Journal of Coastal Research SI 39 151 - 156 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 STATTEGGER, K.; CALDAS, L.H.O. and VITAL, H. 2006. Holocene coastal evolution of the Northern Rio Grande do Norte coast, NE, Brazil. Journal of Coastal Research, SI 39 (Proceedings of the 8th International Coastal Symposium), 151 - 156. Itajaí, SC, Brazil, ISSN 0749-0208. Sedimentological and geochronological studies were carried out along the north coast of Rio Grande do Norte state, NE Brazil, between Ponta dos Três Irmãos and the spit of Galinhos to reconstruct the Holocene coastal evolution of this area. Sedimentation during the Holocene has been controlled mainly by the variation of sea level, longshore currents and the advance and westward propagation of active dunes along this semiarid coast. A new sea-level curve could be constructed based on AMS- C datings of beachrock and lagoonal sediments and on careful elevation measurements. Plant remnants and bivalve shells found in the paleo-lagoon close to São Bento and Caiçara 3 metres below the modern sea-level were dated at 7000 cal yr BP. Passing its actual position at 6500 cal yr BP sea level reached the Holocene highstand 1.2m above the modern mean level at 5900 cal years BP as documented by non-recrystallized bivalve shells from beachrock and one shell midden. Sea level dropped afterwards to its present position as can be observed from elevation and age of regressive deposits. Secondary oscillations could not be detected during the regression. The semi-enclosed lagoon of São Bento experienced episodic marine incursions controlled mainly by the advance of active dunes that ceased the connections between lagoon and open sea. The last marine incursion occurred at 3580 cal yr BP. In contrast, the lagoonal system of Galinhos and Galos has remained active since the sea level reached the actual position for the first time at 6500 cal yr BP. 14 ADDITIONAL INDEX WORDS: Sea-level curve, beachrock, lagoon, radiocarbon dating. ABSTRACT Holocene Coastal Evolution of the Northern Rio Grande do Norte Coast, NE Brazil K. Stattegger†; L. H. de O. Caldas‡ and H. Vital‡ †Institute of Geosciences University of Kiel D-24118 Kiel, Germany kstattegger@gpi.uni-kiel.de ‡ Dept. of Geology UFRN Natal CP 1639 Natal RN 59072-970, Brazil lucianocaldas@yahoo.com.br INTRODUCTION Holocene coastal evolution of the northeastern Brazilian coast between Natal and Fortaleza has been poorly investigated so far. Recent studies focused on modern hydro-dynamics and sedimentation ( and , 1998; , 2002), neotectonics ( 1998), and late Quaternary stratigraphy and sea-level fluctuations ( , 1991; , 2002; 2003). The study of sea level and coastal evolution is enormously improved when accurate paleo sea-level indicators are available. Amongst of the existing sea-level indicators formed by coastal deposits one of the most reliable is beachrock, that means coastal sandstone representing the intertidal upper shoreface “frozen” by early diagenesis. Although some authors believe that beachrock is a more reliable sea-level indicators in microtidal coasts ( , 1995; , 2001; and , 2002), their use in sea-level studies in mesotidal coasts should not be underrated. Beachrock and lagoonal sediments are the most important coastal deposits occurring mainly in the intertidal zone along the entire northern coast of Rio Grande do Norte (Figure1). The description of sedimentary structures and AMS radiocarbon dating allow some considerations to be made about sea-level changes and coastal evolution for this area during Holocene time. The study area is located on the Potiguar basin which is part of the NE Brazilian Rift System together with others basins originated in Cretaceous times The investigated area has been subjected to late Quaternary neotectonics according to (1998). The coastal zone of this area is an open, wave dominated, meso-tidal coast. It is associated with the inner zone of the narrow continental shelf that is shallow (maximum 25 m depth) and has an average width of 18 km. The shelf-break is situated at 50 m water depth 20 km offshore. The continental shelf and coastal zones are strongly influenced by oceanic and wind driven currents from N and E-NE directions, which promote local erosion as well as deposition in the shore zone. A large lagoonal channel system can be recognized on the backside of the sandy spit of Galinhos. Some closed paleo- lagoons filled up with sediments occur near to São Bento do Norte. In the beach zone beachrocks occur frequently along the actual foreshore zone and in the backshore zone (Figure 1). The semidiurnal mesotidal tide has a maximum spring-tide range of almost 3 m and a minimum range of 0.8 m during neap- tides. The tidal measurements performed in the harbor of Macau yielded spring- and neap-tide amplitudes of 2.55 m and 1.27 m, respectively (DHN, 1974). The climate of the area is tropical, hot and semi-arid ( , 1989), and it is subjected to the conditions of the intertropical convergence zone. The dry period of 7 to 8 months lasts from June to January, while the rainy period of 3 to 4 months, lasts from February to May. The mean air temperature is approximately 26.8 C, with minimum temperatures of 25 C occurring at the end of winter (July) and maximum of 28.6 C in February during the summer. In order to obtain a reliable stratigraphic framework for the region, nine vibro-cores were taken in the study area utilizing aluminum barrels with a length of 6 m and inner diameter of 70 mm. 7 cores were drilled in the tidal plan of Galinhos and Galos and two vibro-cores in the swamp landward of São Bento do Norte. Descriptions of 5 water-wells from RN state's water company ( ) located in São Bento were also used. Beachrock outcrops have been sampled at the highest level containing suitable shell fragments, always from the intertidal upper foreshore section with low-angle swash-cross- stratification representing the paleo-beachface. This is the best available sea-level indicator for a mesotidal coast where the upper part of the beachrock has originanally been deposited in the intertidal upper foreshore zone. Here, much higher TESTA BOSENCE VITAL BEZERRA SILVA BARRETO BEZERRA RAMSAY OMOTO RAMSAY COPPER B NIMER CAERN et al. et al. et al. et al. , et al. , EZERRA o o o METHODS Journal of Coastal Research Special Issue 39, 2006 ,