, Brasil eduardo.barboza@ufrgs.br BARBOZA, E. G.; TOLDO Jr, E. E.; TOMAZELLI, L. J.; DILLENBURG, S. R. and AYUP-ZOUAIN, R. N., 2006. Stratigraphic and Holocenic Evolution of the Submerged Platform of the Eastern Margin of the Lagoa dos Patos Lagoon, RS. Journal of Coastal Research, SI 39 (Proceedings of the 8th International Coastal Symposium), 266 - 269. Itajaí, SC, Brazil, ISSN 0749-0208. It was identified along the eastern margin of the Lagoa dos Patos lagoon, southern Brazil, a wide (up to 1 km) and up to 1 m deep platform that occurs along most of the lagoon margin has been interpreted in previous studies as a drowned lagoonal margin. The Lagoa dos Patos lagoon is the largest lagoon of Brazil, has a length of 240 km, an 2 average width of 40 km, and covers a surface of 10,000 km . The lagoon runs roughly NE-SW and its average depth is almost 6 m, and its means tidal amplitude is 0.30 m. The lagoon can be subdivided into three morphological cells partially separated by sand spits that have grown from its margin. Although preliminary, this paper shows new data could change these previous interpretations. Two radiocarbon dating of mollusk shells from a shallow core (2.7 m long) recently collected near the edge of the submerged platform suggest an alternative origin for this morphologic feature. According to the dating the feature isn't a drowned lagoonal margin but it is a result of an active progradation of the lagoon margin under the present dynamics. New data coming from future studies to be carrying in the area would point to what is the most consistent hypothesis to explain this morphologic feature of Lagoa dos Patos lagoon margins. ADDITIONAL INDEX WORDS: Rio Grande do Sul coastal plain, wave-cut terraces, submerged platform. ABSTRACT ‡ Centro de Estudos de Geologia Costeira e Oceânica CECO/IG/UFRGS, Av. Bento Gonçalves, 9500 91509-900, Porto Alegre, RS, Brasil Journal of Coastal Research, Special Issue 39, 2006